2. HIGHLY MIGRATORY SPECIES

2.1 Tuna and tuna-like species

2.1.1 The resources

The sub-order Scombroidei is usually referred to as comprising tuna and tuna-like species, and is composed of true tunas, billfish, swordfish and tuna-like species ranging throughout the tropical and temperate zones of the world (Maps 1 to 6). Many are extremely valuable commercially and for game-fishing.

The tuna (Thunnini) comprises 13 species⁵ and 4 genera: Thunnus, Euthynnus, Katsuwonus and Auxis (bullet and frigate tunas). The most commercially valuable tuna species belong to the genus Thunnus. The northern bluefin tuna (Thunnus thynnus) exist in the Pacific and Atlantic Oceans. The southern bluefin tuna (T. maccoyii) is found in the Atlantic, Indian and Pacific Oceans. The bigeye tuna (T. obesus), the yellowfin tuna (T. albacares), and the albacore (T. alalunga) are found all over the three oceans. The skipjack (Katsuwonus pelamis), a commercially important species of relatively lower value, is also distributed in the three oceans. The above species are usually referred to as the principal market tuna species. They can undertake long-range migrations across oceans (although skipjack and yellowfin tunas may do so to a lesser extent) and are considered as highly migratory in the 1982 Convention.

⁵ Most of them listed in the 1982 Convention Annex I as Highly Migratory Species

Map 1: Distribution of albacore tuna fisheries (from Bartoo, 1987. Reproduced by courtesy of US Nat. Mar. Fish. Serv.)

Map 2: Distribution of bigeye tuna fisheries (from Bartoo, 1987. Reproduced by courtesy of the US Nat. Mar. Fish. Serv.)

Map 3: Distribution of bluefin tuna fisheries (from Bartoo, 1987. Reproduced by courtesy of the US Nat. Mar. FIsh. Serv.)

Map 4: Distribution of skipjack tuna fisheries (from Bartoo, 1987. Reproduced by courtesy of the US Nat. Mar. Fish. Serv.)

Map 5: Distribution of yellowfin tuna fisheries (from Bartoo, 1987. Reproduced by courtesy of the US Nat. Mar. Fish. Serv.)

Map 6: Distribution of marlin fisheries (from Bartoo, 1987. Reproduced by courtesy of the US Nat. Mar. Fish. Serv.)

Other important “highly migratory” tuna species are of a more neritic character, living closer to shelves, around islands and archipelagos and not migrating across oceans. In this category are found less commercially valuable species of the Thunnini group such as the Euthynnus spp. (little tuna, kawakawa, black skipjack) or the blackfin tuna (T. atlanticus) which have, nonetheless, been included in the Annex I of the 1982 Convention.

The longtail tuna, Thunnus tonggol (from the Eastern Indian Ocean), has a similar behaviour but is one of the two Thunnini species (the other being Euthynnus lineatus) that has not been included in the list of highly migratory species.

The billfish (Istiophoridae) includes marlins (Makaira spp.), sailfish (Istiophorus spp.), and spearfish (Tetrapturus spp.). The swordfish (Xiphiidae) includes only one species (Xiphias gladius). With the exception of two spearfish, all billfish and swordfish species have very wide geographical distribution, but not all the species necessarily occur in all the oceans. All the billfish and swordfish species are excellent sea food and are also extremely valuable for game-fishing. They are all included in the 1982 Convention.

Other tuna-like species of importance to fisheries and not referred to in Annex I of the 1982 Convention include two species from the Southern Ocean, the slender tuna (Allothunnus fallai) and the butterfly kingfish (Gasterochisma melampus)⁶, the wahoo (Acanthocybium solandri), the bonitos (Cybiosarda, Orcynopsis, Sarda) and Spanish mackerel, king mackerels, seerfish, and sierra (Scomberomorus spp.). They represent a significant potential for many developing countries and are subject to artisanal and recreational fisheries. Most of these resources are shared by two or more countries, and stocks are likely to straddle beyond the 200-mile limit. Their status is usually unknown or poorly known and it is not clear whether they should be included in Annex I of the 1982 Convention or not.

In the absence of an agreed biological and legal definition of highly migratory species the situation appears confusing and the list of species referred to in the 1982 Convention seems both incomplete and sometimes arbitrary.

2.1.2 The fisheries

Most tuna and tuna-like species are commercially caught with various methods (pole-and-line, purse-seine and longline) all over the oceans (Maps 1 to 6). The first two methods are used for fish found close to the water surface (e.g., skipjack and relatively small yellowfin, albacore and northern and southern bluefin tuna), whereas the last method aims at fish found at lower depths (e.g., large northern and southern bluefin tuna, bigeye tuna, yellowfin, albacore, billfish and swordfish). The last two species are taken mostly as by-catch. Most purse-seine and pole-and-line catches are canned, while longline catches, with the exception of those of albacore, are mainly sold on the sashimi market⁷. Other gears used are troll lines, handlines, driftnets, traps and harpoons. Natural or artificial fish aggregating devices (FADs) are often used in conjunction with purse-seining or handlining. Information on fishing vessels, flags and techniques are given in Table 5.

