Tunas have been important to mankind for several thousand years. Archaeological evidence shows that early humans harvested tuna more than 6 000 years ago, and tuna products may have been among the earliest manufactured fisheries commodities traded among ancient civilizations. Currently, fishermen of nearly 80 nations harvest tuna from the worlds oceans. The harvest is consumed in many forms: raw, cooked, smoked, dried, and canned. More than half of all the tuna consumed is canned.
The following discussion, which is taken mostly from Joseph (2000), deals with the populations of the principal market species of tuna: skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares), bigeye (Thunnus obesus), albacore (Thunnus alalunga), northern bluefin (Thunnus thynnus), and southern bluefin (Thunnus maccoyi).
Between 1975 and 1992 the total world catch of tuna increased (Figure 1), as did those for most of the individual species (Figure 2). However there were periods of slow growth that alternated with periods of fast growth. From 1991 through 1996 catches stayed relatively steady, between about 3.1 and 3.2 million metric tonnes. In 1997 the catch reached 3.4 million tonnes, and it has continued to increase through 1999, when it reached about 3.9 million tonnes.
The large increases in the 1970-1978 period were the result of expansion of the fisheries in the eastern Atlantic and the development of new offshore fishing areas in the eastern Pacific. Subsequently, after six years of little increase in world production, many vessels transferred to the western Pacific and western Indian Ocean, where they developed new fishing grounds. The catches during this period showed the greatest rate of growth seen in the fishery in many decades. No new major fishing grounds have been developed since 1990, and from then until 1997 the fishery showed almost no growth. From 1996 through 1999 the catch increased by about 19%, due mostly to the improvement and increased use of fish aggregating-devices (FADs).
The annual catches of the principal market species, by ocean, during 1970-2000 are shown in Figure 1. The Pacific Ocean currently produces about 2.5 million tonnes, or 64% of all the worlds annual tuna catch and, with the exception of bluefin, also produces the greatest quantities of each of the principal market species (Figures 3, 4, 5, and 6). Of this 2.5 million tonnes, about 65% is taken by purse-seine vessels, slightly less than 14% by pole-and-line vessels, slightly more than 10% by longline vessels, and the remainder by miscellaneous other gears. The fishery in the western Pacific, west of 150°W, accounts for the large majority of these 2.5 million tonnes. The west-central tropical Pacific, the area studied by the scientists of the Secretariat of the Pacific Community (SPC; formerly the South Pacific Commission), has produced more than one million tonnes, or about 30% of world production, during the last few years; of this, nearly 800 000 tonnes, or about 24% of the world total, is caught by the purse-seine fishery, the single largest tuna fishery in the world. The Japanese home island tuna fishery, which operates within a few hundred miles of the home islands of Japan, also produces large quantities of fish; in recent years the Japanese catches of the principal market species in this area have fluctuated between 150 and 300 000 tonnes, while the catches around Indonesia may even exceed those of Japan, and those of the Philippines approach those of Japan.
Prior to the 1980s, the Indian Ocean accounted for less than 8% of world production of tuna. Most of the catch came from artisan fisheries in Sri Lanka and the Maldives, augmented by distant-water longline fleets. In the early 1980s French and Spanish purse-seine vessels, faced by poor catch rates and problems of access in the Atlantic Ocean, moved to the western Indian Ocean, and as they expanded their operations there, catches of skipjack and yellowfin increased rapidly. Over the last several years the catches of tuna from the Indian Ocean have averaged about 20% of the world total. Following this rapid increase in the catch, annual production stayed around 700 000 tonnes until 1999, when it increased to nearly 900 000 tonnes (Figure 1). The catch in 2000 was only slightly behind that of 1999. More than 75% of this is caught in the western Indian Ocean by purse-seine vessels and by the fisheries of the Maldives and Sri Lanka. Two species, skipjack and yellowfin, account for about 80% of the total catch from the Indian Ocean.
Although artisanal and small-scale fisheries for tunas have existed in the Atlantic Ocean for many centuries (significant trap fisheries have existed in the Mediterranean Sea since the 12th Century), large-scale commercial exploitation of tunas in that ocean did not begin until the 1950s. Tunas were caught mainly with pole-and-line and longline gears until purse seining was introduced in the early 1960s. Catches increased slowly until the early 1980s, when they started to decline because of the shift of fishing fleets to the Indian Ocean. They were stable for several years, and then began to increase again, peaking in the early 1990s. Since then they have been relatively stable at around 500 000 tonnes per year (Figure 1). The Atlantic Ocean currently accounts for about 14% of the world production of tuna. The principal species caught in the Atlantic, in terms of quantities landed, are skipjack and yellowfin, with nearly 80% of the landings coming from the eastern Atlantic. Most of this catch is made by large purse-seine vessels, which also catch bigeye.
