National Research Institute of Far Seas Fisheries
This paper is prepared for the Expert Consultation on Interaction of Pacific Ocean Tuna Fisheries. Thus the contents included here are limited to those related to Pacific bigeye tuna. In compiling this paper, the “Synopsis of biological data on the bigeye tuna, Thunnus obesus (Lowe, 1839), in the Pacific Ocean” by Calkins (1980) and “Fishery biology of the bigeye tuna resource in the Pacific Ocean” by Kume (1979a) are major sources of reference.
According to Nelson (1976), Genus Thunnus is classified as follows:
Bigeye tuna was first described by Lowe (1839) based on the specimen caught in the area of Madeira, Portugal (cited in Iwai et al., 1965). The scientific name given to this species varied considerably among the taxonomists until mid-1960s, since then Thunnus obesus has been generally accepted by the scientific community.
The classification of bigeye tuna and other tunas in Genus Thunnus is discussed by Iwai et al. (1965), Gibbs and Collette (1967), Sharp and Pirages (1978) and Collette and Nauen (1983).
3. EARLY LIFE HISTORY
Currently the egg of this species cannot be differentiated from other tuna eggs. The fertilized egg is known to be pelagic and non-adhesive. Kikawa (1953) reported that the egg had an oil globule and the diameter of running ripe eggs was about 1 mm. In Yuen (1955) and Nikaido et al. (1991), the diameter of the most advanced eggs ranges between 0.8 to 1.2 mm. Yasutake et al. (1973) described larval development from hatching to 86 hours after hatching of bigeye tuna eggs artificially fertilized on board a research vessel. It is reported that it took 24 to 30 hours before the first hatching at 25.5–29.0°C. The size of larvae when hatched was 2.5 mm and after 26 hours the larvae reached 2.76–3.12 mm in total length. By 86 hours after hatching the early post-larval stage was attained. The development of the larvae in these stages is shown in Figure 1.
Figure 1. Larvae of bigeye tuna: a) newly-hatched (2.5 mm in TL); b) 24 hours after hatching (3.0 mm in TL); c) 48 hours after hatching (3.1 mm in TL); d) 86 hours after hatching. (After Yasutake et. al., 1973).
The diagnostic keys to the larvae (3 to 12 mm standard length) of Family Scombridae (including Genus Thunnus) based on the morphological characters is given in Nishikawa and Rimmer (1987). Pigmentation is the most important feature for the identification of tuna larvae. The identification of larvae and juveniles from 12 mm to about 60 mm is very difficult because of morphological similarities among the tuna species and the overlapping counts in the meristic characters. Graves et al. (1988) conducted an electrophoretic analysis on tuna larvae and early juveniles in an attempt to separate bigeye and yellowfin. Their samples were all identified as yellowfin in spite of the fact that their samples included larvae which have morphological characteristics for bigeye and yellowfin. This result raised a question on the validity of the pigmentation pattern which is currently used for the species separation between bigeye and yellowfin larvae.
Nishikawa et al. (1985) reported average tuna larvae distribution based on the total of 63,017 net tows during 1956 to 1981. According to their results, bigeye tuna larvae are distributed very widely in the equatorial area of the western, central and eastern Pacific. The area where they occur is very broad in the western Pacific, extending from off the south coast of Japan to the northern Coral Sea. In the eastern Pacific, there are several l-degree squares marked with higher larval densities than in the western and central Pacific. It appears, however, that, despite the huge spawning potential of the Pacific bigeye tuna inferred from the studies of sexual activities, the number of their larvae per unit of water strained is generally smaller than expected, especially compared to other tuna larvae such as yellowfin. This difference may suggest a possible different characteristic of bigeye tuna larvae which makes them less vulnerable to the ordinary sampling procedure.
4. AGE AND GROWTH
Yukinawa and Yabuta (1963) derived the growth curve based on scale samples. A total of 1,622 samples were collected from the central to western Pacific north of 10°S; of the total 463 were readable. In this study up to six rings were counted; the rings were reported to be formed twice a year, one in spring and the other in fall. Yukinawa and Yabuta (1963) reported that the rings were hard to detect for fish over 130 cm in fork length (FL).
Shomura and Keala (1963) estimated the growth curve by sex from the weight-frequency data of the fish unloaded by the Hawaiian longline fishery. They fitted normal distributions to those weight-frequency data with several assumptions, one of which was that fish entering the fishery in September at about 45 pounds were 17 months old. Sexual dimorphism was observed for fish over 130 cm in FL.
For the fish in the eastern Pacific, Kume and Joseph (1966) followed modal progressions of bigeye tuna taken by longline. Their data indicated that males and females grow at approximately the same rate up to 150 cm in length, but that males appear to grow faster at sizes greater than 150 cm. Therefore they did not use modes greater than 150 cm in their growth study.
Suda and Kume (1967) modified slightly the equation estimated by Yukinawa and Yabuta (1963) by taking observations on young bigeye into consideration.
