Several large-scale fisheries operating in the high seas around the world are known to take a substantial bycatch of elasmobranchs, particularly sharks. Although sharks are retained and utilized in some of these fisheries, they usually are dumped, sometimes alive after their fins have been chopped off. The survival of released sharks varies depending on the type of gear used. Trawl and gill nets and perhaps purse seines, almost certainly cause 100% mortality. While longlines permit prolonged survival of sharks by allowing limited movement and thus some respiration, survival rates depend on the metabolism and endurance of individual species. Overall, it is believed that most of the bycatches of sharks in large-scale fisheries have high mortality. This might not be true for batoids which generally have different mobility requirements in order to respire. However, their catches are normally small in large-scale high seas fisheries due to their more demersal habits.
The amount of elasmobranchs killed in large-scale high seas fisheries is poorly known and has not been systematically assessed and an unknown part of the bycatch is discarded at sea. Reports on the sharks taken by the countries involved in these fisheries do not reflect the actual by catches but only the amounts retained. A purpose of this section is to present the available information on the most important large-scale fisheries of the world and evaluate the extent of their elasmobranch bycatches, the amounts taken and the total discards.
Until recently, there were two main large-scale fisheries catching and discarding significant numbers of elasmobranchs in their operations- driftnet and longline fisheries. Due to international pressures, and following UN Resolution 44/225, all large-scale driftnet fisheries were phased out of international waters at the end of 1992. They are discussed here because of the importance of their bycatches. In addition to longline and driftnet fisheries other large-scale fisheries with minor elasmobranch bycatches (tuna purse seine and pole and line fisheries) are briefly discussed. The deep trawl fisheries for orange roughy are also mentioned because of their potential impact on deep water shark populations. Attention is drawn to the assessments of the elasmobranchs caught and their catch rates. Incidental catches are estimated where no figures exist and are compared with reported landings for each fishery, or country, in order to assess the quantities of elasmobranchs wasted each year and which are not included in the official statistics of world fisheries.
For the last few decades, several countries, chiefly Japan, Korea and Taiwan (Prov. of China), prosecuted large-scale fisheries using drift gillnets on the high seas of many oceans. Typically, vessels deployed several kilometres of gillnet which efficiently trapped the relatively dispersed resources they aimed for. Unfortunately, they also captured many other non-target species, sometimes in vast quantities which commonly were not utilized. Concern over the impact of drift gillnets on the world's oceanic animals and ecosystems has been focused mainly on marine mammals, however, it is now known that sharks were among the most frequently caught non-target animals in some of these fisheries though little attention was paid to the effect of this gear on their populations. Although all large-scale driftnet fisheries on the high seas stopped as of December 1992, an assessment is attempted here of the effects on sharks and ray populations. Though most of this mortality has ceased, its effects may still effect subsequent generations of elasmobranchs.
The most important large-scale driftnet fisheries are examined to estimate the quantities and kinds of elasmobranchs that were caught in these global operations. The description of these fisheries is based on Northridge (1991) and bulletins of the International North Pacific Fisheries Commission (INPFC) (Myers et al. 1993, Ito et al. 1993) which give more detailed information.
Until recently, there were main large-scale driftnet fisheries in the North Pacific, the salmon fishery, the flying squid fishery and the large-mesh fishery for tunas and billfishes. As a result of these fisheries the North Pacific was the most heavily exploited area in the world by driftnets. This was probably a consequence of the geographic location of the three large-scale countries involved in driftnetting.
The Japanese fleet was the largest in this fishery. Canadian and US fleet sizes are still considerable but they use small driftnets (<500m per vessel) and fish exclusively in the coastal EEZ waters. There were two Japanese fisheries for salmon: (1) the mothership operation in international waters of the North Pacific south of the Aleutians and in the Bering Sea and, (2), the land based fishery in the high seas East of Japan (Figure 2.31). In general, during the past two decades the Japanese salmon fishery showed a consistent decline in effort that involved reductions in number of vessels, fishing area and fishing season.
Figura 2.31. Generalized area of operation of the Japanese land-based and non traditional (exmothership) fisheries in 1990. (Based on INPFC 1993). Of these fisheries, the mothership processing ships supported some 40 smaller catcher vessels. The fishing grounds were divided in subareas with different opening and closing seasons, although the fishery season only ran from 31 May to 31 July. The fishery contracted its operations primarily due to pressure from the USA, Canada and the former USSR. During 1990 and 1991 operations were converted to a landbased fishery by eliminating the motherships. Catches peaked in 1956 when approximately 9 300 000 tans were set, only 238 700 tans were set in 1991, the last year of the fishery (FAJ, 1991). Tans are independent net panels which are the working unit of driftnets and are typically 45–50m long in the salmon fishery. Driftnets are 8–10m deep and are constructed of nylon monofilament with mesh sizes in the range of 121– 130mm. Each vessel deploys a maximum of 15km of net in a dusk-to-dawn operation.
Two types of vessels were operated in the land based fishery: coastal boats of 30 GT and medium size vessels of 30–127 GT. Effort in this fishery also declined significantly latterly and the fishing area was reduced. There was a peak of 1400 coastal vessels in the mid-1970's but only 678 by 1978–1988 (Northridge 1991). There were 374 vessels over 30 GT in 374 boats in 1972; by 1991 there were only 83 (Myers et al. 1993). The number of sets peaked at approximately 19 700 in 1966 and declined to 4 100 (781 176 tans) in 1989 (FAJ, 1990) only 374 990 tans were set in 1991 (FAJ, 1991). During the last years of the fishery the season ran from late May to the end of June. Gillnets used by the landbased fishery were similar to those of the mothership fishery but with smaller mesh sizes (110–117mm). Coastal vessels of <10 GT set less than 10km of net per night while offshore vessels set up to 15 km.
Detailed reports on the by catches of non-target species in these fisheries (Northridge, 1991) are strongly biased towards studies dealing with marine mammals and birds; sharks are mentioned only as a side issue. However, the Fisheries Agency of Japan (FAJ, 1987, 1988, 1989) reported gillnet by catches of several non-target species in their research cruises for salmon. Table 2.9 show results for 1986-1988 together with the estimated total by catch of sharks taken in 1989 when 1 097 630 tans were set. Blue sharks are the most frequently reported shark species. The total by catch in the fishery for 1989 is estimated at 11 492 sharks consisting of 8 species, for approximately 108t. These estmates should be treated with caution. First, the areas and gear used in these research cruises appear to be different to those of the commercial fishery with at least some very small mesh sizes among the research driftnet effort. This will affect the catch rates of most species, both through changes in efficiency of the gear and availability of each species (e.g. blue sharks are not excepted to be caught in the Bering Sea in high numbers due to their more temperate distribution). Direct extrapolation to the total fishery should be done carefully. Second, most of the catch rates of sharks reported in Table 2.9 seem too low compared with other studies.
|Species||1986||1987||1988||Catch Rate a)||Estimated Numbers in Catch 1989b)||Likely weight (kg) c)|
|(24,549 tans)||(17,056 tans)||(17,805 tans)||(sharks/1000km)||Landbased||Mothership||Total||per shark||in the catch|
|Prionace glauca||142||188||79||137.69||5,378||2,179||7,556||2.42||18,287 d)|
a) assuming 50m tans in research cruises
b) based in effort reported by FAJ (1990)
c) Considering sizes expected for 110–130 mm mesh
d) Calculated from LeBrasseur et al. (1987) length frequency data, Pratt (1979) TL-FL relationship, and Strasburg (1958) L-W relationship.
Although no other direct reports for the salmon fishery were found except those of the FAJ research cruises, results from Canadian research cruises (LeBrasseur et al. 1987) can be used to derive alternative catch rates for sharks. The results obtained for blue and salmon sharks are of an order of magnitude higher than those calculated from FAJ data. They give values of 5275 and 194 sharks/1000 km of net respectively (Table 2.10). These research cruises were designed to assess the salmon by catches of the squid fishery but employed nets nearly identical to those of the commercial salmon fishery. Thus, their results should more accurately reflect the catch rates of sharks in the commercial salmon fishery.
In general terms, the total catches of sharks in the Japanese salmon fisheries is believed to have been relatively small compared with other driftnet fisheries in the north Pacific. Even considering the alternative catch rates of 5502 sharks per 1000km of driftnet derived above, some 300 000 individuals, or approximately 1237t, are estimated to have been caught during the 1989 season in this fishery. This relatively small catch is mainly a function of the size of the fishery, which has contracted year-by-year. As a reference point, according to Shimada and Nakano (1992), some 34 000 large and adult salmon sharks were landed from the salmon driftnet fishery in Japan in 1960. Further, reports for the early 1980's (Paust 1987) indicate that 25 000 salmon sharks (Lamna ditropis) were taken each year by the Japanese salmon fishermen in the central Aleutian region. Considering pertinent effort statistics and the catch rates obtained from LeBrasseur et al. (1987), a total of less than 1600 salmon sharks are thought to have been taken in the area south of the Aleutians in 1989, i.e., a reduction of about 95% in salmon shark mortality accompanied the decline of the fishery. Although there is not enough information to assess the level of catches and discards of sharks that took place in this fishery, it is possible that some of the salmon sharks would have been kept and utilized. This is suggested by reports of specific fisheries for this species taking place in NE Japanese waters off the Oyashio front (Paust 1987, Anon. 1988) which indicate that the salmon shark is sought by Japanese fishermen. However, the incentive to keep salmon sharks probably should be weighted against the availability of space and danger of spoilage of the valuable salmon catch. In July 1991, all Japanese salmon driftnet fisheries in the high seas ceased. Most ofthe fleet was disbanded although a minor part moved to Russian EEZ waters under a Russian joint-venture. There are no data available about this new salmon driftnet fishery but judging from the calculations made above its bycatches of elasmobranchs should be minor.
|Species||Sharks caught (618 tans)||Catch rate per/1000km of net||Estimated numbers in 1989 catch||Likely weight (kg) per shark in 1989 fishery|
|Prionace glauca||163||5,25||289,504||2.42||700,601 a)|
a) Calculated from LeBrasseur et al.(1987) length frequency data, Pratt (1979) TL-FL relationship, and Strasburg (1958) L-W relationship.
In the late 1970's, a major driftnet fishery for flying squid (Ommastrephes bartrami) was started in the Central North Pacific by Japan (in 1978), then Korea and Taiwan (Prov. of China). In 1990 almost 740 vessels from these countries prosecuted this fishery. Yatsu et al. (1993) summarize most of the information available for Japan. Japan limited the number of vessels and the area open to this fishery (Figure 2.32) by a northern boundary which moved through the year to avoid taking salmon - which was prohibited by the flying squid fishery. Japanese vessels were classified in two categories: 60–100 GT and 100–500 GT. The fishing season for these vessels ran from June 1st to December 31st, although two types of licences, for 7 and 4 months, were issued within the season. Nylon monofilament (0.5mm) driftnets were used with a mesh sizes of 100–135mm; 115–120mm sizes were the most common. Tans were 9–10m deep and 33–42m long. Each vessel set between 15 and 50km of net although some reports indicate that the most common sets were close to 50km.