Figure 1. Trends in the catch of the principal market species of tunas by ocean
Figure 2. Trends in the world catch of tunas by species
During the last several years, skipjack tuna has accounted for about 50% of the total world catch of the principal market species of tuna (Figure 2). It is among the most widely distributed of all tuna species, being found in commercial quantities between 45°N and 40°S; it inhabits the upper mixed layer of the ocean, and is caught mostly with purse seines and pole-and-line gear. Most of the catch is used for canning. Skipjack is a short-lived species, with high rates of natural mortality and population turnover. These characteristics of skipjack, together with their wide distribution, results in a huge biomass of fish, and very high levels of potential production. Ever since the beginning of heavy commercial exploitation in the early 1970s, the consensus among scientists had been that the populations of skipjack in all oceans of the world were lightly exploited and capable of sustaining much higher catches. This has been borne out by the fact that annual catches increased from about 400 000 tonnes in 1970 to around 1.9 million tonnes in 1998. They remained near that level during 1999 and 2000.
In terms of weight skipjack is the dominant species in the catch of the Pacific (Figure 3). Genetic studies of the Pacific population of skipjack suggest that there is some mixing of fish across the Pacific Ocean, but for management purposes the stocks in the western Pacific have been considered by most scientists to be independent of those in the eastern Pacific. Tagging data, showing limited movement of skipjack from the eastern Pacific to the western Pacific, supports the same conclusion. The Pacific-wide catch of this species increased from slightly more than 200 000 tonnes in 1970 to highs of about 1.4 million tonnes in 1998, 1999, and 2000.
About 1.2 million tonnes per year was taken from the western Pacific in 1998 to 2000. Studies based on tagging experiments conducted by the SPC suggest the stock of skipjack in the western Pacific is under exploited and that it may be possible to increase catches significantly over levels experienced during 1991-1997, perhaps by as much as 200 to 300 000 tonnes per year (Klieber, Argue and Kearney, 1987). Such increases would, of course, depend on demand for raw material, price, the ability of the fishermen to locate additional fishing areas, and the vulnerability to capture of the fish in these new areas. The 1998-2000 catches did surpass the previous high catch level of 1991, but only slightly. However, if the estimates from the tagging experiments are correct, additional increases in skipjack catch could be sustainable.
Prior to 1999, catches of skipjack from the eastern Pacific ranged between about 40 and 160 000 tonnes, with a peak of about 170 000 tonnes taken during the late 1970s. During the last few years catches have reached record highs: 262 and 208 000 tonnes during 1999 and 2000, respectively. It is likely that catches of skipjack could be sustained at higher levels than those of the 1970s. However, because of the variability of skipjack abundance catches as great as those of 1998 and 1999 could not be expected every year (IATTC, 2001). Purse-seine vessels fishing on FADs, a method which normally catches a very high proportion of small fish, have taken much of this increased catch in recent years. There is concern that increasing fishing effort on FADs in the eastern Pacific, and elsewhere, in order to increase the skipjack catch, could result in increased catches of small yellowfin and bigeye, which might affect the abundance and future catches of those species.
Figure 3. Trends in the catch of tunas from the Pacific Ocean
Figure 4. Trends in the catch of tunas from the Indian Ocean
Catches of skipjack in the Indian Ocean, like those in the Pacific, have shown a steady increase since the entry of purse-seiners from other areas (mostly from France and Spain) into the fishery during the early 1980s (Figure 4). The 1999 catch reached an all-time high of nearly 390 000 tonnes. The Indian Ocean is the only ocean in which skipjack has not regularly formed the greatest proportion of the tonnage landed. Since 1990 the annual average landings of yellowfin and skipjack have been about the same, about 265 000 tonnes each, with the exception of 1999 and 2000 when the skipjack catch substantially exceeded that of yellowfin. Skipjack in the Indian Ocean are considered to comprise a single stock, so that any management and conservation measures enacted would have to apply over the entire ocean. Although studies of the stock do not show clear evidence that it is fully exploited, scientists have expressed some concern about the possibility of increased fishing levels adversely affecting stock abundance (Anganuzzi, Stobberup and Webb, 1996). Nevertheless catches have shown a steady annual increase since 1983, reaching a peak of about 390 000 tonnes in 1999.