⁶ Both species represent a significant potential and are now caught as by-catch in the Japanese southern bluefin fishery

Misc. = mainly flags of convenience; domestic tuna fleets (e.g., Hawaii, Fiji, Solomon Islands, Maldives, Senegal, etc.), are not included.

⁷ Market for high quality tuna to be consumed raw, essentially in Japan

In 1991, sixteen countries, plus the countries grouped in the “Other nei”⁸ category, caught 90% of all tunas listed in the 1982 Convention (Fig. 5). Eighty other countries reported catches, which in total comprised 10% of the catches. Of the top sixteen tuna-catching countries, only five report to FAO on the number and composition of their tuna fleets⁹. There are obvious discrepancies in data provided on catches. For example, Panama reports 17 700 t of catches for 444 vessels listed in Lloyd's Register (about 40 t per vessel per year) while Venezuela reports 82 800 t for 80 vessels in Lloyd's Register (about 1 000 t per vessel), implying an important misreporting from vessels flying flags of convenience.

Figure 5: Catches of tunas by major tuna fishing countries in 1991 and cumulative percentages (for tuna species listed in Annex I of the 1982 LOS Convention)

A summary on the fishery and state of stocks, by species and by ocean, is given in Table 6. The total annual catch of tuna and other tuna-like species has been steadily increasing from an average catch of about 1.9 million t in the early 1970s to 4.2 million t in 1990. The catch of the principal market tuna species increased from 1.4 to 2.8 million t with 66% taken in the Pacific, 16% in the Atlantic and the remainder in the Indian Ocean. Skipjack and yellowfin tuna make up to 45% and 35% of the catch, respectively. The Pacific yields 71% of the world skipjack catches, 66% of the yellowfin, 58% of the bigeye and 53% of the albacore.

⁸ From 1982 catches under this category refer exclusively to tuna caught in the Atlantic by vessels from Cayman Is., Ecuador, Malta, Mexico, Morocco, Norway, Panama, Vanuatu and Venezuela, combined to avoid breaches of confidentiality. Catches in the Pacific refer to combined tuna caught by Cayman Is., Costa Rica, Colombia, Cyprus, El Salvador, Honduras, Panama, Peru, Spain, Vanuatu and Former USSR (FAO, 1993b).

⁹ Ecuador, Ghana, Republic of Korea, Mexico, and Taiwan (Prov. of China, shown in Fig. 5 as “Other Asia”)

Exploitation symbols: ? - uncertain or unknown,
L = lightly exploited,
M - moderately exploited,
F - mainly to fully exploited,
O - overexploited,
D - depleted

The world fishery is highly dynamic. In the early 1980s, part of the Spanish and French purse-seine fleets shifted from the Atlantic to the Indian Ocean, significantly contributing to the increase in Indian Ocean catches which more than doubled between the early 1980s and 1989. Most US purse-seiners have stopped operating in the Eastern Pacific because of incidental catches of dolphins, and some have moved to the Western Pacific where the Republic of Korea and Taiwan (Province of China) have also intiated substantial purse-seine operations. About two-thirds of the principal market tuna species are taken by traditional tuna fishing countries¹⁰, but in many developing countries¹¹, the tuna industry is developing rapidly, including in the artisanal sector.

The markets for tuna are global with products moving freely among regions and nations. Landed values for most of the species are between US$ 1 000 and US$ 2 000/t, with the exception of bluefin (US$ 20 000/t)¹² and bigeye (US$ 8 000–12 000/t). The most important species, in terms of total revenues to the fleets are yellowfin, bigeye and skipjack (Fig. 6) each yielding more than US$1.8 thousand million in 1991.

The catch of tunas other than the principal market species also significantly increased from about 0.5 million t in the early 1970s to 1.5 million t in 1990. Only about 10% of this catch is composed of billfish and swordfish, taken mainly in the Pacific and Atlantic. Because most of the secondary species, other than billfish and swordfish, are mostly taken in developing countries by artisanal fisheries and sometimes discarded by industrial fisheries and frequently misidentified, their catch statistics are less reliable. The very rapid increase of the catches from 1970 to 1990 may therefore partly reflect an improvement in the statistics. The 1990 catch was composed mainly of Japanese Spanish mackerel (247 000 t) frigate and bullet tuna (215 000t), longtail tuna (141 000 t), narrow-barred Spanish mackerel (115 000 t), kawakawa (98 000t), swordfish (77 000 t), Eastern Pacific bonito (54 000 t), seerfish (46 000 t) and Indo-Pacific king mackerel (43 000 t). This gives a total of more than 400 000 t of fish.