Korea joined the fishery in 1979 (see Gong et al. 1993 for a full account). Korean squid driftnet vessels were mostly about 350 GT, but some exceeded 400 GT. The Korean fleet fished from April to early August in an area partially overlapping the Japanese grounds and from early August to mid-December for smaller squid east of Japan (Figure 2.32). Their driftnets had 50m tans with mesh sizes of 76-155mm. In the first fishing area mesh sizes used were 105–115mm while those used in the grounds east of Japan were 86–96mm. According to Gong et al. (1993) Korean vessels deployed about 28km of driftnet in the early 1980's but increased to 45km in 1990.
Information on the Taiwanese squid fishery, which started in 1980, is scarce and most of the information here is based on the brief account of Yeh and Tung (1993). Vessel sizes ranged from 100–700 GRT but most were 200–300 GRT. Driftnetters larger than 400 GRT were introduced mainly in 1984 while those larger than 600 GRT entered during the 1986–1987 season. Taiwanese driftnets for squid are believed to have been constructed of monofilament nylon. Mesh sizes ranged from 76–120mm with each tan measuring between 15 and 40m in length. Typical total lengths of driftnet deployed per boat were 31–41km (Fitzgerald et al. 1993). Taiwanese vessels were allowed to fish year round (Pella et al. 1993) but the fishing season was apparently from June to November (Yeh and Tung 1993) in an area similar to that fished by Korea but west of the Japanese EEZ (Figure 2.32).
Effort statistics for these fisheries have only recently been available. According to data provided by Yatsu et al. (1993), Gong et al. (1993) and Yeh and Tung (1993) the total number of vessels from the three countries prosecuting the squid driftnet fishery during 1988–1990 was 792, 784 and 737 respectively. Statistics on the total number of tans deployed by Japan and Korea are also available. Unfortunately, Taiwanese statistics do not separate effort between the squid fishery and the large-mesh driftnet fishery as their boats carried both gears and deployed either depending on the expected catch. Further, Taiwanese effort statistics are given only in total “vessel-days” fished (Table 2.11).
The total number of standardized tans set by the Taiwanese fleet in the squid fishery can be estimated using comparative data on total length of sets for vessels from each county. Fitzgerald et al. (1993) estimate a total of 51–61 km of driftnet per Japanese vessel and 31–41km per Taiwanese vessel. Data from Yatsu et al. (1993) indicate that Japanese vessels deployed an average of 997.43 tans (50m each) per fishing day during 1989 and 1990. The effort of Taiwanese vessels is assumed to be allocated equally to the flying neon squid and the large-mesh fisheries. Assuming the number of tans per vessel is equal in the Japanese and Taiwanese fleets, total estimated effort was 4 471 678, 5 616 888 and 3 595 855 standardized (50 m) tans for the Taiwanese fleet in the squid fishery for the years 1988–1990 respectively. Total effort for the three countries in this fishery is estimated at 64 782 236 tans (3 239 112 km) for 1989 and 50 922 388 tans (2 546 119 km) for 1990.
There are several sources of information on catches of non-target species, chiefly in the form of research cruises and more recently from observer programmes. Results from some research surveys enable an assessment of catch rates in numbers for blue, salmon and four other species of sharks, size structure and catch rate in kg/m for blue sharks, percentage distribution by mesh size for blue and unspecified shark species and differences in blue shark catches between surface and subsurface squid driftnets (FAJ 1983, Murata and Shingu 1985, Murata 1986, 1987, Rowlett 1988, Murata et al. 1989, Yatsu 1989, lto et al. 1990). However, results from these surveys suffer the same problems as for the salmon fishery research surveys. Japanese and Korean research cruises use a variety of mesh sizes which extended above and below the size of those used by the commercial fishery. The results therefore are limited to assessing total catches of non-target species. More useful information comes from the observer programmes on commercial vessels. Data from Japanese observers for 1988 (FAJ 1989) give catch rates of 536 blue sharks per 1000km of net. However, collective data from Japanese, Canadian and U.S. observers for 1989 (Gjernes et al. 1990) report 814 blue sharks per 1000 km of net.
Figure 2.32. Legal boundaries of the Japanese, Korean and Taiwanese flying squid driftnet fisheries. (Redrawn from Pella et. al. 1993).
Data for the 1990 observer programme (INPFC 1991) are more detailed and show that 12 elasmobranch species were taken as bycatch in the fishery. The catch rates for blue sharks was 718/1000 km of driftnet, followed by salmon sharks, 55/1000km of driftnet. Other large shark species caught, perhaps by entangling, were the thresher shark (Alopias vulpinus), shortfin mako (Isurus oxyrinchus), great white (Carcharodon carcharias) and basking shark (Cetorhinus maximus) (Table 2.12). Observer data from the Korean fleet for 1990 give an estimated catch rate of 32.08 sharks and rays per 1000 poks (Korean tans), equivalent to 641.6/1000km of net. This is slightly low, compared to the 785/1000km of net estimated for the Japanese fishery. Data on fishes in most of the observer programmes for the North Pacific driftnet fisheries are likely to be slightly underestimated. Only decked animals are counted thus unknown numbers of “dropoff” fishes are not included in the records. Despite this, observer programmes provide the best available information.
|Species||Numbers observed (2,281,896 tans)||Catch rate per/1000km of net||Numbers in total catch||Likely mean weight(kg)*||Weight in total catch (kg)|
|Prionace glauca||81,956||718||1,828,915||7 (1)||12,802,407|
|Lamna ditropis||6,263||55||139,764||38.7 (1)||5,408,866|
* considering sizes expected for 100–135mm. mesh
(1) from Yatsu et al. 1993
There are some estimates for elasmobranch by catches in the squid driftnets. Yatsu et al. (1993) estimate a total incidental catch of 723 933 blue sharks, 56 029 salmon sharks and 11 322 various sharks and rays for the Japanese fleet during 1990, making an estimated 7415t. Yatsu and Hayase's estimate considers sources of variability for cruises and sets sampled. However, their estimates of blue shark by catch for 1989 are almost double those for 1990 highlighting the variability in estimates and the changes in fishing effort during the period. Wetherall and Seki (1992) used a stratified estimate to obtain a total of 1.2–1.4 million blue sharks for the Japanese fishery during 1989 while Northridge (1991) estimated the total catch of blue sharks for the entire flying squid fishery during 1989 at 2.44 million individuals assuming the same effort level of 1988 and catch rates derived from Gjernes et al. (1990).
Using the effort statistics for 1990 and the results from the Japan-USA observer programme (INPFC 1991) the number of elasmobranchs caught by species and the estimated weight of their catches in the 1990 are summarized in Table 2.12. About 2 million sharks, an estimated 18 739t, were taken in the fishery. Of these, about 12 802t, or 1.8 million individuals, would be young blue sharks, which according to Nakano and Watanabe (1992), correspond to sharks 1-2 years old. Unless otherwise stated, the estimates of weight for each species are based on approximations made by the author that consider the relatively small mesh size of the nets and might be biased towards small sizes. These results are a minimum estimate of the catch of elasmobranchs by the fishery. Of the 18 700t of sharks caught, 8 400t would have been taken by Japan; Korea and Taiwan (Prov. of China) would have caught 9000t and 1300t respectively.
A great proportion of the elasmobranch bycatches are apparently dumped to the sea. Assessment of shark catches for the Japanese fleet in 1989 using the same procedure gave estimates of 1 800 000 individuals with a total weight of 12 654t. The reported catch of sharks by the squid fleet of Japan during 1989 is 237 734 individuals (FAJ 1990). If this figure is equal to the landed catch of sharks, about 1 560 000 sharks weighing some 10 900t were wasted in the operation. Some of the almost 95 000 salmon sharks estimated to be caught in the fishery might have been used as this species is more valued in Japan. An appraisal of the amount of elasmobranchs actually discarded by the fleets of Taiwan (Prov. of China) and Korea is not possible due to the lack of information on their landings from the squid fishery. The total catch of elasmobranch for the Korean and Taiwanese fleets in 1989, estimated in the same manner, is 9120t and 2067t respectively.
These estimates of elasmobranch by catches are approximate due to limitations of the available data. But they do highlight the problems in determining the size of the elasmobranch by catch and the proportions dumped at sea, e.g., estimates of total weight of the bycatch are sensitive to the average weights for each species used in the calculations. This is particularly true for blue shark which accounts for most of the by catches by number. Yatsu et al. (1993) use an average weight of 7kg for blue sharks but alternative calculations give an average of 2.4 kg/shark. This was estimated from the length frequency reports for blue sharks of LeBrasseur et al. (1987) and morphometric equations for the species provided by Strasburg (1958) and Pratt (1979). The estimate of 2.4 kg/shark is consistent with the results of Bernard (1986), Mckinnell et al. (1989) and Murata et al. (1989) for nets with the same characteristics as those of the commercial squid fishery.
The results derived here appear to be slight overestimates compared with other results. However, considering that observer programmes do not consider “dropoffs” from the nets, the present estimates may be closer to the real mortality caused by the driftnets and serve as an indication of the order of magnitude of the problem. Following this reasoning, previous appraisals of blue shark catches in the whole fishery (Anon. 1988) seem to be highly overestimated.
Efforts to minimize the take of non-target species in the squid driftnet fishery were unsuccessful. Data summarized by Gong et al. (1993) for Korean research experiments shows that shark bycatches can drop by up to 41% when subsurface driftnets are used instead of normal (surface) driftnets. Unfortunately, catches of the target species (neon flying squid) dropped by 73%, probably making operations with the subsurface driftnets unprofitable. As a result of international agreements the squid driftnet fishery of the North Pacific stopped at the end of 1992.
A large-mesh driftnet fishery for skipjack, marlin, albacore and other tunas on the high seas of the North Pacific was started in the early 1970's by Japan. However, this fishery ended on 31 December 1992 together with all other high seas driftnet fisheries in the area. It started in the coastal Japanese bluefin tuna fishery of the 1840's but by the late 1980's it covered an area extending from 140°E to 145°W (Figure 2.33). The fishing grounds were divided into two areas: a southern area open to fishing year round and a northern area with portions closed to fishing during some months to avoid catching salmonids. Recent reports indicate this fishery operated with vessels in the 100–500 GT range. Nets were made of nylon monofilament twines of 1.2mm diameter for smaller meshed nets and multifilament and multistrand for larger meshed ones. Mesh size was greater than 150mm. Meshes as small as 113mm have been recorded though most driftnets used 180mm (INPFC 1992). Tans were commonly 33–36 m long. Japanese boats were restricted to a maximum of 12km of net at a time. Recent figures show that 459 vessels from Japan participated in the large-mesh driftnet fishery in 1988 catching approximately 40 000t. Taiwanese vessels also participated in this fishery, but information is scarce. Apparently, up to 123 vessels from Taiwan (Prov. of China) took part in this fishery during 1989. The Taiwanese fish chiefly from June to December.
According to the most recent data (Fitzgerald et al. 1993), Japanese vessels deployed a total of 4 682 630 standard (50m) tans in this fishery during 1990. Taiwanese effort is assumed to be the same for that estimated for the squid driftnet fishery (see above) due to the combined nature of these fisheries (Yeh and Tung 1993). The total effort of both countries during 1990 was equal to a total of 413 924 km of large mesh driftnet.