In terms of weight of fish caught, skipjack is the most important tuna species in the Atlantic; in both 1999 and 2000, about 165 000 tonnes were landed. This is somewhat less than the average for the previous few years, and well below the record landings of about 200 000 tonnes in 1991 (Figure 5). About 85% of the catch is taken in the eastern Atlantic, and the rest is taken primarily off Brazil. Although there is no conclusive evidence concerning the stock structure of this species, scientists have treated skipjack in the eastern and western Atlantic as separate stocks. No Atlantic-wide assessments of skipjack have been made since 1984, when scientists working cooperatively under the auspices of the International Commission for the Conservation of Atlantic Tunas (ICCAT) concluded that the resource was underexploited in both the western and eastern Atlantic. However, a recent analysis by these scientists, for the central area of the eastern Atlantic fishery, where more than one-half the Atlantic catch is taken, shows decreasing average size and decreasing catch rates (ICCAT, 1999). These scientists considered that, in spite of the high turnover rates of the Atlantic skipjack population and the fact that nominal fishing effort for this species has been declining in recent years over-exploitation of skipjack in some areas of the fishery is occurring and fishing mortality may exceed levels that would maximize the yield per recruit (Even though the nominal fishing effort has declined, the fishing mortality may be increasing, because of increased fishing power of the vessels). Since 1990 the use of FADs has increased greatly. This has most likely led to an increase in the catch of unmarketable skipjack that are discarded at sea dead, so the total catch may be underestimated. For all these reasons, any increase in the fishing mortality of Atlantic skipjack, particularly in the eastern area, should be carefully monitored.
Figure 5. Trends in the catch of tunas from the Atlantic Ocean
In terms of weight of catch, the second most important species of tuna is yellowfin, which accounts for about 30% of the world catch (Figure 2). This species, like skipjack, is widely distributed, but is confined to slightly more tropical latitudes. Yellowfin live longer and reach larger sizes than skipjack. Most of the commercial catch is used for canning, and fish over 10 kg are considered prime raw material for this purpose. Like skipjack, most yellowfin is taken at the surface by purse-seine vessels but, unlike skipjack, significant catches, particularly of large fish, are made in subsurface waters by longline vessels. From the early 1970s until about 1984 world catches of yellowfin increased only slightly, but in 1985, with the development of new fishing grounds in the western Pacific and western Indian Oceans, the catch increased sharply. This increasing trend continued through 1993, but since then catches have shown no upward trend.
Yellowfin tuna are widely distributed throughout the tropical Pacific Ocean, and are caught by longline vessels throughout their area of distribution. However, most of the approximately 750 000 tonnes taken annually is caught by purse-seine vessels, which fish in much of the western Pacific as far to the east as about 170°W, and in the eastern Pacific from the coastline of the Americas to about 150°W.
After relative stability in production from 1972 to 1984, annual catches of yellowfin in the Pacific increased from about 400 000 to about 700 000 tonnes by 1990. Since 1990 they have averaged about 700 000 tonnes, showing no upward or downward trends (Figure 3).
In the western Pacific, yellowfin catches averaged slightly over 200 000 tonnes per year prior to the late 1970s. With the arrival of the distant-water purse-seine fleets in the area after 1980, catches increased rapidly, and during the last several years have averaged about 450 000 tonnes per year. Although the results of mark-and-recapture experiments conducted by the SPC suggest that exploitation rates on yellowfin are low, indicating that the western Pacific stock can sustain increased yields, any expectations of increased yellowfin production should be viewed with caution, as the longline catches and catch rates have declined and catches of small yellowfin appear to be increasing. In fact, recent analysis suggests that the yellowfin stock is probably fully exploited (Hampton, Lewis and Williams, 2000).
In the eastern Pacific, catches of yellowfin have averaged about 250 000 tonnes over the last decade. Analyses by scientists of the Inter-American Tropical Tuna Commission (IATTC) indicate that the yellowfin resource in this area is fully exploited and is producing near the maximum it can sustain, so increasing fishing effort will not result in a sustained increase in catches (IATTC, 2001). Yellowfin in the eastern Pacific are considered to be a separate stock from those to the west. The IATTC has adopted catch quotas and closed areas for yellowfin in the eastern Pacific during recent years. Scientific analyses have also shown that if the fishing effort currently directed at large yellowfin associated with dolphins were to be redirected to fishing on floating objects, particularly FADs, in an effort to minimize the mortality of dolphins in the fishery, the catch of yellowfin would decrease. This decrease would result from a reduction in the yield per recruit, because large yellowfin are taken in association with dolphins, while fishing with FADs takes mainly smaller fish.