2.1.3 Status of the stocks

Details regarding the status of the stocks are given in Table 6. Bearing in mind the significant uncertainties in their status, many stocks of the principal market tuna species appear to be heavily to fully exploited. Some stocks are overfished¹³ or depleted. Of the 20 stocks of principal market tuna species, 14 are in such condition while five are lightly to moderately exploited and the status of the remainder is uncertain. Trends in catches by species and oceans since 1970 are given in Figs. 7–9.

¹⁰ Japan, Taiwan (Province of China), Spain, Republic of Korea, USA and France

¹¹ Indonesia, Philippines, Mexico, Venezuela, Maldives, Ecuador, Panama, Ghana, Solomon Is.

¹² The landed value of bluefin tunas has increased sixfold since 1982 as a result of decreases in catches and current values place increased pressure on the stocks

¹³ Overfishing refers, in this paper, to recruitment overfishing (i.e., the annual recruitment rate has been affected by the depletion of the parental stock)

Total revenues are average landed value times quantities caught.

Figure 6 Total revenue of tuna fleets in US dollars and percentages by species (species listed in Annex I of the 1982 LOS Convention)

Albacore. The stock of albacore in the North Atlantic appears to be intensively exploited. In the South Atlantic, the exploitation exceeds the MSY level, but the extent of overexploitation is uncertain. In the Indian Ocean, the stock of albacore may be heavily exploited, but its assessment is uncertain. In the North Pacific, the albacore stock seems to be fully exploited, but in a stable condition. In the South Pacific, the deepwater strata of the resource (older age groups) is presently heavily fished. On the basis of data from that fishery, MSY was estimated at 31 000–37 000 t, but might be increased by surface fishing, though this could affect longline catch-rates. The combined driftnet, longline and troll catch of the late 1980s might not have been sustainable, but fishing effort has been reduced after the close of the large-scale pelagic driftnet fishery in 1991.

Bigeye. In the Atlantic, bigeye tuna are fished at or above the MSY level, which is estimated at 67 000–76 000 t. In the Indian Ocean, bigeye tuna are heavily fished, but their stock is probably in a healthy condition. In the Pacific, the stock of bigeye tuna is exploited at a level close to MSY (139 000–170 000 t for the longline fishery).

Bluefin. In the Eastern Atlantic and the Mediterranean Sea, reductions in fishing effort, especially that directed at small fish, could increase the yield in the long term. In the Western Atlantic, the stock is considered overfished and the biomass of adults is now only 10–23% of the 1970 level. In the North Pacific, the status of northern bluefin tuna is uncertain, but it appears to be overfished from the yield-per-recruit point of view, and the recruitment highly variable. It is probable that the yield-per-recruit could be increased by increasing the minimum age of capture to 2.5 years. The southern bluefin tuna in the Indian, Pacific and South Atlantic Oceans is overexploited; its parental biomass has been in a continuous decline, at least, until 1990. Most recent analyses have suggested a decline in recruitment from 1975 to 1983, but subsequent recruitment levels could not be determined.

Skipjack. In the Atlantic, skipjack are likely to be moderately exploited, but this is uncertain. In the Indian Ocean, the stock of skipjack looks to be in a healthy condition, but its potential is largely unknown. Catches of skipjack can probably be further increased both in the Eastern, Central and Western Pacific.

Yellowfin. The Eastern Atlantic stock of yellowfin tuna is exploited at about the MSY level. The 1990 catch of 147 000 t was above the estimated MSY of 132 000 t due to strong year-classes, but fishing effort seemed to be slightly below that associated with MSY. The status of the Western Atlantic stock is uncertain. In the Indian Ocean, yellowfin tuna do not appear to be threatened by overexploitation despite the tremendous increase in catch during the last ten years. However, more research is needed before their status can be precisely determined. In the Western and Central Pacific, most recent analyses suggest that a doubling of the present catch would be sustainable. In the Eastern Pacific, the stock declined between the 1970s and 1982 and then increased due to targeting at larger fish (improving the yield-per-recruit) and due to increased recruitment, which has stabilized since 1985. The stock now appears to be fully exploited or close to it.

Figure 6 Total revenue of tuna fleets in US dollars and percentages by species (species listed in Annex I of the 1982 LOS Convention)

Figure 8 Trends in tuna catches in the Pacific Ocean (species listed in Annex I of the 1982 LOS Convention)

Figure 9 Trends in tuna catches in the Indian Ocean (species listed in Annex I of the 1982 LOS Convention)

Billfish and swordfish. Blue and white marlin in the Atlantic could be overexploited. There is some concern about the status of sailfish in the Western Atlantic and an even greater concern about the status of swordfish in the Atlantic. Fishery statistics for billfish and Indian and Pacific Ocean swordfish are incomplete and even basic biological information is limited. Consequently, the knowledge of stock status for the stocks is generally poor. Swordfish in the Indian Ocean seem to be underexploited, and moderately so in the Pacific.