Information on the kinds and numbers of elasmobranchs caught in this fishery has become available through the reports of the international observers programme (INPFC 1992). Catch rates and estimates of the total catches of sharks and batoids based on effort levels reported for 1990 indicate that about 150 000 sharks, equivalent to 1722t, were taken as by catch (Table 2.13). The average weights of some species were obtained from research cruises that used driftnets with mesh sizes 150–180mm (FAJ 1983) while others are estimated from the mesh sizes used.
The estimated elasmobranch by catch of 366 fish/1000km for the large-mesh driftnets is about half that of the squid fishery. This difference is related to the different selectivity of the different nets. Larger meshes allow a greater escapement of small non-target species. Blue shark catch rates are less than half of those observed in squid driftnets and catch rates for salmon sharks are even lower, though the average size of each species tends to be larger in the largemesh fishery. Of the total estimated catch of elasmobranchs in this fishery in 1990, approximately 974t would have been taken by Japan and 748t by Taiwan (Prov. of China).
No estimates of past elasmobranch by catch could be found for this fishery to compare with the present values. Further, there are no data on the amounts of elasmobranchs landed from the large-mesh fishery in Japan or Taiwan (Prov. of China). Judging from the trends in other high seas fisheries, it is likely that most bycatches of sharks were discarded at sea.
Large-scale driftnet fishing stopped in 1991 in the South Pacific. Previously Japan and Taiwan (Prov. of China) fished chiefly for albacore with large-mesh driftnets (Northridge 1991). Due to pressure from coastal states in the area it was agreed to terminate these high seas South Pacific fisheries by 1991. It is unclear if the agreement relates only to the waters of the South Pacific Commission (Figure 2.34) or if it also includes the eastern waters of the South Pacific. Japan stopped all large-scale driftnet fishing in the area in 1990 (Nagao et al. 1993). No information on Taiwanese vessels activities is available however, it appears that elasmobranch bycatch in large-scale driftnets in the South Pacific should at present be little or nothing, even if vessels from Taiwan (Prov. of China) continue to fish there.
Figure 2.33. Area of operation of Japanese large-mesh driftnet fishery (Redrawn from Nakano et al. 1993).
|Species||Numbers observed (513,367 Tans)||Catch rate per 1000km of net||Numbers in Total catch||Likely mean Weight (kg)||Weight in Total catch (kg)|
|Unidentified shark||57||12.00||4,967||25 ?||124,177|
|Prionace glauca||7,692||300||124,040||9.2 (1)||1,141,168|
|Lamna ditropis||136||5.30||2,193||32.5 (1)||71,276|
|Isurus oxyrinchus||592||23||9,547||30 ?||286,395|
|Alopias vulpinus||6||0.23||97||167 (1)||16,158|
|Carcharodon carcharias||35||1.36||564||47.7 (1)||26,922|
|Dasyatis violacea||73||2.84||1,177||12 ?||14,126|
|Dasyatis brevis||8||0.31||129||12 ?||1,548|
|Unidentified ray||69||2.69||1,113||12 ?||13,352|
(1) Derived from F.A.J. (1983).
Some reports of elasmobranch catch rates in the South Pacific are given in Table 2.14 based on data from Sharples et al. (1990) and Watanabe (1990). Their sources of information are two research cruises conducted in the Tasman Sea and the Sub-Tropical Convergence Zone (STCZ) to the east of New Zealand between 30° and 45°S. Catch rates estimated from these data are 181 and 158 sharks/1000km of net for the STCZ and the Tasman Sea respectively, or 5035 kg/1000km of net for the Tasman Sea. While total elasmobranch catch rates seem relatively similar among both areas, strong differences in catch rates for individual species are evident from the detailed information (e.g. blue sharks are more frequently caught in the STCZ than in the Tasman Sea while the opposite is true for mako sharks). Further, the catch rate for the Tasman Sea is high compared to data given by Coffey and Grace (1990). These differences illustrate the difficulties faced in extrapolating from catch rates to total by catches when the data are based on information limited to a particular area, fishery or season.
|Species||STCZ (464 km of net)*||TASMAN SEA (766 km of net)**|
|Numbers Caught||Catch rate (#/1000 km)||Numbers Caught||Catch rate (#/1000 km)||Mean Weight||Catch rate (kg/1000 km)|
* Data from Watanabe (1990)
** Data from Sharples et al. (1990)
Based on commercial vessel activities, Coffey and Grace (1990) estimated catch rates of 48 sharks/1000 km of net and a total bycatch of 3500 sharks from the Tasman Sea area for the 1990 season. Murray (1990) compiled data from several sources and provides information on percentage by weight of sharks in total catches of Japanese research campaigns using 3 types of driftnets along with the total effort for each type of net. Shark catch rates calculated using this information, and assuming 50m tans are 16 362 kg/1000 km, for albacore nets; 14 618 kg/1000km, for slender tuna nets; and 21 781 kg/1000 km, for pomfret nets. Given the lack of estimates of the total amount of nets deployed in these fisheries it is necessary to use the above percentages of sharks as a by catch for the albacore nets and the reported albacore catches for driftnet fleets in 1989 provided by Lawson (1991). Thus, estimates of total shark by catches are: Japan, 3462t, Korea, 48t and Taiwan (Prov. of China), 2871t for a total of 6381t. This corresponds to the reported peak in albacore driftnet catches. Therefore, total by catch levels should have been smaller in the earlier and later years of this fishery. These estimated catches are for the waters of the South Pacific Commission (Figure 2.34) only and are crude estimates limited by the available information. Further, it is unknown if the data cited by Murray (1990) on which the by catch percentages are based, contain information from the whole South Pacific region or only part. Geographical variations in abundance are likely to considerably affect the by catch estimates. Without information about driftnetting activities in the rest of the South Pacific Ocean it is possible that, given the proportion of the South Pacific covered by the SPC (about 2/3), the by-catch of elasmobranchs in the whole Southern Pacific could have been 50% more than that calculated for the SPC zone, or a total of 9572t. There is uncertainty about this estimate, which is about half the driftnet catch of elasmobranchs in the North Pacific Ocean.
Several countries have extensive driftnet fisheries in the Indian Ocean but most coastal states, e.g., India, Pakistan and Sri Lanka, only fish inshore waters in small to medium-scale fisheries (Section 2.2.). The elasmobranch catches of these coastal states are assumed to be landed and reported in FAO statistics. Taiwan (Prov. of China) is the only country known to have large-scale driftnet fisheries in the international waters of the Indian Ocean but only limited information is available on their activities. Their tuna fishery started with one boat in 1983 and increased to 139 vessels by 1988. Fishing apparently occurs from November to March with driftnets of 200–220mm mesh size, 20–24m depth with 20–25 or 37–47km of net deployed per vessel. Fishing occurs in the North West and South Central Indian Ocean. Hsu and Liu (1991) report sharks to be 23 by number and 29% by weight of the total catches for the 1986–1987 season while for 1987–1988 season their contribution decreased to 0.52% and 2.07%. As no significant changes in the fishing area were observed between both fishing seasons, this reduction in shark bycatches is most likely caused by changes in discard practices. By multiplying the percentage composition of sharks by the reported total landings of 18 28lt in the 1986–1987 season (IPTP 1990), 5405t of sharks are estimated to have been caught by the fishery. A total of 6108t of shark is estimated to have been caught during the 1988–1989 season assuming that the number of vessels increased by 13% over the 1986–1987 level.
Until recently, the only known large-scale driftnet fisheries in the Atlantic were a French albacore fishery and an Italian swordfish fishery. However, Taiwanese driftnet vessels were also believed to operate in this ocean during the early 1990's. Many other gillnet fisheries exist in the Atlantic and Mediterranean and in many cases large quantities of nets are deployed nightly. However, most of these fisheries are limited to coastal waters and are not within the scope of this section. A summary of these smaller fisheries is given by Northridge (1991).
The French albacore fishery began in the Bay of Biscay in 1986; 37 vessels operated in 1989. These boats troll during the day and use gillnets at night. Fishing occurs from June to September and extends from the Azores north and eastward, following the albacore. Nets are 20–36m deep with 80–120mm mesh size; a mesh size of 90mm is the most successful. While French reports indicate driftnet lengths of 2–6km per vessel, Greenpeace claims they are up to 20km long. The only available information on shark by catches indicates they were of the order of 6–10%. Woodley and Earle (1991) observed several French boats and report sharks (mostly Prionace glauca) as the most common by catch, amounting to 6.2% of the albacore catch. The sharks caught ranged between 40–250 cm but were most common between 125–200cm. Woodley and Earle estimate catch rates of 1750 to 3520 sharks/1000km of net (including dropouts) for a total catch of 22 015 to 44 282 sharks during the 1991 French albacore fishery. This is equivalent to 430–865t of sharks assuming a mean total length of 175 cm for blue sharks. They reported a discard of 2 sharks at sea but no further information is available on the disposition of the shark by catches in this fishery. These shark catches could be included in the reported “various elasmobranchs” of France which amount to almost 10 000t/yr.
Figure 2.34. South Pacific Commision statistical area (Taken from Lawson 1991).
The use of driftnets in Italian fisheries for tuna and swordfish has a long history, but the fishery expanded only from the 1980's as a consequence of government support. According to Northridge (1991), this was one of the largest driftnet fisheries in the world before it was banned. By 1989, 700 vessels participated, 90% of them used nets of 12–13 km in length with depths of 28–32 m with mesh sizes of 180–400mm. A few vessels used less than 6km of net and a few others, more than 20km. The fishery pursued albacore and swordfish from Sicily and Calabria to the Ligurian Sea. While no information on catch rates of non-target species exists, several elasmobranchs have been reported caught by this fishery. Species commonly caught include thresher, blue and porbeagle sharks as well as manta and common eagle rays. Another three sharks are reported to be infrequently taken and 10 more as occasional taken species (Table 2.15). It is unknown if most of the catches were kept or discarded. It is impossible to estimate the amount of the total catch from available information, however, a large increase in landings of smoothhounds took place concurrently with the expansion of the driftnet fishery and it is known that other sharks are commonly merchandised locally as smoothhounds (De Metrio et al. 1984). It is possible that a considerable part of the shark by catch was landed by this fishery. Recent reports suggest that there are still some driftnetters in the Ligurian Sea using gear lengths above the permitted 2.5km per vessel for this area (ICCAT 1993a).
|Common Name||Scientific Name|
|Species commonly caught|
|Thresher shark||Alopias vulpinus|
|Blue shark||Prionace glauca|
|Manta ray||Mobula mobular|
|Common eagle ray||Mylobatis aquila|
|Basking shark||Cetorhinus maximus|
|Shortfin mako||Isurus oxyrinchus|
|Smooth hammerhead||Sphyrna zygaena|
|Bigeye thresher||Alopias superciliosus|
|Spinner shark||Carcharhinus brevipinna|
|Blacktip shark||C. limbatus|
|Dusky shark||C. obscurus|
|Sandbar shark||C. plumbeus|
|Great white shark||Carcharodon carcharias|
|Sharpnose sevengill shark||Heptranchias perlo|
|Sand tiger shark||Carcharias taurus|
|Smalltooth sand tiger||Odontaspis ferox|
|Hammerhead shark||Sphyran spp.|
|Bull ray||Pteromylaeus bovinus|
Northridge (1991) reviews several reports of Taiwanese vessels fishing with large driftnets in different areas of the Atlantic Ocean. However, apart from accounts confirming these activities in the Atlantic, no other information is available. There are no reports of the fate of driftnet fisheries in the Atlantic Ocean though all large-scale driftnet fishing was prohibited after 1992 in parallel with the worldwide ban on high seas driftnet fishing.