In the Indian Ocean catches of yellowfin, like those of skipjack, increased rapidly after the arrival of the French and Spanish purse-seine fleets. They hit a peak of 330 000 tonnes in 1993, but since then have remained at about that level (Figure 4). Although the scientific evidence is not incontrovertible, it seems likely, based on production models, that the stock in the western Indian Ocean is fully exploited, or perhaps overexploited. Scientists have urged caution regarding expansion of fishing effort in the surface fisheries of the western Indian Ocean, and have expressed concern over the fact that the increased use of FADs has increased the catch of small yellowfin, which could be reducing the yield per recruit, and hence the total potential yield (Anganuzzi, Stobberup and Webb, 1996). It is not known whether yellowfin from the eastern and western Indian Ocean belong to the same stock, but if the two are independent of each other it may be possible to increase yellowfin catches somewhat in the eastern area.
In the Atlantic about 60% of the commercially-caught yellowfin is taken by purse seiners, but significant catches are also made by baitboats and longliners. Fish between about 40 and 170 cm in length are retained, but smaller yellowfin, of low commercial value, are often discarded. With the increased use of FADs the proportion of small yellowfin in the catch has increased. The population of yellowfin in the Atlantic is considered to consist of a single intermingling stock. The fish spawn in equatorial regions of the central Atlantic. Most of the young migrate east to the nursery grounds, where they stay until they are about 65 to 85 cm in length, and then most migrate to the western Atlantic, many returning to the eastern Atlantic fishing grounds at about 110 cm. In the late 1960s catches of yellowfin increased as fleets of purse seine vessels increased their activities in the eastern Atlantic. Catches rose steadily until about 1984, but declined thereafter for a few years due to vessels moving to the Indian Ocean (Figure 5). After 1989, effort in the Atlantic increased again. The peak catch of yellowfin, about 185 000 tonnes, was taken during 1990, but since then catches have been decreasing. The 2000 catch was about 140 000 tonnes. Most of the catch is taken in the eastern Atlantic, with about 20 to 30 000 tonnes coming from the western Atlantic.
Recent analysis for the yellowfin population completed by scientists of ICCATs Standing Committee on Research and Statistics (SCRS), suggests that the stock is capable of supporting yields of about 150 000 tonnes on a sustained basis (ICCAT, 1999). Since catches had been near that level in recent years, it was concluded that the population was fully exploited, and that any increase in fishing mortality would lead to overfishing and reduced catches. The scientists also cautioned that if fishing effort was being underestimated because of changes in efficiency, then the stock was probably being over-exploited. They also noted that if catches of small fish increased, then the potential yield would probably decrease due to a reduction in the yield per recruit. In 1973, ICCAT instituted controls on the catch of yellowfin of less than 3.2 kg, but these have been ineffective in keeping catches of these small fish down; in fact, they have been increasing. However, action was taken by ICCAT to close certain areas of the Atlantic to fishing on floating objects for the period November to January, in an effort to protect small fish. Further recommendations have been made to set limits on the catch of yellowfin.
Bigeye tuna are very similar to yellowfin in appearance, and fishermen and processors often confuse the two species. Bigeye are distributed throughout most of the worlds oceans, but they occur mostly in waters below the thermocline. Among their unique adaptations to life at greater depth is a layer of subcutaneous fat, which insulates them from the cold. This fat makes them very valuable in the sashimi market, and has made them the target of subsurface longline fisheries. In the mid 1970s, with the introduction of deep longlines, world catches of bigeye began to increase, reaching about 250 000 tonnes by the mid 1980s (Figure 2), and remained at about that level until the early 1990s, when purse-seine vessels began to utilize FADs for capturing small bigeye for canning. By 2000 the overall catch of bigeye reached about 475 000 tonnes, with much of this increased catch being attributable to the use of FADs.
In the Pacific Ocean annual catches of bigeye have fluctuated between about 100 and 165 000 tonnes prior to 1999. During 1999 and 2000 catches were 173 and 208 000 tonnes respectively (Figure 3); about 50% of this is taken in the eastern Pacific (east of 150°W). With the exception of an increase during 1999, there has been no observable trend in bigeye production in either the eastern or western Pacific, but the size composition of the catch has changed greatly in recent years.