Parameters of von Bertalanffy's growth equation from the above-mentioned papers are listed in Table 1. These growth curves were plotted in Figure 2 for comparison; the growth curve of Suda and Kume (1967) has been excluded since it is quite similar to that of Yukinawa and Yabuta (1963), particularly for 4 year old and older fish. It is noted that although the growth curves do differ from each other, the growth rates appears to be similar.
Length-weight relationship in the form of W = a.Lb, where W is round weight in kg and L is fork length (FL) in cm, was estimated by Iversen (1955), Kume and Shiohama (1964), Nakamura and Uchiyama (1966) and Morita (1973). Samples taken in those studies were mostly from the western and central Pacific. Parameter estimates of a and b are listed in Table 2. As the original units of measurement were different among studies, equations are modified so as to use FL in cm and round weight in kg. For the conversion from gilled and gutted weight to round weight, the factor of 1.16 (Morita, 1973) was used.
Table 1. Von Bertalanffy's growth parameters estimated for Pacific bigeye tuna. Length is fork length in cm and weight is live weight in kg.
|Length||215||0.10412||-0.01056||Yukinawa and Yabuta (1963)|
|Weight||234.7||0.114||1.07||Shomura and Keala (1963), Male|
|Weight||164.9||0.167||1.06||Shomura and Keala (1963), Female|
|Length||186.95||0.095||2.11||Kume and Joseph (1966)|
|Length||214.8||0.2066||-0.12052||Suda and Kume (1967)|
Figure 2. Growth curves of bigeye tuna estimated by various authors. (After Kume and Joseph, 1966).
These results are very similar in the common size range up to 150 cm, although the difference becomes greater as the fish becomes larger. The maximum difference in length among equations is about 10% when the length is 200 cm; at 200 cm the fish is close to the maximum size reported for bigeye tuna.
Table 2. Length-weight relationship (W = a Lb) estimated for Pacific bigeye tuna. L is fork length in cm and W is live weight in kg.
|a||b||Area in Pacific||Sample||Author(s)|
|1.3504×10-5||3.1056||Western-north||4121||Kume and Shiohama (1964)*|
|1.7265×10-5||3.0475||Western equatorial||2538||Kume and Shiohama (1964)*|
|3.3263×10-5||2.9180||Central||1832||Kume and Shiohama (1964)*|
|3.6562×10-5||2.90182||Central||9144||Nakamura and Uchiyama (1966)|
* Gilled and gutted weight is converted to live weight by a factor of 1.16.
5. MATURATION AND SPAWNING (SEX RATIO)
The minimum size at first maturity of Pacific bigeye tuna is reported as 91–100 cm by Kikawa (1953). Yuen (1955) also reported minimum size at first maturity, but his results were given in weight (14–20 kg). These two values are equivalent and are supported by the later studies (Kikawa 1957; 1961; 1962).
As regards the Gonosomatic Index (GSI=104.GW/L3; GW=gonad weight in g, L=FL in cm) of longline-caught bigeye tuna by latitude, Kikawa (1957) found the main spawning ground in the western Pacific in the area of the equatorial counter current (2°N–10°N). Kikawa (1961) investigated seasonal and areal change of GSI and size composition of longline-caught fish in the equatorial Pacific. Kikawa (1966) further extended his study with additional data extending as far east as 100°W. He reported that the size of fish, CPUE, and group maturity (rate of mature fish) increased from west to east. Mature fish were seen throughout the year in all areas. The seasonality of group maturity was recognized, but it appeared to be similar among areas. Maturity was highest in June and July in the waters west of 140°E, April to July between 140°E and 180°, April to September between 180° and 140°W and February to July between 140°W and 100°W. Kume and Joseph (1966) confirmed a similar trend of seasonality of group maturity for the longline-caught fish in the eastern Pacific, and reported that in the area between the equator and 5°S the proportion of mature fish was considerably less than in the adjacent areas to the north and south.
Nikaido et al. (1991) examined histologically the gonad samples taken by longline southwest offshore of Hawaii during May to July and in the waters off Java in the Indian Ocean during January to March. They limited their study to fish which were alive when they were hauled on the deck in order to investigate the spawning time and the developmental process of gonad with time. Nikaido et al. (1991) noted that most of the fish (>100 cm in FL) were mature, i.e. in tertiary yolk stage or a more advanced stage of maturity. From the degeneration process of postovulatory follicles (POF), the developmental process of gonad and the change of GSI by time, bigeye was determined to be a multiple spawner. Out of the mature fish more than 90% were identified to have spawned within 24 hours. At the same time it was estimated that spawning took place approximately from 19:00 to midnight. Spawning frequency was calculated 1.00–1.57 (days per spawning) taking the inverse of the number of sample with POF divided by the total number of mature samples.
Batch fecundity (number of eggs spawned per day) was also estimated (Table 3) based on the number of most advanced eggs. The estimated batch fecundity was different between two areas studied. For example, in the area off Hawaii the batch fecundity was estimated at 2.2 million eggs for fish at 150 cm in FL, while the batch fecundity was 2.8 million eggs for the same size fish in the Java area. Currently it is not known whether this reflects a difference in stocks. These spawning figures are smaller than those estimated by Yuen (1955). It is thought that Yuen (1955) might have included eggs in pre-tertiary yolk stage in his calculation.
