In general, the outlook for world production of skipjack and yellowfin tuna is mixed. For yellowfin, most of the fisheries, with the possible exception of the eastern Indian Ocean, are probably fully exploited. For skipjack, catches can on the average possibly be increased in the Pacific, but probably not much, if any, in the Atlantic and Indian Oceans
In the eastern Atlantic the catches by the surface fleets targeting yellowfin and skipjack have reached the upper sustainable limit of yellowfin and probably are near that limit for skipjack. This tendency became obvious in the early 1980s, and caused many purse-seine vessels from the Atlantic to transfer their operations to the western Indian Ocean (ICCAT, 1999).
Although the scientific assessments of the stocks in the western Indian Ocean are much less conclusive than those for the Atlantic, it appears that the surface catches of yellowfin and skipjack are at or near the maximum that the stocks can support. The potential for increased production of these two species in the eastern Indian Ocean is unknown (Anganuzzi, Stobberup, and Webb, 1996). Currently, the surface catch in that region is low compared to that of the western Indian Ocean, but it is not known how much, if any, catches can be increased.
In the eastern Pacific Ocean the stock of yellowfin tuna is fully exploited, but the skipjack stock could possibly sustain increased average catches (IATTC, 2001). However, increasing the catch of skipjack could lead to overfishing of yellowfin and bigeye, since small yellowfin and bigeye are often caught together with skipjack, especially on FADs.
In the western Pacific some increase in catch may be possible. This region supports the largest tuna fishery in the world, producing about 60% of the worlds skipjack and 35% of the worlds yellowfin. Analyses conducted by scientists of the SPC, based mostly on data from tagging experiments, suggest that the skipjack stocks of the region can support an increase in catch. However, for this increase to become reality the stocks of currently underexploited skipjack must be identified, there must be a demand for the raw material, and they must be vulnerable to fishing gear. Similar tagging studies of yellowfin tuna suggested that catches of that species could possibly be increased over current levels. However, the declining catch rates in the longline fishery and increased catches of small yellowfin in the purse-seine fishery on FADs suggest that any increases in fishing effort on yellowfin in the western tropical Pacific should be viewed with caution, and may not lead to sustained increases in catch. Increasing fishing effort may lead to increased catches of skipjack but, since in the western Pacific, as in the eastern Pacific, skipjack are often caught together with small yellowfin and bigeye, the problem lies in ensuring that the increase is in catches of skipjack only, and not those of yellowfin and bigeye (Hampton, Lewis and Williams, 2000).
Assessments and management of bigeye tuna present special problems. Due to the fact that they inhabit the deeper layers of the ocean, catches of bigeye have historically been taken mostly by longline vessels. With the expansion of purse-seine fishing throughout the worlds oceans, and the stabilization of production from 1990 to 1997, surface fleets have developed the use of FADs to capture skipjack, and previously unavailable bigeye. Whereas longline vessels capture large bigeye near the optimum size for maximizing the yield per recruit, purse-seine vessels generally catch small bigeye well below that size. In all the major tuna-fishing regions of the world purse-seine catches of bigeye are increasing, while longline catches are generally decreasing. However, it is uncertain whether total catch will increase. The natural mortality rate of bigeye is believed to be lower than that of yellowfin. (Natural mortality almost certainly varies with age, with younger fish having higher rates than older ones, but good estimates of age-specific mortality are not currently available.) If the natural mortality rate of bigeye is as low as 0.4, as it was for some time believed to be, then the current expansion of catch in the surface fisheries cannot be sustained, and overall catches of bigeye will decline. However, if the mortality rate of bigeye is similar to that of yellowfin, then total catches of bigeye can be increased. In either case, longline catches of bigeye will decline. From an economic point of view the effect would be enormous, since the value of the large bigeye caught by longliners and destined for the sashimi market is far greater than that of the smaller fish caught by purse seiners and destined for the canned fish market (Deriso, Bayliff and Webb, 1998).
Because of these uncertainties regarding the biology of bigeye, it is difficult to know with any degree of confidence whether catches of bigeye can be expected to rise or fall in the future. If purse-seine fishing effort for small bigeye is allowed to increase further, it may be possible to estimate the natural mortality rate more accurately and determine what the potential production might be. If increased effort results in increased sustained catches, then 0.4 is most likely an underestimate of natural mortality; however, if total catches decrease, this would confirm this lower mortality rate.
Of the six stocks of albacore harvested commercially, at least one is considered to be overexploited, three are considered to be fully exploited, and the status of the remaining two is uncertain. Judging from the lack of a trend in the world production of albacore over the last 25 years, and the high degree of interannual variability in the catch, it does not seem likely that increased levels of fishing effort would result in significant increases in catch. Continued fluctuations in catch, associated with a changing environment, will likely be the norm for the future.
Of all the principal market species of tuna, bluefin have suffered the most from the ravages of heavy exploitation, for two main reasons: their longevity, and their exceptionally high value in the sashimi market.
Most seriously overexploited is southern bluefin. Production is now about half of what it should be under a proper management regime. To return population abundance to former levels will require, at a minimum, a long-term commitment to keep catches at the currently low levels, or more likely at even lower levels. Judging from the difficulties scientists responsible for the assessment of the southern bluefin stock are having in agreeing on their assessments, it is possible that the current conservation programme could be weakened. If that were to occur, this already heavily depleted stock could be further affected (Deriso and Bayliff, 1991).
Even though total catches of northern bluefin from the Atlantic and Mediterranean are still at high levels, the stocks in both the east and west are considered to be overexploited. Without effective management in the eastern Atlantic and Mediterranean, many analysts consider that overall catches will decline.