As mentioned above, bigeye tuna are creatures of the deep: they spend most of their time in waters below the thermocline, where they are vulnerable to deep-fishing longline gear. Until recently this form of fishing was the principal method of capturing bigeye. However, during the late 1980s new methods were developed for capturing bigeye with purse-seine nets, which involve using FADs, sophisticated sonar, and deeper nets: the fish are attracted to the FADs, identified at depth by the sonar, encircled with the nets, and captured. The bigeye caught with this method are generally small, averaging about 8 kg, whereas the average for the longline fishery is about 55 to 60 kg. Surprisingly, in late 1999 and during 2000 the purse-seine fishery in the EPO captured mostly large bigeye, averaging about 19 kg. This unusual situation was most likely attributable to a series of large recruitments followed by poor recruitment during the last couple of years (Watters and Maunder, 2001).
With this new method, annual purse-seine catches of bigeye in the eastern Pacific have increased from about 2 000 tonnes in the late 1980s to a high of about 70 000 tonnes in 2000. These increasing surface catches of bigeye contributed to the decline in longline catches (heavy exploitation by longline gear may have also contributed to this decline), which went from an average of about 90 000 tonnes during the 1980s to less than 35 000 tonnes in recent years. Moreover, the total catch data tell only part of the story: since the market value of large longline-caught bigeye is far greater than that of the small bigeye caught with purse seines, the economic effect is enormous. Studies indicate that if purse-seine catches continue at current levels, longline catches will decrease even further (IATTC, 2001). These studies also suggest that, depending on the natural mortality rate of bigeye, total production from the two methods of fishing could very well decline after an initial increase. The same patterns of fishing seem to be prevailing in the western Pacific as well.
Studies based on longline data only, indicate that bigeye in the Pacific Ocean are capable of supporting catches of between about 115 to 150 000 tonnes annually. Because longline catches have been near, and in some cases above, this level in recent years, concern has been expressed that future increases in fishing effort on bigeye would result in overexploitation of the species. Adding to this concern is the increase in the catches of bigeye by purse-seine vessels. This concern over bigeye led the IATTC to adopt conservation measures designed to restrict fishing on floating objects of all types, including FADs during part of the fishing year. These measures are designed to limit the catch of small bigeye, because almost all the surface catch of that species is caught on floating objects, but it will also affect the catch of skipjack and small yellowfin. Such measures were first implemented in 1999.
Although the biological relationship between bigeye taken in the eastern and western Pacific is not known, it seems clear that in both regions there is a need to view developments in the fisheries with caution. In both areas longline catches appear to be declining, and will probably continue to do so as long as surface catches continue at current levels. There is some evidence that the combined catches of longline and purse-seine vessels in the Pacific Ocean may not be sustainable.
Prior to 1985, longline vessels were responsible for nearly all the catch of bigeye tuna in the Indian Ocean. Longline catches increased each year until 1985, when surface vessels began to catch more bigeye. After 1985 longline catches levelled off at between about 40 and 60 000 tonnes per year, while surface catches increased, reaching more than 30 000 tonnes by 1997. The status of the bigeye stock in the Indian Ocean is unclear. The relationship among bigeye from different parts of the Indian Ocean is unknown, so for the purposes of stock analysis scientists have assumed that there is a single stock. The most current stock analysis for bigeye in the Indian Ocean has concentrated on fitting the production model to historical catch and effort data (Anganuzzi, Stobberup and Webb, 1996). The conclusions vary, depending on the form of the model used and the data series applied, but the average maximum sustainable yield (AMSY) was estimated by one model to be between about 32 and 45 000 tonnes and by another to be between about 52 and 60 000 tonnes. Since catches have been well over 50 000 tonnes for many years, and during the last three years have averaged about 100 000 tonnes (Figure 4), the lower estimates seem unrealistic. If the higher estimates are correct, then the fishery is currently harvesting bigeye in excess of the AMSY; however, because of the increasing use of FADs in the surface fishery, and the consequent increase in catches of small bigeye, resulting in a shifting vector of age-specific fishing mortality, these estimates are also probably unreliable. Given the similarity of the situation in the Indian Ocean to that in the Pacific, and the results of the stock assessment studies for the latter area, any increases in the surface catch of small bigeye in the Indian Ocean should be viewed with caution, as they will almost certainly reduce longline catches, and could result in a decrease in the total catch. As is the case in other oceans, consideration is being given to limiting the use of FADs in the Indian Ocean by setting season and area closures.
Although information is limited, the stock of bigeye in the Atlantic is considered to be a single intermingling unit. Prior to 1970, most bigeye tuna taken commercially in the Atlantic Ocean were caught by longline or pole-and-line vessels, but since then the use of purse seines has increased, and by 1993 nearly 30% of the catch of bigeye was taken with this gear. Total catches rose steadily from 1950 to 1985, when they peaked at about 75 000 tonnes, remaining there until 1990 (Figure 5). Since 1992 catches increased, averaging about 120 000 tonnes annually. The increases since 1990 have been due to greater longline fishing effort and increased use of FADs by purse-seine vessels, the latter resulting in increased catches of small fish.