Table 3. Batch fecundity (number of eggs spawned per day) by size. (After Nikaido et al., 1991).
|Fork length (cm)||Off Java||South-western offshore of Hawaii|
Currently information is not available on the length of the spawning season, thus it is not known how long the spawning season lasts for a fish and how many times the same fish spawns during a spawning season.
Kikawa (1966) calculated the spawning potential of bigeye and yellowfin tunas in the equatorial Pacific by two-month intervals utilizing the following equation:
K = a · p · s · D
where a = mean weight of ovary, p = mean group maturity, s = mean sex ratio, and D = mean hook rate weighted by area surveyed. It was pointed out that the annual spawning potential was much higher in the eastern Pacific (between 140°W and 100°W) than in the western and central Pacific; the eastern Pacific comprising roughly 70% of the total spawning potential of bigeye tuna. This was attributable to not only the high hook rate but also the larger size of fish and the higher rate of group maturity in the eastern Pacific. The seasonal difference in spawning potential, which was considered to reflect the group maturity, was observed with its maximum in April-May in the area north of the equator, and in February-March in the area south of the equator. The minimum spawning potential occurred in October-January in the area north of the equator and August-November in the area south of the equator. It is also indicated that the seasonal difference in spawning potential in the longitudinal direction is similar both north and south of the equator, respectively.
5.3 Sex Ratio
Kikawa (1966) and Kume (1969b) analysed sex ratio of longline-caught bigeye tuna from the broad area of equatorial Pacific. Both studies summarized the data by area and quarter-of-the-year; however, Kume (1969b) provided the data in smaller units. There was a general tendency of predominance of male fish over the entire size range encountered. The dominance of males became more prominent as the size increased. They reported no discernible seasonal change in sex ratio by size. However, Kume (1969b) noted that in the eastern equatorial Pacific the sex ratio was close to 0.5 where sea-surface temperature (SST) was low (<24°C) and fishes were found to be immature, whereas male predominated over female (less than 40 % were female) in the adjacent areas where sea surface temperature (SST) was high (>25°C). For fish of all sizes caught by longline in the Pacific north of 28°N between 140°E and 180° the sex ratio was almost equal to 0.5, although a predominance of males was observed among fish smaller than 130 cm in the area east of 180° (Kume 1969a).
6. DISTRIBUTION, MIGRATION AND STOCK STRUCTURE
Bigeye tuna inhabit the tropical to temperate waters of the Pacific Ocean. This species is found across the entire Pacific between northern Japan (45°N) and the north island of New Zealand (40°S) on the western side, and from about 40°N to 30°S on the eastern side (Figure 3). Significant bigeye catch has not been reported so far from the area along the coast of Mexico and Central America between about 10°N and 20°N. Isolated specimens have been reported from as far north as 47°10'N on the coast of North America (Radovich, 1961; Meehan, 1965).
Most of the information on this subject is obtained from the fisheries, i.e. through seasonal and areal change of fishing ground, catch, CPUE, size, etc., assuming that these changes reflect the migration and/or movement of the fish.
Figure 3. Distribution of bigeye tuna in the Pacific. (After Calkins, 1980).
Kawasaki (1958; 1960) and Kume and Morita (1967) reported the seasonal shift of baitboat fishing grounds and the size of fish caught around Japan. The Japanese baitboat fishery starts operations in April in Izu-Bonin Islands waters. The fishing grounds move progressively northeastward and reach the Tohoku area (north of 35°N, 140–165°E) in July and August. Subsequently, the fishery turns south to southwest until the end of the fishing season. Similar north-south movements of the fishing grounds are also reported for the longline fishery in higher latitudes of the North Pacific along 30°N from 130°E to 120°W (Kume, 1967), waters around north island of New Zealand (Kume, 1967), and waters off Chile (Miyabe and Bayliff, 1987). Since the main component of the bigeye tuna caught by the Japanese baitboat fishery is small (45–80 cm in FL), it is considered that this represents the movement of young bigeye tuna around Japan (Honma and Kamimura, 1955; Kume and Morita, 1967).
In the North Pacific area along 30°N, the majority of fish caught by longline are between 90–140 cm with more fish prevailing in the smaller size of this range (Kamimura and Honma, 1953; Kume, 1969a). Similar-sized fish are also caught in the area off Chile as shown in Miyabe and Bayliff (1987). In both areas fishing takes place in their respective winter. On the other hand, the fishing grounds in the equatorial Pacific are formed throughout the year and the size of fish caught is larger (110–160 cm). It is a well-known fact that those fish caught in the tropical Pacific are sexually mature but those encountered in the temperate waters are immature. Based on these observations, many investigators hypothesized that the spawning-feeding migration between equatorial waters and temperate waters must be in existence since fish in the both areas have to be intermingled through spawning.