In summary, some tuna species are still in good state with relatively low to moderate exploitation rates such as for skipjack in all oceans, albacore in the Atlantic, yellowfin in the Western and Central Pacific. Other stocks are under more severe pressure such as yellowfin in the Eastern Pacific and Atlantic. Other stocks are exploited beyond their MSY, such as northern bluefin in the Eastern Atlantic and the Mediterranean Sea, albacore in the South Atlantic and, possibly, bigeye in the Atlantic. Finally, some tuna resources are considered depleted such as northern bluefin and southern bluefin in the Atlantic. The status of yellowfin in the Western Atlantic and northern bluefin in the Pacific is unknown and the status of the remaining stocks can be classified as heavily to over exploited. This assessment suggests that the longest-lived species of tunas, which require most careful protection, are either overexploited or depleted while the species with the shortest lifespan are still in relatively good condition.

Information on the stock status for coastal tuna-like species, for which little research has been generally conducted, is generally lacking.

2.1.4 Fisheries management

International arrangements

A full review and analysis of international tuna management bodies of the world has been prepared by FAO as a background to the creation of the Indian Ocean Tuna Commission (Shomura and Jayewardene, 1992). Some of the States whose nationals fish tunas cooperate in specialized international fishery commissions such as the Inter-American Tropical Tuna Commission (IATTC) in the Eastern Pacific, the International Commission for the Conservation of Atlantic Tunas (ICCAT), the FAO Indian Ocean Fisheries Commission, the FAO Indo-Pacific Fisheries Commission and the Southern Bluefin Trilateral Arrangement (between Australia, Japan and New Zealand). The South Pacific Commission (SPC) carries out scientific studies on tunas in the Commission's area of competence while the South Pacific Forum Fisheries Agency (SPFFA) is responsible for regulating access to tuna resources within the jurisdiction of its Member States. The Wellington Convention entered into force in 1991 in the South Pacific in reaction to the driftnet fishery for southern albacore and has led to negotiations for an arrangement for the management of the South Pacific albacore. There are also several ongoing activities to formally establish additional tuna management bodies in the world, including the Eastern Pacific Tuna Fishing Agreement (EPTA), the Eastern Pacific Ocean Tuna Organization (OAPO), the Western Indian Ocean Tuna Organization (WIOTO) and the FAO Indian Ocean Tuna Commission (IOTC).

IATTC, ICCAT and IOFC collect data and make cooperative assessments for the entire area of distribution of the stocks. This is, however, not the case in the Pacific where IATTC and SPPFFA/SPC cover only part of the relevant area. Full mandate over the entire distribution range is a necessary (albeit not sufficient) condition for proper management and the situation in the Pacific could be improved in this respect.

Management concerns

The prime concern is for the conservation of the depleted resources of bluefin. Concern for the bluefin has been expressed by some countries and it has been discussed within the context of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). It is the prime example of a highly migratory species with the most extensive migration, much of which is through coastal waters where it is highly vulnerable to a cumulative risk of fishing at a large number of discrete fishing locations, constituting a classic “gauntlet” fishery at all sizes of its life history for both juveniles and large 30-year-old adults. This situation, where risk of capture accrues throughout its life cycle, requires a special mechanism for management if the stocks are to be protected and enhanced. The recovery of such long-lived species will require time and a long-term management strategy with drastic measures to reduce exploitation rates and protect small sizes before their sexual maturation and substantial contribution to the reproduction of the stock.

At the FAO Technical Consultation on High Seas Fishing, Rome, 7–15 September 1992, IATTC reported that “there is a high degree of overlap among the extant tuna organizations and this will increase as more regional bodies are created”. Greater efficiency could be achieved if the regional bodies worked together more closely through for example, an “umbrella mechanism”. The rationale for a world tuna management mechanism is largely the result of the fact that most of the fishing vessels involved in tuna catching can target at practically any of the stocks shifting between regions and oceans. Nevertheless, it would appear that a world tuna body would perhaps face the same difficulties as single commissions if management regulations are ignored by States not contracting parties to the Convention.

Another serious potential problem in most tuna fisheries is the lack of tradition and mechanisms for direct effort controls, overall fleet sizes limitations and explicit resources allocation. Despite this, serious overfishing has been avoided, up till now¹⁴, in most many cases thanks to limitations imposed by a fairly saturated market as well as to wide possibilities of expansion on other species or areas. In the future, with increasing demands and rising prices, the lack of effort controls and allocation mechanisms may result in extension of overfishing and depletion of smaller and presently less valuable tunas.

In recent years, some coastal countries have started exerting control on the access of tuna long-range fleets in their EEZs (e.g., Mexico and, more recently, USA). Others are charging distant-water fishing nations for tuna fishing in their EEZs (e.g., South Pacific countries, Senegal, Morocco), fixing unilaterally the number of vessels to be authorized and the quantities to be caught. The implications are not trivial. On the one hand, this is taken by some scholars as an implicit recognition of a privileged right of the coastal State on the utilization of highly migratory species when they are in its EEZ (Munro, 1993). On the other hand, this reveals a management problem because, in the absence of an international agreement on total allowable catch (TAC), overall fleet size and State-by-State allocation of resources, the competition between coastal States trying to obtain as much as possible from an internationally common resource is likely to lead to unsustainable patterns of exploitation.