Estimates made by SCRS of AMSY obtained from production models indicate that under optimum conditions the population of bigeye in the Atlantic can sustain catches of between 80 and 95 000 tonnes per year. These analyses indicate that the bigeye stock in the Atlantic is over-exploited, and that at current levels of fishing effort catch levels will decline in the future (ICCAT, 1999). Age-structured models generally corroborate the results of the production models, and indicate that if the fishery on FADs continues to catch large quantities of small fish, the result will be growth overfishing and a decrease in catches. If catches of small fish could be reduced, the total catch would increase. In this regard, ICCAT instituted a minimum size limit of 3.2 kg for bigeye a number of years ago, but it has not been effective, since in recent years about 55% of the bigeye captured in the Atlantic have been below that size. According to SCRS scientists, strict enforcement of the minimum size limit would lead to a 35% increase in the catch, and they have called for a catch limit less than the estimated AMSY and for limitations on fishing for bigeye with FADs.
This concern over the heavy exploitation of small bigeye (and skipjack and yellowfin, as well) in the FAD fishery is not limited to scientists: French and Spanish vessel owners voluntarily imposed their own restrictions on the use of FADs in the eastern Atlantic. Since this initiative by the industry, the governments have also taken action to limit catches by restricting fishing with floating objects during a three-month period in 1999 and prohibiting the use of tender vessels, which maintain, repair, and replace the FADs, as necessary. Governments have also taken action to limit the entry of vessels greater than 24 meters in overall length into the fishery for bigeye.
Albacore has the distinction of being the tuna that lead to the development of the present-day world market for canned tuna. Early marketing slogans in the United States, where the first canning of albacore took place, emphasized its white flesh, comparing it to chicken (almost like chicken, Chicken of the Sea, and Breast of Chicken). Demand for the product grew rapidly, which led to the development of the canned light-meat market for yellowfin and skipjack. Because of the high demand for its white flesh, and the fact that supplies of raw material are limited, never exceeding 260 000 tonnes, canned albacore has always fetched a premium price. Albacore is a temperate species, concentrated mainly in the cooler temperate and subtropical waters of the worlds oceans, but undertake extensive migrations, seeking optimum conditions for feeding and reproduction. Surface fishing with hooks and lines in temperate and subtropical regions accounts for most of the catch of younger fish, while longline fisheries in more tropical waters capture the older fish. Purse-seining accounts for only a very small portion of the total albacore catch. Because of the wide distribution and highly-migratory characteristics of this species, levels of catch vary a great deal from year to year: annual catches have ranged from 170 and 255 000 tonnes over the last 25 years, with an average of about 200 000 tonnes (Figure 2). Catches show no trends, up or down, but both 1999 and 2000 showed increases catches in all oceans.
In the Pacific there is a northern stock of albacore that occurs between the equator and about 40°N, from Japan to North America, and a southern stock that is found between 15° and 40°S, from off Chile to around New Zealand. Total catches for these two stocks have fluctuated between 90 and 150 000 tonnes during the last 20 years, with no visible upward or downward trend (Figure 3). On average, about 60% of the catch comes from the northern stock. Most of the albacore harvested commercially in the Pacific Ocean are captured by surface trolling gear and by longlines.
Scientific studies have indicated that the northern stock was possibly overexploited during the mid 1980s but, due to natural fluctuations in abundance, it is currently above the level of abundance necessary to sustain the AMSY (Sakagawa and Hsu, 2000). Based on past experience, it does not seem likely that there will be sustained increases in catch, but rather that environmental variability will play an important role in future production.
Firm conclusions regarding the status of the southern stock are difficult. On the one hand, studies based on tagging data and age-structured models suggest low exploitation rates, whereas production models suggest that the stock is fully exploited and incapable of sustaining increased catches. The former studies are considered more reliable, so it is probable that the stock is not overexploited.
In general, it appears that catches of albacore from the Pacific Ocean will continue to show a great deal of variability in the future.
Between 1970 and 1985 the annual average catch of albacore from the Indian Ocean was about 15 000 tonnes (Figure 4). With the introduction of large pelagic gillnets in 1985 catches increased to nearly 30 000 tonnes, where they remained until this form of fishing was banned on the high seas in the early 1990s. Catches subsequently declined to as low as 17 000 tonnes; they increased in 1999 to about 40 000 tonnes. Scientific studies of the effect of fishing on the albacore of this region have been very limited, and it is uncertain whether the stock is fully exploited at recent levels of fishing effort, or whether increasing effort will result in sustained increased catches.