Tagging data are also available but the amount of information is scarce because of fewer releases and resultant fewer recaptures compared to other tunas such as yellowfin. In Table 4, tagged bigeye tuna which were recaptured after substantial time at liberty are listed. Although there is a tendency for tagged bigeye to stay in the vicinity where they were tagged, some fishes did exhibit long-distance movement. It is worth noting that two fishes tagged in the Coral Sea were recaptured in the central Pacific after more than 900 days at liberty. The sizes at recapture of those two fishes are 131 and 160 cm, respectively, and are the largest of those listed in Table 4. The movement of adult bigeye may be more extensive than previously considered.
Table 4. Recapture records of tagged bigeye tuna with long time at liberty. Length and distance are given in fork length (cm) and nautical miles, respectively. LL-longline; HL-handline. (Modified from Calkins, 1980).
|Release||Recapture||Distance||Days Free||Direction (Deg.)||Reference|
|Jan. 31, 1955||122.3||32°41'N|
|Nov. 24, 1955||126.8||LL||785||298||82|
Otsu and Uchida (1956)
|Jan. 31, 1955||109.0||29°50'N|
|Feb. 2, 1956||127.5||LL||348||368||259||Kume (1967)|
|May. 31, 1958||82||29°15'N|
|Nov. 11, 1959||110– 115||LL||540||530||255||Kume (1967)|
|May. 31, 1958||81||35°27'N|
|Jan. 17, 1960||119||LL||182||597||324||Kume (1967)|
|May. 6, 1967||80||2°12'S|
|Jan. 13, 1968||?||?||664||253||117|
|May. 12, 1967||50||4°32'N|
|Jun. 18, 1969||128.0||?||1,020||769||274||IATTC (1970)|
|Dec. 30, 1981||42||2°-4°N|
|Feb. 15– Mar. 18, 1986||126||LL||200– 700||1,508– 1,539||330– 360||FSFRL (1988)|
|Nov. 19, 1986||96||16°58'S|
|Oct. 29, 1987||112||HL||129||345||148|
|Nov. 19, 1986||109||7°17'S|
|Jun. 2, 1990||160||LL||3,408||1,292||89|
|Nov. 19, 1986||108||16°43'S|
|Nov. 2, 1987||127||HL||110||349||148|
|Nov. 21, 1986||78||3°42'S|
|Jun. 17, 1989||131||LL||2,591||940||79|
|Nov. 21, 1986||98||16°42'S|
|Nov. 1, 1987||124||HL||94||346||143|
*1 Cited in Calkins (1980).
*2 Ward, Peter. Personal Communication. Fisheries Resources Branch. Bureau of Rural Resources. Australia.
6.3 Stock Structure
Except for Fujino and Kang (1968), there have been no other studies on the genetic differences among bigeye tuna population(s) in the Pacific. Fujino and Kang (1968) analyzed serum-esterase groups of bigeye tuna taken in the Hawaiian waters, and found two phenotypes in the sample. This study, however, is not sufficient enough to draw any conclusions regarding the population structure of Pacific bigeye tuna.
The stock structure of Pacific bigeye tuna has been inferred by indirect evidence such as geographical distribution of fish, CPUE and fish size, and maturity and spawning by area and time. Evidence that supports a single stock includes: (a) continuous distribution of bigeye throughout the Pacific, (b) similar size frequencies in neighbouring areas, (c) broad spawning area and time, (d) the concurrent appearance of a dominant year class throughout the North Pacific as observed by the length-frequency studies. On the other hand, the existence of subpopulation(s) is supported by the cline shown in the size of bigeye tuna in the east-west direction and the limited movement of tagged fish; both features suggest a low level of intermingling of fish.
In summary, it might be appropriate to repeat the following interpretation by Suda and Kume (1967) that “It is rather difficult to consider that the mixing of Pacific bigeye is active enough to ensure the unified stock in the whole Pacific. At the same time there is no clear evidence to support the existence of plural subpopulations. Presumably, it seems reasonable to assume a single population in the Pacific where mixing of the fish takes place gradually through the whole life history.”
7. NATURAL MORTALITY
Applying the Paloheimo method (Paloheimo, 1961), Suda and Kume (1967) estimated the instantaneous coefficient of natural mortality (M). Looking at the age composition of the fish taken by the longline fishery, they concluded that bigeye tuna are fully recruited to the fishery after age 4. Therefore the effective effort (f) and CPUE for age 5 and older fish during 1957–1964 were used, and fitted to the following equation assuming catchability (q) is constant among age and year for fully-recruited fish:
log e S = q · f + M
where S is survival rate calculated from the ratio of CPUE of the fish older than 4 years old in one year and that of older than 5 years old fish in next year. They obtained the annual coefficient of 0.361.
In general it is likely that natural mortality is high at the younger ages, lower at the intermediate ages and again higher at the older ages.
8. OCEANOGRAPHIC FEATURES ASSOCIATED WITH SPECIES
In the developing stage of the Japanese longline fishery, Federation of Japan Tuna Fisheries Cooperative Association (1959) hypothesized that (a) tuna species distribute in different current systems, (b) even in the same species, tuna might have a habitat in the different current systems according to their developmental stages, and (c) the change of developmental stage and resultant change in tuna longline fishing grounds take place around March and September. Nakamura and Yamanaka (1959) also discussed the tuna distribution in relation to the ocean current and noted that bigeye tuna had a characteristic to aggregate around the boundary of currents. Yamanaka and Anraku (1962) discussed the relation between the distribution of tunas and water types in the central and western Pacific based on the T-S diagram.