¹⁴ Except for bluefin stocks

The overall potential yield from the tuna and tuna-like resources depends on the combination of fishing techniques used, because different methods have different ability to target the various age groups. Improvements in the yield can be achieved in some cases (e.g., albacore and yellowfin in the Atlantic and northern and southern bluefin tuna) by protecting small or immature fish and targeting more precisely older age-groups. Compliance with the size regulations recommended by ICCAT, especially for northern bluefin tuna in the Mediterranean Sea, need to be improved. Protection of small sizes may be globally ineffective for stocks with high natural mortality like skipjack, or locally ineffective (or unacceptable) in areas with intense emigration. In such cases, bio-economic interactions among the various segments of the fishery must be studied to assess the overall and local impacts of management measures. Proposals for a worldwide tuna management mechanism have been made.

Dolphins caught incidentally by purse-seiners are one area of concern, primarily for yellowfin in the Eastern Pacific. Research, monitoring and management have been aiming at reducing, with great success, the incidental mortality of dolphins. Many canneries have stopped accepting tuna that are caught with dolphins. This embargo has had implications on the fishing operation in the Eastern Pacific, where the fishing intensity has decreased (some vessels moved to the Western Pacific). The need to protect dolphins will lead to avoidance and lower catches of large yellowfin which are usually aggregated with dolphins. This may result in concentration of effort on smaller individuals, characteristic of “dolphin-free” schools leading to sub-optimal tuna fishing.

2.2 Oceanic sharks

2.2.1 The resources

There are about 350 species of sharks and most of them venture far offshore or have some phase of their life-cycle inshore (Compagno, 1984). Maps 7 and 8 show the distribution of some major neritic and oceanic sharks. Neritic sharks are often found from coastal area to the outer shelf and on slopes, and may straddle “out” into the high seas when the shelf is sufficiently wide. Large oceanic sharks are essentially distributed in the high seas and may undertake long-range migrations of more than 1 000 km but they can be found quite close to shore and as such are likely to straddle “in” EEZs, particularly those of island countries.

Map 7: Distribution of neritic sharks (from Garcia and Majkowski, 1992. Reproduced by courtesy of the Law of the Sea Institute, Univ. Hawaii)

Map 8: Distribution of oceanic sharks (from Garcia and Majkowski, 1992. Reproduced by courtesy of the Law of thr Sea Institute, Univ. Hawaii)

Sharks are taken as a by-catch of many oceanic and inshore fisheries, but now they are being aimed at directly by longlines, hand-lines and sometimes bottom trawl fisheries. They are also targeted by sport fishermen. Shark fisheries have increased in number recently as the market for shark meat and shark products has expanded.

2.2.2 The fisheries

There are few well-documented shark fisheries. Major fishing nations are from Asia and Europe and include Taiwan (Province of China), Japan, Republic of Korea, France, Norway, Russia, Spain, and the UK, and consumer interest is increasing in the USA. In general, catch statistics reported to FAO do not properly reflect actual catches, and species breakdown of those reported is inadequate. Because of the possible extent of by-catch under reporting¹⁵ and discards, the time-series available are not reliable indications of total removals from the sea and might at best be considered as minimal values, perhaps indicative of trends. The recreational catch of sharks is also substantial, especially in some countries, and the lack of information on their magnitude contributes to the uncertainty of the level of worldwide shark catches.

Reported catches have been increasing steadily since the 1940s to the present. According to data reported to FAO (1993b) in 1991 around 700 000 t of cartilaginous fishes were landed, of which some 220 000 t were identified as skate and rays, 170 000 t as sharks and 310 000 t remained unidentified. Overall, landings of identified sharks have increased from 148 000 t in 1970–74 to 170 000 t in 1991. Unfortunately, statistics show that the greatest increases in landings correspond to the group reported to FAO as unidentified elasmobranchs. This exemplifies the difficulty to analyse stock-specific trends of elasmobranch fisheries on a worldwide basis. However, it is interesting to note that reported landings of pelagic sharks have been decreasing slightly over the last 20 years while, over the last ten years, reported landings of demersal sharks have slightly increased. This may be an indication of global overfishing of oceanic sharks and of an increased utilization of the by-catch of elasmobranchs from demersal fisheries. Considering the importance of unidentified catches, interpretation of these global trends remains, however, very speculative.

Dogfish sharks represent a major portion of shark landings. This group dominates shark landings in the North Atlantic and North Pacific and, although most catches come from the inshore shelf area, some dogfish sharks are caught as deep as 900 m. The piked dogfish is part of the multispecies demersal fishery off the Northwest Atlantic and may constitute a straddling stock there as well as off the coast of Canada. Landings of piked dogfish in the Northwest Atlantic (FAO Fishing Area 21) reported to FAO in 1990/91 were around 14 000 t. The worldwide landings of dogfish sharks amounted to about 47 000 t in the same period.