Annual catches of northern bluefin in the Pacific have fluctuated between about 15 and 30 000 tonnes over the last several decades. During the last decade catches have declined, but this may be due in part to the decrease in fishing effort in the eastern Pacific. Studies show that if catches of small bluefin taken in the troll fishery of the northwestern Pacific could be reduced, overall production could be increased on a sustained basis as a result of increased yield per recruit.
Judging from current trends in these fisheries for bluefin, and the difficulties in implementing effective management measures, it is possible that world catches of bluefin will not increase, but rather decline.
Because of increasing demand, declining production, and the high price of bluefin, a number of attempts have been made to rear the species in a controlled environment. Bluefin ranching, which involves capturing wild bluefin, holding them in anchored pens, and growing and fattening them for the sashimi market, is being developed in various regions of the world, including Japan, where the methods were originally developed, Australia, Morocco, Spain, Croatia, and Mexico. This industry is expected to grow, but output in the foreseeable future will never match or replace production from the wild. Because of the lower value of skipjack and yellowfin, and the high potential cost of artificially rearing them, mariculture does not seem an economically viable alternative in the near future, particularly with respect to the canned fish market.
In view of all the above, it seems likely that the combined world production of yellowfin, bigeye, albacore, and bluefin will not change very much in the future from what it has been during the past few years. Annual production of those species has averaged slightly more than 1.8 million tonnes over the last several years, and will probably stay near that level in the future. Skipjack, however, has shown a significant increase in production, going from about 400 000 tonnes in 1970 to an average of slightly more than 1.9 million tonnes since 1998, although fluctuations in the catches of skipjack due to natural causes will continue in the future, as they have done in the past. Catches of all of these species could move in either direction in the future however, depending on a number of factors.
On the one hand, if more purse-seine vessels enter the fishery in the western Pacific, either through new construction or transfer from other areas, and if the scientists estimates of population abundance are correct, and if these vessels are able to realize the potential increases in catch that the skipjack stock in that region may be able to support, then world tuna catch might increase by as much as 10 to 15%. On the other hand, there is a real possibility that, unless effective management controls are implemented for presently fully-exploited tuna resources, overfishing of those species could occur and world catches decline. These two possibilities are not mutually exclusive. Newly-directed fishing effort in the Pacific could increase the catch of skipjack in that ocean, while overfishing could reduce the catches in other areas.
Complete and timely information on the catches of tunas is fundamental to the monitoring and assessment of the stocks. This information is also essential for many other purposes of a more political, economic, or management nature, e.g. compliance with regulations, evaluating supply and demand in the market place, and forecasting tax revenues. The history of data acquisition has been one of progressive improvement, but there is a great deal of room for more improvement. The most complete set of data on world tuna catches is compiled by FAO. The FAO data are from information that is provided to them by governments and international fisheries organizations. The data are not complete in many cases, because some governments and/or international organizations do not collect complete nor timely data, so FAO must make estimates of catches for which no data are available. Neither is the data timely; the FAO reports usually appear with a two-year time lag.
The question then arises: if the data needs improvement, how can that be accomplished? One possible way is to use more effectively the international bodies with responsibility for the management of tuna. Whereas only a few years ago there where many ocean regions where important tuna fisheries existed, but where there were no regional fisheries bodies, there are now regional tuna bodies covering all areas of tuna fishing except for the western Pacific (Although not Article 64 type tuna bodies, the FFA and SPC do collect data for parts of the western Pacific). The establishment of a regional tuna organization in that area is near completion. Each of these bodies has various degrees of responsibility for the collection and/or compilation of tuna statistics. The degree to which they do this varies. At one extreme, some of these organizations are mandated to directly collect, archive, and distribute such data, while at the other extreme, others are mandated to be a central repository where such information can be received from member governments. The detail and quality of the data varies, as do these specific responsibilities. Perhaps by giving them more specific responsibilities for the collection of data the organizations could work more closely with both the tuna fleets and the governments in giving them technical and financial assistance in creating national data collection centers.
Because the data and the means of collecting them are the same in nearly all tuna fishing areas, it would be efficient and beneficial to coordinate and standardize the data collection among the regional bodies. This coordination could be accomplished in two ways. One way would be to request the FAO to act as the coordinating agent. The other way would be for the bodies themselves to create a coordinating committee, comprised of representatives of each of the regional bodies. In addition to improving the quality of the data, such an approach could lead to the establishment of a system for estimating the world catch of tuna on a real time basis. Most major tuna processing companies, vessel owner organizations, and other industry organizations have information on the catches of the vessels while still at sea. These estimates are made on a daily or weekly basis. It is possible that the regional bodies could collect this sort of information. In fact, one regional tuna body, the Inter-American Tropical Tuna Commission (IATTC), has collected such data for many years and publishes a weekly report of the catches of tuna in the eastern Pacific Ocean. The report has a wide distribution and is well known. It is used for a variety of purposes by governments and industry, and by the IATTC to insure compliance with its conservation programmes for tuna. Because of the fundamental importance of reliable, timely, and accurate statistics of tuna catch on a global, as well as regional, basis, a high priority should be set on the establishment of ways and means for collecting such data. In March of 2000, the FAO in conjunction with regional tuna bodies held an Expert Consultation on Implications of the Precautionary Approach for Tuna Biological and Technological Research in Phuket, Thailand. In the report of that meeting (FAO, 2001) it was noted that current data collection programmes for tuna do not provide complete and accurate sets of data for determining the status of the stocks of tuna. A number of recommendations are made for improving data collection and the reader is referred to the report of the Expert Consultation for the details of the recommendations.