High seas driftnet fisheries have been an important source of elasmobranch by catches. The estimates here suggest that the total elasmobranch by catch could have been between 3 280 000 and 4 310 000 sharks and rays per year during 1989–1991, i.e., of the order of 20 000–38 000t/yr. Total discards of elasmobranchs at sea from driftnet fisheries could have been as high as 30 500t/yr. If all Taiwanese and French catches were kept, discards could have been as low as 20 803t/yr. These results are derived from the estimated totals for the previously described fisheries and thus are highly uncertain. They should be used only as an approximation of the amount of elasmobranchs taken by driftnets worldwide.
Even though these figures are approximate, a clear picture arises from the analysis of information from global high seas driftnet fisheries (Table 2.16). The North Pacific fisheries were the most intensive and therefore the most important in terms of waste of sharks and rays. In particular, the flying squid fishery, with its high catch rates and massive effort, killed more elasmobranchs than any previous high seas driftnet fishery. Of the world by catch of elasmobranchs by driftnets, the North Pacific fisheries accounted for the largest proportion of the total and were also the best studied.
|Fishery||Total catch in tonnes||Total catch in||Catch rates|
|Lower level||Upper level||number of individuals||(sharks/1000 km nets)|
|North Pacific Ocean|
|squid(90)||5,905||-||18,739||2.0–2.44 Million||536 - 814|
|South Pacific Ocean(89)||6,381||-||9,572||56,000–841,500*||48–181|
|Atlantic Ocean(91)||430||-||865||22,000 –44,000||1,750 –3,520|
* from extrapolation of average weight of large mesh fishery
Blue sharks were the most common animal caught in driftnet fisheries because of their abundance in pelagic habitats; in 1989 an estimated 2.2–2.5 million sharks were caught worldwide. Blue sharks may be the elasmobranch most affected by these fisheries but more information is needed to confirm this.
The uncertainty about the estimated catch rates for each fishery highlight the importance of cooperative observer programmes in high seas fisheries: only fisheries with observers provided enough information confidently estimate elasmobranch bycatch and determine the species affected. Hence, the best estimates are those for the North Pacific squid and large-mesh fisheries, the only fisheries which had observers. Greater uncertainty exists in the estimates of catch and mortality rates for the other fisheries.
With the recent closure of large-scale high seas driftnet fisheries, the mortality caused by these fisheries has ceased providing relief to many populations of birds, mammals and other marine fauna. Unfortunately it provides only partial respite for elasmobranchs, particularly sharks, which continue to be caught incidentally in large numbers in other high seas fisheries.
The most important large-scale longline fisheries are those for tunas and billfishes. These fisheries are prosecuted by several countries and occur in all of the oceans. As a consequence of technological innovations such as deep longlines and blast freezing, some of these fisheries supply the most valuable world markets such as that for sushimi. These fisheries target several species and often sharks account for a large part of the bycatches. Sharks are regularly discarded if freezer space, which is limited, is insufficient for the more valuable species. The amount of elasmobranch by catch in these fisheries is unknown and is difficult to assess as most of the international bodies managing these fisheries (i.e. ICCAT, IPTP, SPC, IATTC) do not explicitly include sharks in their statistics or undertake research on elasmobranchs.
Japan, Taiwan (Prov. of China), Korea and Spain have the most important large-scale longline fleets operating in the Atlantic Ocean. Several countries, e.g. Canada, Cuba, USA, Italy, Morocco and Brazil have longline fisheries in their own waters but their efforts are small and in some cases the elasmobranch by catch is utilized and included in official statistics. Most of the information on Atlantic high seas fisheries comes from documents produced by the International Commission for the Conservation of Atlantic Tunas (ICCAT). However, their information is of variable quality; this should be considered when interpreting the results.
Japanese longliners have fished albacore (Thunnus elalunga) and yellowfin tuna (Thunnus albacares) in the Atlantic Ocean since the mid 1950's and bigeye tuna (Thunnus obesus) since at least 1961. The fleet expanded their range from the western Atlantic equatorial grounds in 1956 to virtually the entire Atlantic by 1970 (Figure 2.35) (Susuki, 1988). Most recently, bigeye tuna made up more than half of the catches and is targeted by deep longlines year-round between 45°N and 45°S. Deep longlines were introduced by the Japanese fishery in 1977 and they also take yellowfin tuna and swordfish (Xiphias gladius). Additional effort is directed towards bluefin tuna (Thunnus thynnus) in the Mediterranean Sea (ICCAT 1991a).
The number of Japanese longliners in the Atlantic during 1988 and 1989 was reported as 183 and 239 (NRIFSF 1992) and used 68 444 716 and 91 395 915 hooks respectively (ICCAT 1992). Recent data show that the Japanese fleet's effort is increasing in the Atlantic Ocean with 96 651 000 hooks set during 1990 (Uozumi 1993). Japan reported 366 and 500t of “other species” caught in 1988 and 1989 but there is no indication if this includes sharks or other elasmobranchs.
Figure 2.35. Effort distribution of Japanese longline fishery in the Atlantic Ocean in the 1980's. Keys indicate accumulated nominal book numbers in thousands, (Redrawn from Nakano 1993).
Hooking rates of sharks in the different areas of the Atlantic Ocean where the Japanese longliners operate are poorly documented. With one exception, most available information relates only to Japanese longlining activities in the North West Atlantic. Witzell (1985) estimated hooking rates of sharks by Japanese longliners at 1.31 sharks/1000 hooks (107 kg/1000 hooks) for the Gulf of Mexico and 5.98 sharks/1000 hooks (378 kg/1000 hooks) for the US Atlantic Coast. These are minimum estimates as they are based on Japanese logbook information and under-reporting is known to occur (Nakano 1993). Reports from observers in Japanese longliners fishing in the Gulf of Mexico indicate higher hooking rates of 1.74 sharks/1000 hooks (Lopez et al. 1979). Au (1985) documents catch rates of between 1 and 5 sharks/1000 hooks as the most frequently recorded for Japanese longliners in US waters based on observers' data. Au reports about 20 shark species to occur in the by catch.
Hoff and Musick (1990) provide monthly numbers of fish caught for 10 shark groups and numbers of sets made by Japanese longliners in the US EEZ in 1987. They report 8330 sharks, from more than 8 species, taken as by catches in this fishery. Blue sharks comprise about 85% of the total numbers followed by porbeagle and shortfin mako. No indication of sizes or weights is given. Assuming an average of 2206 hooks per set (derived from data of Lopez et al. 1979) the total hook rate is 7.04 sharks/1000 hooks.
Hooking rates reported by Nakano (1993) for sharks in Japanese Atlantic operations range between 1 and 4.5 sharks/1000 hooks with an average of 2.1 sharks/1000 hooks. Nakano lists 11 elasmobranchs (10 sharks and 1 ray) caught during a research cruise in the Atlantic during the 1960's but does not give hooking rates by species. Although Nakano derives separate estimates for the North and South Atlantic, these hooking rates are underestimated because of the common under-reporting of sharks in logbooks. Most skippers do not report sharks catch and some only record sharks of economic value (Nakano 1993).
Information on shark by catches by other longline operations confirms the order of magnitude of hook rates estimated above for the Japanese fishery. Research cruises by the USA in the North Atlantic are documented by Sivasubramaniam (1963) and Brazilian tuna longliners in the Equatorial West Atlantic by Hazin et al. (1990). From Sivasubramaniam (1963) hook rates of 10.35 sharks/1000 hooks can be derived for an area inside 0–80°W and 30–40°N. A smaller area within this had catch rates for blue and oceanic whitetip sharks (Carcharhinus longimanus) of 3.32 and 2.3 sharks/1000 hooks respectively. For Brazilian longliners, averages can be calculated from the hook rates for 6 shark groups provided by Hazin et al. for 1° squares off Rio Grande do Norte. The results give an overall rate of 8.66 sharks/1000 hooks, 3.94 for blue sharks, 4.17 for grey sharks (genus Carcharhinus), 0.27 for mako sharks, 0.08 for thresher sharks, 0.14 for crocodile sharks (Pseudocarcharias kamoharai) and 0.06 for oceanic whitetip sharks. In coastal areas higher hooking rates of up to 41.6 sharks/1000 hooks occured (Berkeley and Campos 1988).
Extrapolating from these hooking rates for specific areas to the total Atlantic is dangerous as the distribution of sharks is not homogeneous in space and time. Also, two different kinds of gear (regular and deep longline) are used in commercial longlining which have different effects on the catches (Gong et al. 1987, Gong et al. 1989). But, the reported range of hooking rates places bounds on the uncertainty. From the reports listed above there appears general agreement that the hooking rate for the Atlantic Ocean is ranges between 1 and 10 sharks/1000 hooks.
Because of the scarcity of information, hooking rates derived from Hoff and Musick (1990) are used here to estimate total catches of Japanese longliners in the Atlantic. They constitute the most recent data based on Japanese longliners and are well within the overall range of hook rates available. But as the species composition of the shark bycatch changes with location of the fishing grounds, no extrapolation is made to the whole Japanese Atlantic fleet because of the limited areal coverage of Hoff and Musick's data. The figure of Hazin et al. (1990) of 40.91 kg per shark is used to estimate the weight of the catch. The total catch of sharks by Japanese longliners during 1989 in the Atlantic Ocean is estimated as outlined above at 643 427 sharks or 26 322t. The estimates for 1990 are 680 423 sharks or 27 835t. However, some uncertainty is associated with these assessments. The estimates for 1989 could be substantially smaller (14 619t) if calculated using the 30% ratio of sharks to total tuna catches suggested by Taniuchi (1990), or larger (40 149t) if the average weights reported by Witzell (1985) for the South East Atlantic USA area are used. However, the average weight of 40.91 kg/shark seems to be supported by Rodriguez et al. (1988) who found an average weight of 48.9 kg/shark for the bycatches of the Cuban longline fleet operating in the tropical Atlantic during 1973–1985.
The percentage of sharks killed in the Japanese longline fishery is only 7.2% along the Atlantic U.S. coast because of the mandatory release of all bycatches and probably because most of the catches are blue sharks (Witzell 1985). This species, as well as other carcharhinid sharks, survives better when caught by longlines than lamnoid sharks (Sivasubramaniam 1963, Hoff and Musick 1990, Hazin et al. 1990). If this mortality rate is common for the whole Japanese Atlantic fishery, then between 1052 and 2890t of sharks died during their 1989 operations. However, other reports indicate that the U.S. enforced release of all shark by catches in this fishery is not observed for the entire Atlantic (Nakano 1993).
Moreover, the species composition of the by catches changes with latitude and this could alter survival rates. Additional errors in the estimated by catch of elasmobranchs are expected arising from the multiple areas and types of gears used by the Japanese longliners across the Atlantic Ocean. However, as better data on areal, seasonal and gear-specific hooking rates are unavailable it is impossible to obtain better estimates.
The reported catch of elasmobranchs by Japan in the Atlantic Ocean in 1989 is 1540t (Section 2.2). This is close to the lower limit of the range of elasmobranch catch estimated here. However, if the average of the different estimates provided above is taken then at least 15 466t of sharks would have been dumped with most finned prior to discard (Nakano 1993).