Among oceanographic features associated with bigeye, water temperature is the most common feature that has been addressed by many workers to explain the spacial/temporal distribution of fish or fishing grounds. Uda (1957) cited the optimum water temperature (17.5°–22°C) for bigeye. This range of optimum water temperature for bigeye was supported by Suda et al. (1969) as coinciding with the temperature of the permanent thermocline, in which bigeye were considered to inhabit. In later studies (Saito and Sasaki, 1974; Saito, 1975; Hanamoto, 1976), it was found that bigeye were caught more efficiently in much deeper waters (133–245 m in waters southwest of Hawaii and 290–380 m in the western waters off Fiji) by the experimental vertical longline. In fact, the Japanese small-size class longliners introduced the so-called “deep longlining” to the western equatorial Pacific during the mid-1970s (Suzuki et al., 1977) and it immediately prevailed among other boats and areas thereafter (Suzuki and Kume, 1981; Miyabe, 1989).
Hanamoto (1987) analyzed the oceanographic data (water temperature, dissolved oxygen, salinity) in relation to longline catch data. The latter included information on branch number, from which the depth at capture was estimated on the assumption that the main line formed a catenary curve. The results (Figure 4) showed the optimum water temperature to lie between 10° and 15°C which was much lower than that previously considered. Hanamoto (1987) stated that the same finding on water temperature could be applied to salinity, since, in the above-mentioned optimum temperature range, there was positive correlation between temperature and salinity. The optimum salinity was between 34.5 and 35.5 per mill in the South Pacific, between 34.0 and 34.7 per mill in the North Pacific, and between 34.7 and 35.2 per mill in the equatorial Pacific.
Figure 4. The catch, catch rate of bigeye tuna and the number of hooks used by water temperature at the depth of capture. (After Hanamoto, 1987).
As shown by Sund et al. (1980) and Sharp (1978), the minimum dissolved-oxygen requirements for bigeye are considered to be 0.5–1.0 ml/l. Hanamoto (1987) constructed the diagram showing the depth of the 1 ml/l surface of dissolved oxygen in the Pacific, and found that the poor catch of bigeye based on the long-term average of longline catch coincided with the shallow depth area (about 100 m) of the 1 ml/l surface. These poor catches were located in the eastern Pacific between 10°N and 20°N and in the area off Chile to 120°W in the south of equator.
It has been mentioned from fishing experience that the swimming depth of bigeye is deeper than that of other tuna species. Suda et al. (1969) compared the areas of high hook rate by longline with the depth of thermocline and found a significant correspondence between areas of high hook rate and the location of thermocline at the depth of 100–150 m, which was identical to the depth of hook set at the longline operation (Figure 5). Taking these into consideration, Suda et al. (1969) postulated that bigeye inhabit waters in or just below the thermocline. It is thought this assumption could explain to some extent the observed west-east cline in both size of fish and CPUE. If the assumption holds, ordinary longline gear is less effective and tends to catch smaller fish since hooks do not reach the thermocline in the western Pacific where larger fish are supposed to inhabit. Hanamoto (1987) also discussed the effect of oceanographic condition on the longline catch along this line. He suggested the possible distribution of this species in the area where the optimum water temperature was located. Those suggested areas were located in the middle latitudes along 20° in both hemispheres and high latitudes between 30°S and 40°S in the South Pacific.
Figure 5. Illustration of the area in which effective catch of bigeye tuna by longline is supposedly expected. Shaded area indicating permanent thermocline is well developed and located at depth between 100 and 150 m. This range is approximately eqivalent to that of longline hooks. Dotted area shows that thermocline is not permanently formed and the habitat of bigeye tuna ranges from surface to some depth. (After Suda et al., 1969).
Sonic tracking may give the answer to the above-mentioned hypotheses on the behavior of bigeye tuna. Several tracking studies have been done on this species, however, most tracks have been on small fish less than 70 cm. One exception was a 70 kg bigeye which was tracked by Carey and Lawson (1973) off the Atlantic coast of Canada. Generally speaking, although the results were different among fishes tracked, the swimming depth ranged from the surface to 300 m and fish tended to stay in deeper waters during the day than during the night (Holland et al., 1990; Koido and Miyabe, 1990).
The effect of large-scale oceanographic change on bigeye and its fishery in the eastern Pacific, known as El-Niño, was briefly analyzed after the very strong 1982–1983 event. Kume and Miyabe (1987) compared the location of the Japanese longline fishing grounds in the eastern tropical Pacific during El-Niño with that during normal conditions. It was reported that, during El-Niño periods, good fishing grounds were formed around the equator between 95°W and 130°W; fishing seldom took place in these areas during normal conditions. The possible influence of the El-Niño event on bigeye tuna stock was studied by Kiyota et al. (1988) based on CPUE and cohort analysis. It was suggested that cohorts born in the El-Niño year or one year before El-Niño were larger and that CPUEs were higher during El-Niño than during normal conditions. These effects might be caused by the changes of oceanographic condition, such as water temperature and the depth of thermocline, although the mechanism involved in the process is not well known.