Total catches of albacore from the Atlantic show no trends, varying between about 60 and 80 000 tonnes per year over the last three decades (Figure 5). The information available is limited, but the population is considered to consist of three independent stocks, one in the North Atlantic, one in the South Atlantic, and one in the Mediterranean Sea.
In the North Atlantic various gears are used to exploit the stock, including longlines, pole-and-line gear, trolling gear, gillnets, and paired trawls. Most of the longline catch is taken in the central-western north Atlantic, while much of the surface catch occurs around the Bay of Biscay. Overall catches have generally been declining since the early 1950s, when they were about 65 000 tonnes per year, due mostly to decreases in longline and trolling effort, although pole-and-line and gillnet effort has been increasing. Recent catches from this northern stock have varied between about 30 and 40 000 tonnes per year. Although reliable estimates of potential sustainable yields are not available, it is generally considered that the northern stock is probably fully exploited, and that increased fishing effort would not result in sustained increased catches. Concern has also been expressed over the observed declines in the biomass of spawning fish, thought to be at about 16 to 20% of its pre-exploitation level. These concerns have led to the conclusion by scientists working cooperatively under the auspices of ICCAT that the fishing mortality of the northern stock needs to be limited to current levels (ICCAT, 1999). Based on this scientific advice, the member governments of ICCAT have agreed to limit the number of vessels fishing northern albacore to 1993/1995 levels.
The southern stock of albacore is harvested mostly off West Africa by longline and pole-and-line vessels. The stock was first exploited on a commercial scale during the early 1950s; catches had risen to about 30 000 tonnes per year by 1985, and generally remained somewhat above that level until 1994. Since 1994 the catches have averaged nearly 30 000 tonnes per year. The AMSY for the stock, estimated with production models, is about 30 000 tonnes, but for nine of the past twelve years the catch has exceeded this level. The current biomass is thought to be above that which would produce the AMSY, and fishing effort below the level needed to harvest the AMSY. In short, the population does not appear to be overfished, but, because of the great uncertainty in the estimates of fishing mortality and biomass, fishing effort should not be increased (ICCAT, 1999). The governments of ICCAT agreed to set a catch quota for 1999 equivalent to the current replacement yield of 28 200 tonnes.
No conclusive assessments have been made for the Mediterranean stock.
There are two species of bluefin tuna, southern bluefin, found throughout the temperate waters of the southern hemisphere, and northern bluefin, found in the north Pacific and the north Atlantic (Some taxonomists consider that the northern bluefin of the Atlantic and the Pacific are separate species). They are a slow-growing and long-lived species, with some individuals reaching more than 25 years of age. In terms of tonnage landed, bluefin is the least important of the principal market species of tuna; however, these low tonnages belie the commercial importance of the species. Because of their large size, and the colour, texture, and high fat content of their flesh, they are the most sought-after species for sashimi, and command a higher price than any other species of tuna. Southern bluefin spawn in the eastern Indian Ocean, and as they grow they migrate through Australian coastal waters to the high seas, where they are found in the southern parts of all three oceans. In the Pacific Ocean northern bluefin spawn in restricted areas off Formosa and southern Japan, and in the Sea of Japan; some of them migrate across the Pacific to off North America, and then return to the spawning grounds in the west as they approach sexual maturity. A few individuals make southerly migrations to areas below the equator in the western Pacific. In the Atlantic northern bluefin occur in most waters north of the equator and in the Caribbean and Mediterranean Seas. Spawning occurs in the Mediterranean Sea and the Gulf of Mexico. World catches of the two species combined have declined from over 100 000 tonnes during the 1960s to less than 65 000 tonnes in recent years.
Pacific Northern Bluefin
In the northwestern Pacific, around Japan, northern bluefin are taken throughout much of the year by a variety of gears, including purse seines, trolling gear, longlines, fixed traps, and pole-and-line gear. In the eastern Pacific purse-seine vessels take almost all of the catch, mostly in nearshore waters off northern Baja California, with some lesser catches off southern California. During the 1960s catches averaged about 25 000 tonnes per year, about 40% from the eastern Pacific and the rest from around Japan; during the 1970s they averaged about 20 000 tonnes, but varied a great deal from year to year (Figure 6). During the 1980s effort directed at bluefin declined, resulting in a reduction in catches: during that decade annual catches averaged about 14 000 tonnes. The portion of bluefin that migrate to the eastern Pacific is highly variable, and reduction in that migration may also have had something to do with the reduced catches of bluefin in the eastern Pacific. If fishing in the eastern Pacific is resumed at pre-1980 levels catches could be increased, perhaps to former levels. However, much of the current catch consists of small fish, and if these could be protected until they reached a larger size, total production of bluefin from the Pacific could increase.