The Korean longlining fleet had 29 vessels operating in the Atlantic Ocean in 1988 and 33 during 1989 (NFRDA 1992). This fleet uses deep longlines which since 1980 have been directed mainly at bigeye tuna. Both the number of vessels and the catches of Korea in the Atlantic have decreased since 1977. These vessels reported an effort of 21 968 198 hooks and a total “others” catch of 944t for 1989 (ICCAT 1992). No information is available on the species composition of the “others” category and no reports of elasmobranch by catches for this particular fishery are known.
An examination of the reported Atlantic fishing grounds of the Korean fleet during 1983– 1985 (NFRDA 1988) shows that most of the effort was between 20°N–20°S (Figure 2.36). Thus, it is more appropriate to use the hook rates derived above from Hazin et al. (1990) for the equatorial Atlantic. It is estimated that 190 245 sharks (86 554 blue sharks, 91 607 grey sharks, 5932 mako sharks, 1758 thresher sharks, 3076 crocodile sharks and 1318 oceanic whitetip sharks) or some 7783t were caught during 1989 by Korean longliners in the Atlantic Ocean. This estimate is high compared to the reported 143t of elasmobranchs reported taken in that year by South Korea in the Atlantic Ocean (FAO 1993). Presumably an elasmobranch discard of at least 97% occured in this fishery. The proportion of sharks released alive and the extent of finning practices in the Korean fishery are unknown.
Taiwan (Prov. of China)
Longliners from Taiwan (Prov. of China) have fished for albacore in the South Atlantic since at least 1967 and in the North Atlantic since at least 1972. More than 80% of their catch is of albacore, Bigeye tuna is the next most common species taken. During 1989, 3 600 000 hooks were deployed in the North Atlantic by Taiwan (Prov. of China) compared to 68 700 000 in the South Atlantic (ICCAT 1991b). According to Hsu and Liu (1992) in 1990 this increased to 99 800 000 hooks, 17.4 and 82.4 million in the North and South Atlantic respectively. Of these, 17 500 000 hooks were used by deep longlines fishing bigeye and yellowfin; the remaining 82 200 000 hooks were on longlines fishing for albacore principally in the South Atlantic (Figure 2.37). The Taiwanese catch of sharks was 736t for 1990 and during 1991 the number of vessels operating in the Atlantic fell about 10% though the reported shark bycatch increased to 1486t (Hsu and Liu 1993). Hsu and Liu (1993) note that the variations in the reported by catches of sharks from this fishery are determined by the catch success for target species. When tuna catches are low, vessels keep a larger proportion of the shark by catch.
Figure 2.36. Distribution of Korean longline catches, no units given. (Redrawn from NFRDA 1988).
The reported catch of sharks in this fishery is small for the number of hooks deployed by the Taiwanese longlining fleet. The Taiwanese fish predominantly in the South Atlantic and thus the hooking rates derived from Hazin et al. (1990) are more appropriate. Nevertheless, much of the effort occurs in temperate waters so the amount of by catch can not be separated by species. Under these assumptions, Taiwanese longliners caught an estimated 864 268 sharks in 1990 (equivalent to 35 357t). The actual catch of elasmobranchs by Taiwan (Prov. of China) from the Atlantic Ocean is unknown, thus this analysis can only be approximate. However, it indicates an alarmingly discard of 34 000t of sharks from the fishery! As in the other fisheries discussed here, the number of sharks released alive or discarded dead is difficult to determine with the available information.
The Spanish longline fishery for swordfish in the Atlantic can be traced from 1973 (Garces and Rey 1984). Fishing grounds for 1988–1991 were centred in the Eastern Atlantic between 55°N and 15°S (Figure 2.38) though some activity was reported in the Mediterranean. Surface longlines are used in the North Atlantic but deep longlines were used in the Southeast Atlantic. The deep longlines consist of baskets of about 1200 m of line between floats having some 33 branch lines 15m long with the deepest hooks between 360 and 470 m (Rey and Muñoz-Chapuli 1991). The Spanish fleet set 35 850 078 hooks in the Atlantic Ocean and 7 683 580 in the Mediterranean during 1989 with increases of 6.75 and 7.3% in 1990 respectively (ICCAT 1991a, 1992).
De Metrio et al. (1984) give catch rates of blue sharks in swordfish longlines in the Mediterranean of 0.014/1000 hooks. However, they do not consider other shark species or discards at sea and the estimates are thus biased downwards. Rey and Alot (1984) give catch success rates for the Spanish swordfish fleet in the western Mediterranean of 6.34 blue sharks, 0.32 shortfin mako sharks, 0.21 smooth hammerhead sharks (Sphyrna zygaena) and 0.005 pelagic rays, per 1000 hooks. Mejuto (1985) reports CPUE values of 138.8, 17.5 and 1.1 kg/1000 hooks for blue, shortfin mako and porbeagle sharks respectively in the north and north western grounds of the Spanish Atlantic swordfish fleet based on a sample of 200 trips during 1984. Based on Mejuto's report, this gives catch rates of 13.7, 0.259 and 0.016 sharks/1000 hooks respectively for those species. These catch rates include discards of blue sharks, which Mejuto estimates at 68.4% in weight. Mejuto also found a linear relationship between swordfish catch and discards of blue sharks due to limited storage capacity and low value of blue sharks. He notes that in many cases the shark fins were removed before discarding. More recently, Mejuto and Iglesias (1988) report on exploratory swordfish longlining during 1986 in the Western North Atlantic. Their data gives catch rates of 13.5 and 2.05 sharks/1000 hooks or 168 and 61.7 kg/1000 hooks for blue and shortfin mako sharks respectively.
Figure 2.37. Distribution of nominal CPUE of bigeye tuna (a) and albacore (b) in the deep and regular longline fisheries of Taiwan (Prov. of China) in the Atlantic Ocean, 1990. (Redrwan from Hsu and Liu 1992).
Figure 2.38. Distribution of effort (in thousands of hooks) by the Spanish swordfish longline fishery in the Atlantic Ocean during 1988-1991. (Redrawn from Mejuto et al. 1993).
The elasmobranch bycatch of Spanish longliners includes more than the 3 species mentioned above; Muños-Chapuli (1985b) report 16 species of sharks occurring in the landings of the fleet fishing between Cape Verde Island and the Azores. The blue, the shortfin mako and the smooth hammerhead shark, Sphyrna zygaena, were, in order, the most abundant sharks in the catches (Table 2.6, Section 2.2.2).
The limited information from the southern Atlantic fishing grounds of the Spanish swordfish fishery, where deep longlines are used, suggest important changes in the species composition. Rey and Muñoz-Chapuli (1991) report 14 elasmobranch species in the catches of this area from 16 nights fishing of a single commercial longliner. Their data give average shark hook rates per 1000 hooks of 20.6 for night sharks, Carcharhinus signatus, 6.3 for silky sharks, 3.4 for bigeye thresher sharks, 2.9 for blue sharks, 2 for devil rays Mobula sp., 1.8 for shortfin mako sharks, 0.3 for common hammerhead sharks and less than 0.3 for Sphyrna couardi, S. mokarran, S. zygaena, Centrophorus granulosus, Galeocerdo cuvieri, Isurus paucus and Carcharhinus plumbeus. The overall catch rate of elasmobranchs is estimated at 38.8 fish/1000 hooks which is high compared to those for Spanish swordfish longliners in the North Atlantic. The different areas fished and gears used may cause these discrepancies, but the limited period and few operations observed by Rey and Muñoz-Chapuli could also be a significant source of bias.
The total catch of sharks by the Spanish fishery for 1989 can be estimated using the results of Mejuto (1985). His report, which considers the discards of blue sharks and provides catch rates in weight, is based on a larger time frame and geographic coverage than other reports. It is estimated that with the effort level in 1989, more than 608 000 sharks weighing 6856t were caught by this fishery (5646t in the Atlantic and 1210t in the Mediterranean (Table 2.17). Mejuto also estimates the discard rate to be 68.3% for blue sharks in the Spanish swordfish fleet and finds an inverse relationship between blue shark discards and swordfish catch. The total discard of blue sharks from the Spanish fishery during 1989 was estimated at 4134t. These results
|Information from Mejuto (1985)||Estimated total catch 1989|
|Numbers||Weight (t)||Hook rate||CPUE||Mediterranean (7.68 M hooks)||Atlantic (35.8 M hooks)|
|Species||(17.344 M hooks)||(sh/1000 h)||(kg/1000 h)||Numbers||Weight(t)||Numbers||Weight(t)|
|Prionace glauca *||237,660||2,408||13.703||138.8||105,286||1,067||491,244||4,977|
* includes estimated discards (68.4%)
should be used with caution as they are based on estimates from only part of the geographical area fished by the Spanish fleet. But they do provide a general indication of the elasmobranch by catches and discards.
The three principal longline fleets fishing tunas in the Indian Ocean are from Japan, Korea and Taiwan (Prov. of China). They started fishing in 1952, 1963 and 1966 respectively. Indian longliners started fishing for tunas in 1986 but their catches, along with those from the few other fishing countries, was small in comparison (IPTP 1990). Most of the information about longline fisheries in the Indian Ocean is documented in reports of the Indo-Pacific Tuna Development and Management Programme (IPTP).
The Japanese fleet fished tropical areas for yellowfin, albacore and bigeye tunas at the beginning of the fishery but shifted to higher latitudes to target southern bluefin and bigeye tuna during the 1970's, introducing deep longlining in tropical waters at the same time. Judging from data given to the IPTP, Japanese longliners decreased their effort from 106 649 999 hooks in 1986 to 74 861 000 hooks in 1989. The data records of Japanese longliners in the Indian Ocean do not include sharks, so they are not reported by Japan as being caught in the fishery. However, FAO yearbooks cite Japanese catches of 675t of “various elasmobranchs” from the Indian Ocean during 1989. As the only Japanese fishery in those waters is the tuna longline fishery (except for 3 newly introduced purse seiners), the elasmobranch catches reported by FAO, although small, can be attributed to shark bycatches of the longliners.
Taiwanese vessels take the largest catches of albacore but also fish for yellowfin and bigeye tunas primarily using deep longlines in tropical waters (Figure 2.39). A total of 199 vessels participated in the fishery in 1983, decreased to 127 in 1985 and then increasing to 187 in 1988. The total effort, in nominal hooks, during 1988 was 107 million (IPTP 1990). Unpublished data from IPTP show 33 052 sharks with a total weight of 1216t were caught by Taiwan (Prov. of China) in this period using 130 235 742 hooks. For 1989 these values were 188 615 sharks or 7 474 t with an effort of 136 418 296 hooks.
Korean longliners operate primarily in the tropical Indian Ocean (Figure 2.36) targeting bigeye and yellowfin tunas with deep longlines. The number of vessels peaked in 1975 at 185, decreased to 62 in 1985 then increased to 112 in 1988 (IPTP 1990). The most recent data from IPTP, shows they caught 10 851 sharks in 1987 with an effort of 35 748 292 hooks.