9. INTERACTION WITH OTHER SPECIES
Based mainly on observations during fishing, many authors report that bigeye tuna are mixed with yellowfin, skipjack, kawakawa, and frigate tuna, particularly when they are young (<100 cm). Around Japan, bigeye are caught by baitboat and purse-seine fisheries in pure or mixed school with yellowfin and/or skipjack. It is known bigeye tuna is seldom caught with albacore in the same operation. In the Coral Sea, small to medium bigeye (50–120 cm) as well as yellowfin are caught by handlining by the Japanese longliners during October to December (Hisada, 1973). It is reported that the types of school sought after are shark-associated, and, to a lesser extent, free-swimming schools with birds.
Recently the bigeye catches by the industrialized purse-seine fisheries in the western and eastern Pacific (Table 10) have accounted for about 5–10% and 0.2–5.0% of the total tuna catch by weight for the two areas, respectively. The former value is a rough estimate because purse-seine catch data of bigeye tuna for Korea, Philippines, Taiwan, USA, etc., are lacking.
10. GENERAL DESCRIPTION OF FISHERIES
The longline fishery has landed the largest share of bigeye tuna catch since the initiation of the fishery in the early 1950s. The baitboat fishery has also accounted for a significant share of the bigeye tuna catch and has a long history as well. The purse-seine fishery, which was developed in later years (during the early 1960s in the eastern Pacific and during the mid-1970s in the western Pacific) and has replaced the baitboat fishery thereafter, has landed a substantial amount of the bigeye catch. Other local fisheries which catch small amounts of bigeye tuna exist in the coastal countries throughout the Pacific; these fisheries include trolling, hand-lining and small-scale purse seine.
10.1 Longline Fishery
Japan, Korea and Taiwan are the major countries catching bigeye tuna by longline gear. Some very minor catches are recorded by Australia, New Caledonia and Tonga. The size of distant-water longliners now operating ranges from 50 to 1000 gross tonnages (GT), and is mostly between 200 and 500 GT. In Japan and Taiwan there have been coastal to offshore tuna-longline fisheries carried on by the small-size vessels up to 50 GT.
The Japanese longliners operate in almost the entire Pacific; the principal exception is the central part of the South Pacific Ocean, south of 20°S. Fishing grounds are located in both tropical (10°N–15°S) and temperate (around 30°N and 30°S) waters extending in an east-west direction (Figure 6). In tropical waters the fishing takes place almost year around while in temperate waters it takes place in winter. Bigeye tuna is the most important tuna species for the Japanese tuna fishery in terms of the catch in weight (except skipjack) and value (Ministry of Agriculture, Forestry and Fishery of Japan, 1990). Bigeye tuna is the species most frequently aimed at and there are by-catches of yellowfin, albacore and billfishes. Catches by distant-water longliners are deeply frozen to below -60°C, and consumed as sashimi in the domestic market. In this fishery a limited-entry system has been adopted and vessels larger than 20 GT are required to obtain a permit from the Government. All longliners which have the permit are not allowed to unload catches in foreign countries except as transshipment to Japan; the total amount of transshipment by longliners is limited by the Government. The numbers of longliners fishing since 1977 are shown in Table 5. Smaller-sized boats (20–100 GT) are decreasing, while numbers of 100 to 200 GT class have doubled during that period. The number of longliners in the largest size class has been stable.
Figure 6. Distribution of bigeye catch (in number) for Japanese longline fishery by 5-degree square in 1987.
(After Miyabe, 1991).
Table 5. Annual change in the number of tuna longliners by country, 1977–1989.
|Japan||Korea (100–500 ton)||Taiwan (50–1000 ton)|
|20–50 ton||50–100 ton||100–200 ton||200< ton||Total||Home based||Foreign based||Total||Pacific based||Takao based||Other||Total|
Japan : MAFFJ (1978–1990).
Korea and Taiwan : FAJ (1989).
The geographical distribution of fishing effort is shown in Figure 7. The main fishing ground is located in the South Pacific (0°–30°S, 160°E–120°W) and bigeye tuna is the target species. Less extensive operations are conducted in the North Pacific around Hawaii in winter. The number of longliners has decreased from 501 in 1977 to 268 in 1988 (Table 5).
Figure 7. Distribution of fishing effort and bigeye CPUE for 1981 by Korean longline fishery. (After NFRDA, 1986).
Fishing grounds for Taiwanese longliners are similar to those of Korean longliners, but the centre is located from the central to western South Pacific (Figure 8). Seasonal change of fishing ground is clear for the fleet fishing in the southern part (20°S–40°S) during winter in the southern hemisphere (TRC, 1983). This is because albacore is the target rather than bigeye tuna. The number of longliners has also declined; in particular the number of Pacific-based boats declined from 319 in 1977 to 62 in 1988 (Table 5).