¹⁵ Especially when landed as shark fins. Exports of shark fins have increased 60% in volume and 180% in value in the last ten years. Imports of shark fins have similarly risen 70% in volume and 250% in value in the same period (Source: FAO FISHDAB, 1993).

Mackerel sharks are mostly oceanic species. The porbeagle (Lamna nasus), which belongs to this group, is caught in substantial quantities by line gear and gillnets in the northern and southern oceans. Catches from northern oceans are regularly reported to FAO but catches from southern oceans are probably much greater than reported. Several North Atlantic stocks of porbeagle seem to have collapsed in the 1960s because of intensive fishing. In fact reported catches of mackerel sharks have been recently decreasing worldwide from a peak of 44 000 t in 1984 to landings of 14500 t in 1991. The map giving the porbeagle distribution (Map 7) indicates that this shark represents a straddling resource all over the North Pacific and Atlantic shelf, as well as in Australia, New Zealand, Peru, Chile, Argentina and South Africa.

Salmon sharks (Lamna ditropis) are common in the North Pacific. They are taken as by-catch and often discarded in driftnet fisheries for squid and salmon as well as on Asian longlines. Interest in the species is increasing in Alaska. The stocks may have also been depleted since the mid-1950s (Compagno, 1988).

Mako sharks (Isurus paucus and I. oxyrinchus) are taken as by-catch of tropical pelagic longlines. The shortfin mako (I. oxyrinchus) is widely distributed from 40°S to 50°N and represents an important species for gillnet and longline fisheries for its high quality meat. It is also appreciated by sport fishermen because of its energy when caught. Catch figures are poor.

Thresher sharks (Alopias vulpinus, A. superciliosus and A. pelagicus) are another important group which includes both coastal and oceanic species. These are caught by Russian and Japanese longliners in the Central Pacific and Northern Indian Ocean. Other fisheries for thresher sharks occur off southern California and northeastern USA.

The blue shark (Prionace glauca) has an oceanic circumglobal distribution in temperate and tropical waters and is considered the most abundant oceanic shark in the Eastern Pacific and in the Atlantic where is a common by-catch of tuna longlines and driftnets (Litvinov, 1989). It migrates northward in summer and southward in winter and concentrates on seamounts. It is known to migrate regularly in a clockwise gyre across the North Atlantic. It is caught in the North Pacific as a by-catch in the giant flying squid fishery with driftnets in which it becomes entangled while preying on squids. It is also widely caught with hook-and-lines, pelagic trawls and bottom trawls. This species represented the main species in the 7 000 t of shark landed in Japan in the early 1970s (Gulland, 1971).

Oceanic whitetip (Carcharhinus longimanus) and silky sharks (C. falciformis) are common by-catch of longline, purse-seine and handline fisheries worldwide. Other species significantly present in the high seas and caught essentially by longlines for tuna or billfishes include the crocodile shark (Pseudocarcharias kamoharai) and the tiger shark (Galeocerdo cuvier), although the latter is closely associated with the continental shelf.

The huge (up to 18 m in length) and unmistakable whale shark (Rhincodon typus) is found in the intertropical area (from 30°N to 30°S). This highly migratory species, the world's largest fish, feeds on plankton and small pelagic species. It is the only shark which is never caught on longlines and is usually taken in nets or harpooned (e.g., in Senegal, Pakistan, India and Taiwan (Province of China). The basking shark (Cetorhinus maximus), another filter-feeder, is heavily fished in China and Japan; Compagno (1988) mentions that fisheries on this coastal-pelagic species have never been sustainable.

2.2.3 Status of stocks and management concerns

The potential and state of most stocks are unknown. Most of the oceanic sharks listed above are viviparous or ovoviviparous with uterine cannibalism (except for the genus Carcharhinus and the filter-feeders). They have a low fecundity, slow growth and long lifespan. As a consequence they have a low resilience to fishing and present a high risk of recruitment overfishing. Holden (1977), on the basis of available case studies, raised doubts as to the sustainability of shark fisheries. The examples of collapse of fisheries on the porbeagle in the North Atlantic, soupfin shark (Galeorhinus zyopterus) in California, the Scottish-Norwegian dogfish, the Australian school shark (NMFS, 1985) clearly illustrate that risk. Many experts believe that most shark stocks can only produce very low sustainable yields, often at rates lower than ten percent of their biomass. Unfortunately because sharks are often only a by-catch, it is very difficult to manage their stocks to achieve such levels of sustainable production. Sharks would require as careful management as small cetaceans but have certainly been given less attention by the conservationist groups and the media. This may not be entirely unrelated with the fact that the image of sharks with the public is not very positive.