The Japanese bycatch of sharks must be estimated as no data are available. Further, the apparent hooking rates derived from the Korean and Taiwanese operations are too low compared with results from the Indian Ocean (see below) and similar fisheries in other oceans (e.g. the Atlantic). The estimated rates were 1.38 sharks/1000 hooks for Taiwan (Prov. of China) in 1989 and 0.3 sharks/1000 hooks for Korea in 1987. The high-grading of catches and discard of sharks in high seas tuna fisheries is common. The results here for these two countries probably reflect considerable under-reporting.
Information on shark by catches in the Indian Ocean longline fisheries is relatively abundant and allows geographical partitioning of the catch in some cases. However, few reports include data on hooking rates by species. The only species composition data is that given by Taniuchi (1990) who reports the percentage of each species in the shark by catches of research tuna longliners from Japan. He shows that 76.6% are blue sharks, 6.6% silky sharks, 6.5% shortfin mako sharks, 3.4% oceanic whitetip sharks and 6.8% unidentified sharks. Sivasubramaniam (1963) provides data on early research operations by Japanese and Taiwanese vessels that indicate bycatches of 10.83 sharks/1000 hooks for the eastern Indian Ocean (E of 60°E). Sivasubramaniam (1964) reports on commercial and research operations for six areas of the Indian Ocean and notes that about 20 species of sharks occur in the bycatches, 11 of these sharks (mainly carcharhinids) are common (Table 2.18). The results of Sivasubramaniam indicate latitudinal changes in species composition of sharks and higher hooking rates for sharks north of the equator. Frequency distributions of hooking rates for sharks are given for 6 areas of the Indian Ocean and show a range of 0–4 to 44.1–49 sharks/1000 hooks. The modal class corresponds to 4.1–8 sharks/1000 hooks. Mimura et al. (1963) provide hooking rates by area and season that average 5.1 sharks/1000 hooks (range 2.6–7.3).
Pillai and Honma (1978) provide monthly catch rates for pelagic sharks in 10°×20° squares of the Japanese fleet in the Indian Ocean that range between 0.1 and 50 sharks/1000 hooks. Varghese (1974; cited by Pillai and Honma, 1978) reports hooking rates as high as 84 sharks/1000 hooks and an average weight of 57 kg/shark in the Lakshadweep Sea. According to Silas and Pillai (1982), hooking rates of sharks in the Indian Ocean vary from year to year and between areas, the highest being between 0.6 and 10 sharks/1000 hooks. They also report that in the Southeast Arabian Sea, sharks were 63.8 and 57.8% of the total catch in number and weight respectively and had an average weight of 30kg. Sivasubramaniam (1987) summarizes data from Fisheries Survey of India tuna research cruises off the south west coast of India during 1983–1986. These results indicate catch rates of 17.6 sharks/1000 hooks. James and Pillai (1987) review additional research cruise result from areas of the Southeast Arabian Sea, Andaman Sea, Western Bay of Bengal and the Equatorial Region south of India. They found the percentage contribution of sharks to the total catch averaged 39.8% (range 30.9–43.7%). They also refer to average catch rates of 16.4 sharks/1000 hooks (range 7.4–29.7) in the Southeast Arabian Sea. James and Jayaprakash (1988) report on two studies of several areas around India. The results indicate catch rates of 8.43 sharks/1000 hooks (range 3.3–14) and a contribution of sharks to the catches of 32.1% (range 19.6–44.8) in one case and catch rates of 7.6 sharks/1000 hooks (range about 1.5–9.5) and contributions of sharks to the catch of 17.4% in the other. Stevens (1992) reports catch rates of 8.3 blue sharks and 3.5 mako sharks per 1000 hooks for a Taiwanese research longliner in south Western Australia.
|Scientific name||Approximate mean weight|
Strong variations occur in catch rates across the Indian Ocean depending on area and season. Ideally, an estimate of elasmobranch by catches is desired but the aggregated nature of effort statistics for each of the fleets of Japan, Korea and Taiwan (Prov. of China) makes it impossible to apply the appropriate hooking rates for the different regions. However, there seems to be agreement of around 1–10 sharks/1000 hooks as the most common hooking rate. Total catches of sharks in numbers for the whole Indian Ocean can be roughly estimated using a catch rate of 7.96 1000 hooks obtained by averaging the values derived from Sivasubramaniam (1963) and Mimura et al. (1963). These values come from data pertaining to most of the Indian Ocean and also agree with the most common hooking rates reported by different sources. The average weight of sharks taken in the fishery is estimated at 38.2kg derived from the weight and numbers of sharks reported for Taiwanese longliners during 1988 and 1989. The estimated shark by catches for the last available effort levels are: 596 267 sharks or 22 783t for Japan during 1989, 248 735 sharks or 10 879t for Korea during 1987 and 1 086 572 sharks or 41 518t for Taiwan (Prov. of China) in 1989. Thus, a better estimate of the catch of sharks in the Indian Ocean tuna longline high seas fishery is 1 931 574 sharks or 75 180t.
Figure 2.39. Distribution of Taiwanese catch per unit effort of lbacore by (a) regular and (b) deep longline fisheries during 1988 in the India Ocean. (Redrawn from Hsu and Liu 1990).
Based on the reported catches of elasmobranchs from each county, the corresponding discards of sharks is estimated at 22 108t by Japan, 9089t by Korea and 34 044t by Taiwan (Prov. of China). The percentages of sharks that survive being hooked and those wasted are unknown but the reports of Sivasubramaniam (1963; 1964) indicate that about 70–80% of the discards of carcharhinid sharks may survive if released alive whereas hammerheads and mako sharks usually die on the line. The rate of finning is also expected to be high. The validity of the estimates are limited by the variability of hooking rates reported for the Indian Ocean and the uncertainty in the effort statistics. Thus they should be used as a first approximation of the amount of elasmobranch by catches and discards in these fisheries.
Numerous fleets fish for tuna in this area which is home to several small island countries. Most of the longlining is done, in order of importance, by Japanese, South Korean, Taiwanese and Australian vessels. In general, these fisheries are poorly documented making it difficult to ascertain the elasmobranch by catch. Most of the available information for the central Pacific area is that submitted to the South Pacific Commission (SPC) and made available through the Forum Fisheries Agency (FFA)1. Australian and New Zealand provide some information about catches in their EEZs but there seems to be no information about areas of the eastern Pacific where neither Australia nor New Zealand have jurisdiction. Further, the cover of the fleets by the FFA data is partial (Lawson 1991). Hence, effort levels for the central and south Pacific area are unknown and are probably larger than those given by the sources used here. The area treated here as Tropical and South Pacific is that south of 20°N.
Japanese fishermen started experimenting with longlines in the western central Pacific as early as the 1920's and 72 vessels were active by 1939. However the peak expansion of this fishery occurred during the late 1960's and covered most of the central and south Pacific (Suzuki 1988, Lawson 1991). At present, at least 406 vessels may operate in the region. The FFA database shows that Japan deploys more than 70% of the total effort in the area, 31 143 fishing days in 1989. The South Korean longline fleet started fishing in 1958 and is reported to have 124 vessels active in the area. According to NFRDA (1988), their longliners fish largely for tunas in the South Pacific (Figure 2.36). The South Korean effort in the FFA zone was 6312 fishing days in 1989. Activities of the Taiwanese fleet are less well documented and not even approximate numbers of active vessels in the region are available. They operate in the waters north of Papua New Guinea and around Fiji and American Samoa (Lawson 1991). According to FFA data, the Taiwanese fleet effort was 4163 fishing days in 1989. The Australian longline fisheries for tuna date back to the 1960's. It expanded in the 1980's to more than 91 vessels by 1989, with a total of 2244 fishing days. In addition to these fleets, a few vessels from China, Fiji and Tonga also operate but in 1989 their effort only accounted for 558 fishing days. The geographical distribution of total longline effort during 1990 available to the SPC is shown in Figure 2.40. Most of the fishing effort occurs between 15°N and 15°S.
1 P. Tauriki, FFA, P.O. Box 629, Honiara, Solomon Islands, pers. comm. June 1992)
Figure 2.40. Distribution of longline effort in the SPC area during 1990, units not given. (Taken from Lawson 1991).
The reported catch of sharks for 1989 in this area was 426t; 375t by Taiwan (Prov. of China), 35t by South Korea and 12t by the Japan. Although numbers of hooks deployed by country were not available, the total for all longliners was 98 832 500 during 1989. The number of hooks per country can be estimated using the reported fishing days of each fleet. The corresponding estimated catch rates in kg/1000 hooks are 0.167 for Japan, 2.5 for South Korea and 40.5 for Taiwan (Prov. of China). This is equal to an overall catch rate of 4.31kg of shark per 1000 hooks. Such minuscule catch rates, equivalent to less than 0.5 sharks/1000 hooks, are almost certainly a result of under-reporting, presumably due to discarding. This is evident in the comparison of the estimated catch rates for each of the countries.
Saika and Yoshimura (1985) plot hooking rates for the most common sharks taken by Japanese research longliners in the western equatorial Pacific. These are approximately 0– 14/1000 hooks for oceanic whitetip and for silky sharks, 0–16/1000 hooks for blue sharks and 0–2/1000 hooks for shortfin mako sharks. An overall rate of 20.45 sharks/1000 hooks can be obtained for waters below 22°N from the report of Strasburg (1958) on research and commercial cruises in the eastern equatorial Pacific. This can be further split into 4.14 for blue sharks, 5.46 for oceanic whitetip sharks, 10.07 for silky sharks and 0.78 for unidentified sharks, per 1000 hooks.
Stevens (1992) gives by catch data for blue and mako sharks by longliners fishing off Tasmania from observers onboard Japanese vessels targeting mainly southern bluefin tuna (Thunnus maccoyii). These data show catch rates of 10.4 for blue sharks and 0.5 for mako sharks per 1000 hooks. Stevens estimates that 1594 mako and 34 000 blue sharks weighing 24 and 275t respectively, are caught each fishing season in this fishery. Hooking rates for other species are not available but Stevens mentions that thresher, porbeagle, school (Galeorhinus galeus), black (Dalatias licha), crocodile (Pseudocarcharias kamoharai), hammerhead, velvet dogfish (Zameus squamulosus) and grey (Carcharhinus) sharks are also present in the by catches of Japanese longliners in the Australian Fishery Zone. He also provides data for the by catches of blue sharks in New Zealand waters where the Northern New Zealand Japanese and Korean fisheries had catch rates of 4.8 and 1.3 blue sharks/1000 hooks respectively and the southern New Zealand Japanese fishery, catches 5.4 blue sharks/1000 hooks. Stevens notes the underreporting of shark by catches in Japanese logbooks and reports that fins are removed from the sharks before being discarded. If so, the mortality in this fishery would equal the total estimated by catch.
Ross and Bailey (1986) provide hooking rates for mako sharks in the northern New Zealand Korean and Japanese fisheries for albacore and for the southern New Zealand Japanese fishery for southern bluefin tuna. Averages are 0.43 and 0.34 sharks/1000 hooks for the northern and southern fisheries respectively. Based on their data, the estimated catch of mako sharks is 334t processed weight. As about 50% of a shark's weight is lost during processing, the estimated live weight of the mako shark by catch is 668t. Ross and Bailey provide no further information and this estimate may only represent the reported catch and not discards.