Figure 8. Distribution of bigeye catch (in number) for 1981 by 5-degree square for the Taiwanese longline fishery. (After TRC, 1982).
10.2 Baitboat Fishery
The Japanese distant-water baitboat fishery operates in the wide area of the western Pacific (40°N–10°S, 120°E-170°W, Figure 9). Bigeye tuna is a by-catch of this fishery which targets skipjack and albacore. The fishing takes place all year round in the tropical area while it occurs during spring to fall in the higher latitudinal waters. The number of licensed boats decreased in the 1980s, and in 1989 the fleet declined to less than one-third of the 1977 total (Table 6). In addition to this, a coastal to offshore baitboat fishery (vessels smaller than 100 GT) has been operating in more nearshore waters than the distant-water fishery.
In the eastern Pacific the baitboat fishery has been operating since the early 1900s primarily directing its fishing effort at yellowfin and skipjack. Starting from the early 1960s, purse seiners have replaced baitboats in the fishery. The baitboat fishing operations are confined to coastal waters and to the vicinity of offshore islands.
10.3 Purse-seine Fishery
There are two major fishing areas for this fishery, one in the western Pacific and the other in the eastern Pacific. In this fishery bigeye is a by-catch as in the baitboat fishery.
Figure 9. The fishing ground of the Japanese baitboat and purse-seine fisheries in the tropical Pacific. Large and small circles show purse-seine and baitboat fishing areas, respectively. (After Tanaka, 1989).
Table 6. Annual change in the number of fishing vessels for the Japanese distant-water baitboat and purseseine fisheries, 1977–1989.
|Baitboat Distant-water fishery||Purse-seine Tropical fishery|
Data source : MAFFJ (1978–1990).
10.3.1 Western Pacific
The industrial purse-seine fishery commenced operation in the late 1970s, and by the mid-1980s vessels from Japan, Korea, Philippines, Solomon islands, Taiwan and the USA were operating in the South Pacific (SPC 1990a). At present (1st quarter of 1990), the USA has the largest fleet size (34 boats) followed by Japan (31), Taiwan (25), and Korea (8) (SPC 1990b).
Table 6 shows the annual change in numbers of the Japanese purse seiners operating both in the Pacific and the Indian Ocean. The Japanese distant-water purse-seine fishery started fishing in the tropical western Pacific in 1973 (Tanaka, 1989). In 1980, 14 vessels were counted. A total of 18 vessels was added during the following 2 years and thereafter the number of vessels operating has been constant in the Pacific. In the vicinity of Japan the offshore purse-seine fishery has operated for various pelagic fish species. Tunas and skipjack are caught seasonally during spring to early fall in the Pacific off Japan. A few of these offshore purse seiners (five to seven boats) are allowed to operate in the tropical waters seasonally (February to May). Based on the species composition of the catch unloaded at Yaizu fishing port (Table 7), the catch of bigeye tuna by the industrial purse-seine fishery is considered to be very minor (0.8–4.2% of total catch) in terms of catch in weight. A preliminary survey (National Research Institute of Far Seas Fisheries, internal data) conducted at the cannery showed that, in terms of numbers of fish, about 10% of fish sold as small yellowfin (smaller than 2 kg) were identified as bigeye.
Table 7. Species composition (%) of the Japanese purse-seine catch (in weight) in the tropical Pacific unloaded at Yaizu fishing port. (From Tanaka. 1989).
*1 Large and small yellowfin are larger than 10 kg or less than 10 kg, respectively.
*2 Over 2 kg in round weight.
*3 This category includes juvenile yellowfin and bigeye (less than 2 kg) as well as small amount of damaged fish in other categories.
10.3.2 Eastern Pacific
Table 8 shows the numbers of vessels and capacities in the eastern Pacific purse-seine fleet during 1950–1988. In 1987 the fleet was composed of vessels from 10 countries. Major countries were the USA (54 vessels), Mexico (54), Ecuador (28), and Venezuela (25) (IATTC, 1989). In the early 1960s the purse-seine fishery replaced the baitboat fishery and to date has been the dominant fishery. The fishing grounds were confined to the coastal area in the early stage of the fishery, but it began to expand into the offshore areas in the mid-1960s. By the mid-1970s the fishery covered the coastal waters between about 30°N to 15°S and extended offshore to about 145°W between 5°N and 15°N (Calkins et al., 1988). The geographical distribution of catch by the surface fishery during 1982–1986 is shown in Figure 10. The most concentrated area of catch was adjacent to the coast of South America from the equator to about 7°S and from the coast to about 87°W. The geographical catch distribution during 1967–81 was similar to that during 1982–86. The catch was small (less than 3,000 short ton (ST) until 1974 (Table 10) when it jumped to over 4,000 ST). During 1976–81 it was the highest (8,000–17,000 ST) and then decreased to 2,000–6,000 ST during 1982–86.
The length distributions for 1975–1986 show that the length for the fish in nearshore areas ranged from 30 to 180 cm. In general, most of them occurred between 60 to 120 cm with major modes between 80–110 cm (Figure 11). The size range in the offshore area is about the same as in the nearshore area, but more of the catch appears to fall in the 30 to 70 cm range.