Large pelagic sharks were, until recently, considered undesired by-catch, and at the most only their fins were kept. Thus, historical catch records were hardly ever kept. Recently, the trend towards landing whole sharks, has probably improved catch data collection; however, for most fisheries data are insufficient for stock assessment. The fragility to exploitation exhibited by shark stocks is not necessarily in contradiction to the growth in landings seen over the last fifty years, an increase which can be explained as a combination of:

• increased use of by-catch, and therefore increased reporting;

• expansion of fishing areas by long-range fleets;

• development of markets and target fisheries in developed countries;

• ecosystems shifts towards scavenger species (dogfish sharks, rays).

In fact, there are signs that many stocks have collapsed due to fishing or are been severely overharvested.

Gulland (1971) indicated that the world potential of sharks might not be more than a few hundred thousand tons, and that comparison of world catches with this very crude estimate of stock potential in order to obtain some idea of the present fishing pressure was not very useful for many reasons. First, statistics are inaccurate. Second, such comparison would not detect the likely local depletion of the various species and populations and, without a good time-series of catches, the ratio of catches to potential could not be interpreted (in particular, low ratios could indicate underfishing as well as depletion). Considering the importance of the shark in the oceanic ecosystem, the lack of interest of the scientific community and of regional fisheries bodies for these species should be a matter of concern.

2.3 Marine turtles

Although marine turtles were not considered in the 1982 Convention list of highly migratory species, their geographical distribution is very wide and their migrations extensive. Most catches are in the coastal areas, directly on beaches when spawning or in the coastal areas in gillnet fisheries. Large-scale pelagic driftnet fisheries are also a potential source of mortality.

The exact structure of the populations is not known with precision but many are considered endangered species. They are either gathered by hand when beaching or accidentally-caught in trammel nets, traps, trawls and even tuna and billfish longlines. Measures of abundance are problematic and population assessments practically non-existent. Catch statistics for turtles are probably very poor and egg catches are not reported. Reported catches which had reached about 5 500 t in 1970–74 and peaked at about 7400 t in 1975–80 decreased steadily since then to only 1255 t in 1991 (Table 1). The reasons for such a decrease is not known. In general, catches should not be considered as good indexes of abundance and the decrease could be the effect of market restrictions. However, because of the fragility of the animals and in the absence of a satisfactory alternative explanation, the information should probably be taken as an indication of dwindling resources. The FAO Catalogue Sea Turtles of the World (Marquez, 1990) shows that four species have a definite oceanic (high seas) character:

The loggerhead (Caretta caretta) probably undertakes long migrations following warm currents (Gulf Stream, Kuroshio, California Currents). It has been sighted in the open seas but little is known about its high seas distribution. Important nesting areas are known in all oceans. It is commonly exploited for its meat, leather, fat and eggs (illegally). Total reported catches decreased from 480 t in 1970–74 to 185 t in 1988, exclusively in the Central Atlantic. In recent years, 32 000 were killed off the US Atlantic coast and 10 500 in the Gulf of Mexico. Accidental kills by blasting of an old petroleum platform has been reported.

The green turtle (Chelonia mydas) can migrate from rookeries to feeding grounds, thousands of kilometres away. Some populations (e.g., from Ascension Islands) migrate 2 000 km across the ocean and have demonstrated homing behaviour. They are caught by a large number of gears and by hand, essentially for meat, and egg-poaching is pervasive. Reported catches have decreased from 400–500 t in the 1970s to 300–350 t in the late 1980s. The species is considered endangered and is listed in CITES. International commerce has been banned since the late 1970s but local “subsistence” fisheries persist in many areas around the world.

The hawksbill (Erethmochelys imbricata) is the most tropical of all marine turtles. Part of the population may not be migratory. Few data are available on potential transoceanic migrations although these are suspected among groups of islands in Oceania. The species is exploited for its “shell”, meat and eggs. It is, however, considered as endangered by IUCN. With the CITES restrictions, exports have decreased substantially since 1979. The total import of carey scutes (worked tortoise shell) in Japan were of 25 and 29 t in 1987 and 1988 respectively. Japan's worldwide trade in tortoise shell from 1970 to 1986 represents the equivalent of 600 000 adults (Ogren, 1989, p.108). Reported catches have oscillated between 240 and 370 t but are probably not accurate. Nesting aggregations are rare.

The leatherback (Dermochelys coriacea) is adapted to colder waters, has a very broad distribution, nesting in tropical and subtropical areas while extending its adult distribution from 40°S to 60°N. There is no evidence that the overall range of distribution has been affected. Upward trends in nest abundance have been observed in the Western Atlantic (Ogren, 1989, p.149) but the interpretation is not clear. Accidentally caught in driftnets and longlines, it is still considered an endangered species throughout its range and included in Annex 1 of CITES. The species has been hit particularly hard in Southeast Asia and its future looks bleak in the Western Atlantic (Ogren, 1989, p. 151–152). There are no identified catches reported for that species although it is certainly caught throughout its range.