The total by catch of sharks in the SPC zone can be estimated using figures estimated from Strasburg (1958) and a conservative estimate of 20kg/shark to calculate the total weight. Even though this catch rate might be too high, the distribution of effort in these fisheries (see Figure 2.40) justifies the use of the hooking rates from the Equatorial Pacific. The results (Table 2.19) indicate that approximately 2 021 711 sharks, or 40 434t, were caught in 1989 and almost 50% of these were silky sharks. Japan takes the majority of the elasmobranch catch and also has the highest discard rate. Total discards are estimated at 40 000t.
|Species||Strasburg's data||Estimated Catch in 1989|
|Numbers caught||Hook rate||Total||Japan||S. Korea||Taiwan||Australia|
|(216,172 hooks)||(#/1000 hooks)||numbers||weight (t)||weight (t)||weight (t)||weight (t)||weight (t)|
Shark by catches for the entire tropical and South Pacific might be higher. Judging from the size of the statistical area covered by the SPC (Figure 2.34) and the maps of CPUE of the South Korean longline fleet for 1983–1985 (Figure 2.36) and considering the partial coverage of the SPC area by FFA statistics (SPC 1991), it is estimated that the South Korean fleet deployed twice as many hooks in the whole central and South Pacific as those reported by the FFA; similarly for the Japanese and Taiwanese fleets. In this case, the estimated catch of sharks in the central and south Pacific outside the SPC zone is 1 097 288 sharks or 21 946t; 16 422t by Japan, 3328t by South Korea and 2196t by Taiwan (Prov. of China). These figures assume an extra effort of 92 598 173 hooks (1989) and a total catch rate of 11.85 sharks/1000 hooks. This rate considers the possible effort less higher in latitude areas and is calculated by averaging the hooking rates obtained from Strasburg (1958) for the equatorial zone, those of Stevens (1992) for Tasmanian waters and those of Ross and Bailey (1986) and Stevens (1992) for New Zealand waters. The same weight of 20 kg/shark is used. Thus, it is estimated that 62 380t of sharks were caught as by catch of longline fisheries in the whole central and south Pacific in 1989. According to FAO statistics, the total reported catch of elasmobranchs from the West Central, South Western and South Eastern Pacific of Japan, Taiwan (Prov. of China) and Korea was only 4409t for 1989. These figures suggest that some 58 000t of sharks may be discarded. These estimates could be less uncertain than those calculated in previous sections for other high seas longline fisheries. Because of the limited information available about the real effort levels of each fleet and the hooking rates in the South Pacific.
This is another area where longline fisheries activities are poorly documented. CPUEs of Korean longliners published by NFRDA confirm that there was some effort by this fleet in the central north Pacific during 1983–1985 (Figure 2.36). Figures from Suzuki (1988) show that the Japanese longline fleet operated in the north Pacific. Though, Taiwan (Prov. of China) does not have a high seas longline fishery in this area (Nakano and Watanabe 1992). No statistics are available, at least in English, on the amount of effort deployed by longliners in the North Pacific.
Nakano and Watanabe (1992) estimate the longline effort of the Korean fleet at 14–19 million hooks/yr for 1982–1988. Using this estimate and statistics from the Fishery Agency of Japan they estimate a total effort of 258 422 780 hooks deployed during 1988 in the entire North Pacific by Japan and Korea. Their estimate of 3 274 609 blue sharks caught by longline fisheries in the North Pacific during 1988 is based on latitudinal stratification of effort and hooking rates. Because of the geographical coverage considered in the previous section for the Tropical and South Pacific, only waters north of 20°N are considered here as “North Pacific.” From Nakano and Watanabe's data is estimated a total effort of 105 885 418 hooks and a by catch of 2 964 500 blue sharks for the North Pacific during 1988.
Data in reports of Strasburg (1958) gives an overall hooking rate of 18.45 for blue sharks, 0.07 for oceanic whitetip sharks and 0.84 for unidentified sharks (total of 19.36 sharks/1000 hooks) for the eastern Pacific north of 22°N. Data given by Sivasubramaniam (1963) indicates hooking rates of 6.79 for blue sharks and 0.35 for oceanic whitetip sharks/1000 hooks for two combined areas of the Pacific north of 20°N. Saika and Yoshimura (1985) present data on shark by catches of Japanese research cruises from 1949–1979 in the Western Pacific. Their maps of hooking rates indicate values of approximately 0–3 oceanic whitetip, 0–0.5 silky, 0–2 short fin mako and 0–30 blue sharks per 1000 hooks for the region north of 20°N. Catch values plotted for blue sharks appear to be around 10 sharks/1000 hooks whereas the other species probably average to less than 1 shark/1000 hooks. Nakano et al. (1985) provide numbers of blue sharks caught and number of stations sampled for longline cruises during 1978–1982 in the western north Pacific. The longlines utilized had between 1500–1800 hooks. Assuming a mean of 1650 hooks per station, then hooking rates averaged 17.62 blue sharks/1000 hooks which is similar to the estimate derived from Strasburg's data.
The estimated by catch of sharks by tuna longlines in the North Pacific is comparatively high. Based on the hooking rates derived from Strasburg (1958) and the estimated effort from Nakano and Watanabe (1992), a total of 2 050 136 sharks are estimated to have been caught during 1988 in the North Pacific. Roughly 1 950 000 of these would be blue sharks, 7250 oceanic whitetip sharks and about 90 000 sharks (Table 2.20). These estimates for blue sharks taken in the same area are conservative compared to those of Nakano and Watanabe. Assuming an average weight of 20 kg/shark regardless of species, the estimated total by catch is 41 000t. National catch is difficult to estimate since it is impossible to separate, the estimates of effort of Nakano and Watanabe. A crude estimate based on proportions indicates that 7.35% of the catches could be South Korean and the rest Japanese.
|Species||Strasburg's data*||Estimated Catch in 1988|
|Numbers caught||Hook rate||Total|
|(87,595 hooks)||(sharks/1000 hooks)||numbers||weight(t)**|
* for cruises north of 21 N
** assuming 20 kg/shark
There is no information on discards of sharks from these fisheries or the amounts released alive. Given the manner of partitioning FAO statistical areas in the Pacific it is difficult to assign to area, catches of elasmobranchs reported by Japan and Korea. Even considering the total reported “various elasmobranchs” catch of 15 537t for Japan and 2927t for Korea, which correspond to a much larger FAO areas 61, 67 and 77 of the Pacific Ocean, the estimated discard would be of about 22 000t.
High seas longline fisheries for tunas and billfishes are a large source of by catch and discards of elasmobranchs. Despite the uncertainty of the different estimates, it is evident that the amount of effort exerted by longline fleets (worldwide total of about 750 million hooks) is the main cause of the high by catch. The best estimates given in Table 2.21. The total high seas catch by longlines worldwide is estimated at 8.3 million fishes, equivalent to 232 425t! This almost a third of the world catch of elasmobranchs reported by FAO in 1991.
The by catch of blue sharks from longline fisheries is large. Although a species breakdown was not always possible, an approximation can be done for areas where only total shark by catch was estimated if a conservative estimate of 40% of the total is used for blue sharks. Adding this estimate to the numbers of blue sharks caught where a species breakdown is done, gives a total of 4 075 162 blue sharks caught incidentally by world high seas longline fisheries.
|Area||Number of individuals||Total catch in tonnes|
|South/Central Pacific Ocean||1,996,350||39,927|
|North Pacific (above 20N)||2,050,135||41,000|
The relative importance of shark by catches, in number of fishes is almost equally distributed in the longline fisheries of the world. The fisheries of the Atlantic, Indian, Tropical and South Pacific and North Pacific Oceans each account for about 2 million elasmobranchs. However, the total weight of by catch in the Atlantic and Indian Oceans is estimated to be almost double that for the whole Pacific Ocean (Table 2.21). Because of the different mean weights used in the calculations and does not necessarily represent a real difference in weight of the catches. Specifically, the mean weight of 20kg/shark used for Pacific fisheries is conservative.
The amount of discarded sharks and survival rate of released sharks are also uncertain. The accumulated estimates of discards from the longline fisheries treated above amount to 204 347t. It is unknown what proportion of these discards survive but some reports indicate it could be as high as 66% (Berkeley and Campos 1988). Nevertheless, numerous accounts of finning exist in the literature (e.g., Mejuto 1985, Nakano 1993) and given the rise in shark fin prices in the late 1980's it would be naive to think that released sharks are not finned. Further research is needed to determine the mortality of sharks due to longline fisheries.
The present estimates seems to be in agreement with previous assessments. As a reference, Taniuchi (1990) estimates a total shark catch from Japanese longliners of 90 000t using an estimate of the ratio of shark-catch/target-species catch for the tuna and billfish longline fishery. The world elasmobranch by catch estimated here for Japanese longliners is 115 441t. But there is a good degree of uncertainty introduced by the low quality of the baseline information that is available. For example, the hooking rates used here ranges between 7.04– 20.45 sharks/1000 hooks whereas Taniuchi (1990) plots rates for Japanese research longliners that range between 2.7 and 8 sharks/1000 hooks. Only reliable regional effort figures and updated hooking rates representative of each region will provide better estimates of the by catches.
In contrast to driftnet fisheries, there are no observer programmes for high seas longline fisheries in the world. This results in much the uncertainty surrounding the estimates of nontarget species caught in longline fisheries. Most of the international tuna organizations and the governments of longline fishing nations requiring logbook reports from longline fleets still do not require, or enforce, reporting of by catches of sharks or other elasmobranchs though some organizations are starting to change (ICCAT 1993b, Nakano 1993). This will reduce uncertainty about the levels of by catches and discards in the future. Considering the common underreporting of elasmobranchs in longliner logbooks (Stevens 1992, Nakano 1993), observer programmes are undoubtedly the best way to provide this crucial information.
Most the large-scale purse-seine fisheries for tuna occur in tropical waters where the relatively shallow schooling behaviour of some tunas makes them vulnerable to this type of gear. The main species targeted by this method of fishing are yellowfin (Thunnus albacares) and skipjack (Katsuwonus pelamis) although other species of tuna, other fish and marine mammals) commonly associated with the schools of tuna, are also frequently caught. Major tuna purse seine fisheries are fairly localized activities. They are centred in four areas: the Eastern Tropical Pacific (ETP), Mexico to the north of South America; the Western Central Pacific (WCP), from the Philippines and Papua-New Guinea to Polynesia; the western Indian Ocean, around the Seychelles and the eastern tropical Atlantic around the Gulf of Guinea (Figure 2.41). Some tuna purse seining also occurs off Venezuela in the western Atlantic Ocean.
The ETP fishery began during the 1950's and expanded in the 1960's and 1970's. In the early 1980's it suffered a temporary decline and today about 280 000t of yellowfin tuna are caught by purse seiners in this region (Sakagawa and Kleiber 1992). The fleet used to be dominated by US vessels but since the early 1980's many of these switched to the WCP fishery and now Mexican vessels are dominant. Tuna purse seining was started in the WCP by Japanese and USA vessels in the 1970's. In contrast to the ETP, the effort here is largely directed towards skipjack although yellowfin are also caught in large amounts. The Japanese fleet mainly fishes log-associated schools while US boats concentrate on free-swimming schools (Sakagawa and Kleiber 1992). Korean and Taiwanese purse seiners joined the fishery in the late 1970's (Suzuki 1988). A smaller number of Australian, Indonesian, Philippine, Marshall Island, New Zealand, Solomon Island and the former USSR vessels also participated. The total purse seiner tuna catch in the WCP during 1989 was 576 204t; at least 73% was skipjack (Lawson 1991).