Atlantic Northern Bluefin
Bluefin tuna are distributed widely throughout the Atlantic Ocean. Historically they were taken in the western Atlantic as far north as Nova Scotia and as far south as southern Brazil. In the eastern Atlantic they were taken off Norway in the north and as far south as North Africa and throughout the Mediterranean Sea. For management purposes, the population has been divided into an eastern and western stock, with the stock boundary approximately equidistant from the two continents. There is some mixing between the two stocks, however, and some scientists think that the bluefin of the Atlantic Ocean and Mediterranean Sea should be considered as a single stock for management purposes (National Research Council, 1994).
Figure 6. Trends in the world catch of bluefin tunas
Between 1950 and 1970 the total annual catches from the entire Atlantic ranged between 30 and 35 000 tonnes. Catches declined to about half that level during the early 1970s, and then increased to the 1994-1996 level of about 48 000 tonnes (Figure 6). By 2000 catches decreased to less than 35 000 tonnes. Judging from these trends, it would seem reasonable to assume that the fishery for bluefin in the Atlantic is healthy and capable of sustaining the current levels of catch. However, examination of more detailed information leads to the opposite conclusion.
For the western stock, catches were at their maximum (10 to 20 000 tonnes per year) during the early 1960s, after which, in the face of increasing fishing effort, they declined to around 3 to 7 000 tonnes per year. Because of these declining catches and a declining biomass, in 1982 ICCAT implemented catch limits, and annual quotas of between 2 and 3 000 tonnes have been in effect since. Current assessments suggest that catches of 2,500 tonnes would be sustainable for the western Atlantic, but at that level of exploitation the biomass of the stock, which is considered to be at a very low level, would not change. In order to ensure any increase at all in biomass the quotas would have to be set lower than current levels of catch, and to increase biomass to AMSY levels within 20 years they would have to be reduced to 500 tonnes per year.
For the eastern stock (which includes the Mediterranean Sea), catches fluctuated around 20 000 tonnes during the early 1960s, hovered around 10 000 tonnes until the mid-1970s, and then increased steadily until 1996, when they reached more than 46 000 tonnes. Since then they have declined. Until 1974 about 70% of the catch came from the eastern Atlantic, but then the catches in that area began to decline while those in the Mediterranean increased, and now comprise the major share of the catch. SCRS scientists have conducted extensive studies of the status of the bluefin stock in the eastern Atlantic, and they have estimated that current catch levels are not sustainable, but that a catch of about 25 000 tonnes per year would halt the decline of the biomass. Catch quotas of 32,000 and 29,500 tonnes were set for 1999 and 2000, respectively. Other conservation measures have been agreed to in the past, but they have not been effective. For example, a minimum size limit of 6.4 kg (with a 15% tolerance) was approved several years ago, but in recent years over 40% of the catch has consisted of fish smaller than this limit. There is grave concern over the status of the bluefin stocks in the Atlantic. Because the eastern stock is so much larger than the western stock, even with low rates of mixing the effects of overfishing in the east could adversely impact the success of the conservation programme in the west (Deriso and Bayliff, 1991 and ICCAT, 1999).
As mentioned above, southern bluefin spawn in the eastern Indian Ocean, and as they grow they migrate through Australian coastal waters to the high seas. Catches have declined considerably over the last decade, from nearly 50 000 to about 15 000 tonnes (Figure 6). The decline is due to overexploitation of younger fish, and possibly a decline in recruitment attributable to a reduced spawning stock; some scientists have suggested that recruitment is in danger of falling to critically low levels unless the spawning biomass is increased substantially while others believe that recruitment is independent of stock size for the range of stock sizes observed in the fishery (Deriso and Bayliff, 1991). Catch limits have been placed on the harvest of southern bluefin by the Commission for the Conservation of Southern Bluefin Tuna (CCSBFT). Australia, Japan, and New Zealand, the principal nations involved in the management of the fishery for southern bluefin, have had catch limits placed on the harvest of this species throughout its range, although there is some dispute as to the status of the stock and what those limits should be. The current annual catch quota is about 12 000 tonnes, but about 17 000 tonnes are actually being taken, the excess mostly by nations other than the three mentioned above. The quota was set to allow the population to recover, but there is some disagreement on whether the level is low enough to ensure an increase in abundance.