Efforts are being made (e.g., in the Gulf of Mexico, in Indonesia) to reduce incidental mortality of turtles in shrimp trawl fisheries through the use of Turtle Excluder Devices allowing selective escapement of turtles accidentally entering trawls.

2.4 Pomfrets

The pomfrets reported in the FAO statistics refers to a heterogenous grouping of different families (Bramidae¹⁶, Formionidae and Stromateidae). Aggregated catches have increased from 37 000 t in 1970–74 to 115 000–140 000 t at the end of the 1980s. The Bramidae (Brama spp.) is an important family of oceanic fish on which very little information is available. Brama japonica, in the Pacific, has an antitropical distribution. It is an important by-catch in the driftnet fishery for the squid Ommastrephes bartrami and probably also in the salmon fishery but is often discarded. Beamish and McFarlane (1989) roughly estimated these discards at 300 000 t on the basis of a 1:1 ratio between squid and pomfrets in the nets. Brama brama, in the Atlantic, has also an antitropical distribution and is exploited around Spain and West Africa. Reported landings increased from 200 t in 1970–75 to 7 000 t in 1988. Sold fresh in Spain, it is said to be reduced to fish meal on industrial trawlers off West Africa. Brama dussumieri is tropical, may be the most “high seas” of all the Brama, and is taken as by-catch by deep-sea prawn trawlers. Other important species taken as by-catch of longlines include Taractichthys spp. (taken with Brama brama off Spain and on Japanese longlines in the Pacific) and Taractes spp. The potential and status of these stocks are totally unknown.

¹⁶ Only the Bramidae are included in the 1982 Convention Annex I

2.5 Sauries

The Pacific saury (Cololabis saira¹⁷) extends its distribution offshore even though most catches are taken close to the shelf where it spawns. There are three stocks in the North Pacific, one in the Northwest along the Japanese Pacific coast, one in the Japan Sea and one in the Central Pacific. According to Chikuni (1985) this last oceanic stock has never been commercially exploited. The northwest stock appears intensively exploited and catches have declined from about 500 000 t in the late 1950s to 235 000–350 000 t in the 1980s (Table 1). The presently exploited stocks seem to be fully fished and no further increase in catches is to be expected. Potential of the open ocean stocks is unknown.

2.6 Dolphinfish

The dolphinfish (Coryphaena hippurus and C. equiselis) are both included in the Annex I of the 1982 Convention. They are distributed widely in all oceanic waters, close to shore off islands and farther offshore off continental masses, beyond the area of influence of water runoff. They are caught by tuna troll lines and occasionally by purse-seines and driftnets. Fishing is developing with the widespread use of fish aggregating devices (FADs). Gulland (1971) indicated, on very limited data, that the world potential might be around 1 million tons. Present catches reported to FAO relate only to C. hippurus underlining most probably the inaccurate determination of the species composition of the landings. Total reported landings (Table 1) increased from about 19 000 t in 1970–74 to 23 000 t in 1975–80 and 30 000–37 000 t in 1988. Reported catches are essentially from the North Pacific (60 – 70%).

2.7 Potential highly migratory species

Section 2.1 on tuna and tuna-like species mentions already a number of species of importance to fisheries (Thunnus tonggol, Allothunnus fallai, Gasteroschisma melampus, Acanthocybium solandri, Scomberomorus spp.) which are not included in the 1982 Convention despite the fact that they share many characteristics with some of the species included.

¹⁷ It is included in the 1982 Convention Annex I together with Scomberesox spp.

The flying fish (Exocoetidae) is a potentially very important group, especially for island countries; it is not, however, included in the 1982 Convention, despite the fact that they seem to belong to the same oceanic species assemblage as the dolphinfish (Coryphaena spp.). The important genera are: Exocoetus, Cypselurus, Hirundichthys, Cheilopogon and Prognichthys. Some species are neritic but many are definitely oceanic. Their migration patterns and stock structure are not well understood. Total reported landings have varied between 60 000 t and 90 000 t in the last two decades with no clear trend. About 70% come from the Pacific, 20% from the Indian Ocean and 10% from the Atlantic. Mahon (1990) reports 4 000–5 000 tons for Eastern Caribbean Islands, essentially Hirundichthys. The potential is unknown. Suda (1973) indicates that the potential world catch could be around 100 000 tons in the coastal areas plus an unknown potential for the oceanic areas.

The sunfish or headfish of the family Molidae are strange representatives of the oceanic fauna of which very little is known. Only three species exist throughout the world. It is taken as by-catch in gillnet fisheries and consumed, at least in the Mediterranean. They will never be of importance to fisheries but are of interest for biodiversity conservation purposes. The snake mackerel (Gempylus serpens), escolar (Lepidocybium flavobrunneum) and oilfish (Ruvettus pretiosus) of the Gempylidae family, are species which are caught close inshore but migrate far offshore. They are all part of the regular by-catch of the tuna longliners together with the lancetfish (Alepisaurus ferox and A. brevirostris).