The purse seine fishery was initiated in the western Indian Ocean (WIO), by a Mauritius-Japan purse seiner in 1979 followed by French vessels in 1980. By 1984 the French fleet together with part of the Spanish fleet moved from the Atlantic to the WIO. During 1989, France, Spain, Panama, Japan, Mauritius, U.S.S.R. and Cayman Island had 49 purse seiners operating in this fishery. The first two countries dominate the fleet. The total catch in the WIO for 1989 was 220 000t, mainly yellowfin and skipjack but also some bigeye (IPTP 1990).
Purse seine fishing for tunas in the tropical Atlantic was initiated by the French in the early 1960's in the coastal waters of the Gulf of Guinea. African coastal states, Spain and US fleets joined later. The fishery expanded to offshore areas at the end of the 1970's and it currently accounts for more than 80% of the Atlantic yellowfin tuna catch (Suzuki 1988). The majority of the catches are now taken by Spanish and French-Ivorian-Senegalese-Moroccan (FISM) fleets with small amounts by Venezuelan, U.S.S.R. and Japanese boats. Yellowfin and skipjack are the main species taken with minor amounts of bigeye tuna taken incidentally. A total of 167 800t of tunas was caught by purse seiners in the tropical Atlantic during 1989, at least 90% of this from the eastern Atlantic (ICCAT 1991a, 1991b, 1992).
Information on elasmobranch by catches in purse seine tuna fisheries is scarce and poorly documented. Even though the presence of sharks in the purse seine catches is documented, at least since the mid-1960's, it has received little attention in the literature. Bane (1966) reports several large silky, as well as other, sharks and devil rays in a set off Gabon in 1961. Bane also mentions that C. limbatus, C. plumbeus and Rhizoprionodon acutus are associated with tuna schools in the area. Yoshimura and Kawasaki (1985) report 183' silky sharks caught by purse seine in the WCP and length frequency histograms indicate that most silky sharks were between 60 and 170cm TL with the mode at 110–130cm TL. In the Indian Ocean, LaBlache and Karpinski (1988) based on observer's data, give rates of 6% of the total catch for purse seiners that had shark bycatches. They consider various teleosts, including undersized and damaged tuna, to comprise the by catch. Oceanic whitetip sharks were the second major by catch (12%).
Figure 2.41. Major areas of Tuna purse-seine fisheries in the world.
The most detailed account of sharks associated with tuna schools for the ETP is that of Au (1991). He notes that sharks associate with yellowfin, perhaps as opportunistic predators or scavengers. The percentage of sharks associated with yellowfin measured as percentage of sets having sharks is 40% for log-associated tuna schools, 6–21% for free swimming schools and 13% for dolphin-associated schools. Apparently, these associations are limited by the swimming speed of sharks. Silky sharks were the most common elasmobranch in the by catches with up to 500 individuals caught per set. Various other carcharhinids, oceanic whitetip, sphyrnid, alopid, lamnid, blue and whale sharks were also caught together with various batoids and mobulids. Au's report does not provide any useful measure of the numbers of sharks caught by purse seine fisheries (i.e. catch of sharks per unit of effort, or the relation between tuna catch and elasmobranch catch. Although he lists average numbers of sharks per set by species, these values are based on purse seine sets that caught the pertinent species. Without reference to the total numbers or weights of sharks in the full sample, his results are of limited use for estimating shark by catch rates although they give the species composition of the elasmobranch catch.
The total by catch of elasmobranchs in purse seine fisheries can be estimated using the information on shark and tuna catch provided by Lablache and Karpinski (1988). Their data permits an estimate of shark catch of to 0.51 % of the tuna kept by purse seiners. Using this proportion and the reported tuna catches listed above, the estimated total catch of sharks in purse seine fisheries during 1989 is of 6345t: 856t in the tropical Atlantic, 1122t in the fisheries of the Western Indian Ocean, 2939t in the Western Central Pacific fisheries and 1428t in the Eastern Tropical Pacific. These estimates assume that the amount of sharks caught is proportional to tuna catch. Purse seining is an active fishing method that takes advantage of the schooling behaviour of fish. Sharks gather around tuna schools, especially certain types of schools such as those associated with log (Au 1991). Unlike passive gears, shark catches in purse seine fisheries are not possible without tuna catches. Thus it is appropriate to relate the shark catch to the tuna catch rather than to a measure of effort, e.g. days at sea, where, for passive gear, competition occurs for hooks or space in the gillnet. The main weakness of the present estimates are the calculations of shark catch rates in tuna purse seine operations and the extrapolation from Western Indian Ocean data to other geographical areas.
There are no records of the condition of the elasmobranchs caught in tuna purse seine operations, but it is likely that they die either by suffocation or crushing if they do not bite their way out of the nets. Bane (1966) reports that shark catches were sold in the Gulf of Guinea but this seems to be an exception for an experimental fishing campaign. Most shark catches in tuna purse seine fisheries are probably discarded though this has not been confirmed.
Other fisheries take elasmobranchs incidentally and although they are either of minor scale or their bycatches are insignificant, it is worth mentioning some of these which might, with time, affect particular elasmobranch stocks. Pole and line fisheries for tunas take some shark bycatches while fishing tuna schools (Anderson and Teshima 1990). Almost nothing is known about the catch rates. Bane (1966) mentions sharks taken by “tuna clippers…at the surface on live bait”, which suggests pole and line fishing: 131 sharks were taken at 6 stations by this method. It is possible, due to the global scale of pole and line fisheries for tunas, that their bycatch of sharks could be significant, perhaps in the order of that from purse seiners. Alternatively, pole and line gear may avoid the capture of sharks and survival of discards could be high. These hypotheses could be verified by interviewing skippers from this type of fishery.
The orange roughy (Hoplostethus atlanticus) fishery of New Zealand takes deep water squaloid sharks and other elasmobranchs. Although no estimates of catch rates are available, some information exists from research vessels. At least 21 elasmobranchs (11 selachians, 4 batoids and 6 holochephalans) have been identified in deep water trawl surveys around New Zealand (Robertson et al. 1984). There are 8 squaloid sharks of potentially commercial importance, of which Deania calcea is the most abundant in the North Island, Etmopterus baxteri in the South Island and Centroscymnus spp. in the central areas. Surveys carried out in the North Island show that Deania calcea constitutes a larger part of the total catches than either the orange roughy or the hoki (Macruronus novaezelandiae), currently the most important commercial species (Clark and King 1989). Although catch rates in commercial trawling should be smaller than those of research cruises due to more targeted fishing, it is possible that the by catches of elasmobranchs constitute between 10 and 50% of the orange roughy catches. According to FAO statistics, orange roughy catches in New Zealand waters were of around 44 000t/yr in 1984–1989. The total bycatch of squaloid sharks could therefore be between 4400 and 22 000t/yr in this fishery. King and Clark (1987) estimate the MSY for these sharks as 2250t/yr. Evidently, the current catches far exceed the MSY. Most of the catch is discarded as there is no market though small quantities are used for fishmeal and liver oil extraction. Given the depth at which these sharks are caught (600–1200 m) and the gear employed, all will be dead when returned to the sea.
The impact of this level of bycatch on the local stocks sharks is unknown but it must be highly damaging and likely to lead to unsustainable exploitation. But this is difficult to verify as little information exists about the biology and population dynamics of these species. More research is needed on the levels of by catch, survival of discards and the deep sea shark populations themselves.
The amount of elasmobranchs caught and discarded in high seas fisheries worldwide is uncertain as neither process is adequately documented. Discard and survival rates are unknown. There are large uncertainties about the catch rates for each region and sometimes also about effort levels. Qualitative and quantitative variations in the elasmobranch bycatches within each ocean due to areal and seasonal changes in availability of the different species should be expected. Present results indicate that a large amount of elasmobranchs are caught incidentally in the high seas fisheries of the world. The estimated annual elasmobranch by catch at the end of the 1980's is around 260 000 and 300 000t or 11.6–12.7 million fish. Most of the catch are sharks, predominantly blue sharks.
Longline fisheries are the most important source of shark kills in the high seas, mainly because of the magnitude of their effort. They contribute about 80% of the estimated total elasmobranch by catch in weight and about 70% in numbers of fish. There is great uncertainty around the estimates for this type of fisheries, but the figures are based on the best available information and seem to compare well with the few reference points available (Section 188.8.131.52). The former high seas driftnet fisheries ranked second in their contribution to the elasmobranch by catches. Since their activities were stopped at the end of 1992 they are now one less worrisome in terms of sea-life conservation. It is conceivable that this effort has been redirected to fisheries which might still affect elasmobranchs and the other species previously affected by their gillnetting activities.
Discards from high seas fisheries also are high. Up to 230 000–240 000t of elasmobranchs are discarded annually by various high seas fisheries. Most discards, certainly those caught by the driftnet, purse seine and orange roughy fisheries, probably die. For longline fisheries, survival depends on whether fishermen release sharks quickly and unharmed, though finning will prevent survival. The little information available on purse seine and pole-and-line tuna fisheries and the deep trawl fisheries for orange roughy make it very difficult to assess the importance of their by catches of sharks and rays. Another source of by catch and waste of sharks and rays is the incidental catch by bottom trawling vessels fishing for shrimps and fishes on continental shelves. The assessment of the impact of these fisheries is difficult because of the difficulty in gathering information about them. These fisheries have high local impacts on populations especially in the case of rays. Some of the elasmobranchs caught are landed and reported under official statistics of the fishing country but a large proportion is discarded and never recorded.
Blue sharks are the most common elasmobranch caught incidentally in high seas fisheries; an estimated 6.2–6.5 million blue sharks are taken annually. Although this is apparently the first estimate of total catches for blue sharks in high seas fisheries, some partial estimates are available for comparison, e.g. Stevens (1992) estimates that the Japanese longline fisheries annually take a total of 433 447 blue sharks. His figure is small compared with that estimated here. However, he uses a hooking rate of only 1 shark/1000 hooks. Nakano and Watanable (1992) estimate that the high seas fisheries of the North Pacific Ocean caught 5 million blue sharks during 1988, an estimate higher than was derived here.
Lack of knowledge prevents an assessment of the impact of the removal of 6 million blue sharks annually on high seas ecosystems or on the blue shark populations. Little is known about the size of the stocks of blue sharks in the world and the biology of most populations is poorly understood. Nakano and Watanabe (1992) provide the only assessment known of the impact of high seas fisheries on blue shark stocks. By estimating bycatches and using cohort analysis, they believe that the catch levels during the late 1980's did not have a significant impact on the populations of the North Pacific. However, Wetherall and Seki (1992) and Anonymous (1992) consider that appropriate information is lacking for an assessment of this kind. Regardless, research is needed to assess the real by catch levels in each fishery and their impacts on the different populations.
Silky sharks are probably the second most commonly caught species, especially in longline and purse seine fisheries. As for blue sharks, little information is available to assess the impacts of removals. Silky sharks have slower growth, later sexual maturation and are less fecund than blue sharks (Pratt and Casey 1990) and hence will be less resilient to exploitation. Local stocks of Deania calcea, Etmopterus baxteri and Centroscymnus spp. in New Zealand could also be threatened by large-scale fisheries.