11. TRENDS IN FISHING EFFORT, CATCH, AND CATCH-PER-UNIT OF FISHING EFFORT
11.1 Trends in Fishing Effort
Not all the data from the various fisheries are available. Information which is available is the number of fishing vessels operated or registered (Tables 5, 6 and 8). The annual change in fishing effort by the Japanese distant-water longline fishery in terms of the number of nominal hooks exerted is shown in Figure 12. The Japanese distant-water longline fishery is the largest component among fisheries for bigeye tuna. The nominal effort increased dramatically from 1952 to 1963 when it reached a high of 3.4×108 hooks. Thereafter it fluctuated between 2.5 and 3.0×108 hooks up until the mid-1970s, and then went up to a record high in 1981. Since then it has stayed at the highest level at around 3.5 × 108 hooks. Because of the limited-entry system adopted for this fishery, the recent change in the fishing effort is attributed to the increase in the number of hooks set per operation (day), and to the movement of the fleet into the Pacific Ocean or to other Oceans.
11.2 Trends in Catch
The annual catch of Pacific bigeye tuna is shown in Table 9 by country (FAO, 1965–1988), and in Table 10 by major gear, and in Figure 13. There is some difference between these two statistics (Tables 9 and 10) reflecting the difference in the source of data. Generally the catch shown in Table 10 is slightly larger except for 1987. As the Japanese longline catch accounted for over 70% and over 80% of the total catch and of the total longline catch, respectively, the trend of total catch is similar to that of the Japanese longline fishery (Table 10). Before 1957 the annual total catch was less than 50,000 mt. It increased to between 82,000 and 92,000 mt during 1958–1960, and then showed a sudden increase in 1961 to 136,000 mt. After a high of about 150,000 mt in 1963, it remained at a lower level between 73,000 mt and 100,000 mt up until 1974 except 1969 and 1973. Since then it has reached a higher level of over 100,000 mt. The catch was higher during 1976–1980 and 1985–1987.
Table 8. Numbers and carrying capacities, in short ton, of vessels of the eastern Pacific tuna fleet. Bolicheras are small purse seiners with limited ranges. There is no information available on bolicheras or trollers for 1950–1954. The 1988 data are preliminary. (After IATTC, 1989).
It should be noted that in the western Pacific there has been no catch record of this species in the FAO statistics from countries such as the Philippines and Indonesia where substantial catch was reportedly made. Several possible reasons for this may be considered. One is lack of a system whereby catch may be estimated. The other is that species separation between tunas, especially for the small fish, is often difficult because of the similarity of their morphological appearances and because, due to the nature of the catch, bigeye tuna sometimes get caught with other tunas at the same time. In the near future this should be clarified and catches from these countries included in the statistics.
Figure 10. Annual average of distribution of bigeye catch by the eastern Pacific surface fishery, 1982–1986. (After Calkins et al., 1988).
11.3 Trends in Catch-per-Unit of Fishing Effort (CPUE)
The series of studies (Suda and Schaefer, 1965; Kume and Schaefer, 1966; Kume and Joseph, 1969; Shingu et al., 1974; Miyabe and Bayliff, 1987) on the Japanese longline fishery in the eastern Pacific analyzed the relationship between nominal fishing effort and catch (CPUE). It is shown in Figure 14 for the equatorial area east of 150°W. From this figure three periods can be separated, i.e. 1957–1961, 1962–1964 and 1965–1980. During 1957–1961 CPUE was high at about 3 fish per 100 hooks. In the following period in 1962–1964 it quickly decreased to less than half of this. Thereafter, it has been very constant at slightly less than one fish per 100 hooks.
Miyabe (1991) estimated the standardized CPUE for the total Pacific stock, applying the Honma method (Honma, 1974) based on the Japanese longline catch and effort data. Deep longlining, a newly-introduced longline gear setting, was also adjusted to the conventional one employing the ratio of CPUEs from both gear settings and the proportion of fishing effort deployed by deep longline. The results showed that CPUE decreased from the late-1950s to the mid-1960s, but after that it fluctuated moderately without any appreciable trend until 1988 (Figure 15). The current level of CPUE is about 45% of the initial level in the 1950s.
Figure 11. Combined annual length-frequency distributions for 1975–1986, by school type and area, of bigeye tuna caught by the surface fishery in the eastern Pacific. (After Calkins, et al., 1988).
Kume (1979a) presented CPUE by age and area for 1955–1976 (Figure 16). Nominal CPUEs by area and 5-year intervals averaged for 2 succeeding years were constructed into age-specific CPUEs applying the length frequency data. It is observed that CPUE decreased greatly between 1960–61 and 1965–66 for fish older than 3 years in all areas. After that, CPUE for older age group is generally decreasing although the trend is not so clear as before.
Figure 12. Annual trend of fishing effort by Japanese longline fishery. (After Miyabe, 1991).
Figure 13. Annual catch trend of Pacific bigeye tuna.