Research Centre, MAFF Fisheries
P.O. Box 287
Wellington, New Zealand
This paper relies heavily on the recent work of a number of scientists active in improving the information available for stock assessment. While most of the published information has been included in this review, only a portion of the more recent work presented in the South Pacific Albacore Research (SPAR) workshops could be included.
The classification scheme used follows Klawe (1980) in modified form:
Authority (Bonnaterre, 1788)
3. EARLY LIFE HISTORY
Albacore larval development and the morphological features distinguishing them from other tunas are described by Ueyanagi (1969). He notes that albacore larvae resemble yellowfin tuna when smaller than 9 mm but can be distinguished on the basis of melanophore patterns; a full complement of fin rays is developed when the larva is about 13 mm total length. Ueyanagi (1969) presents data suggesting albacore larvae are slightly more abundant in night tows and in tows taken at 20–50 m than at the surface. Murray (1986) summarised surface circulation in the South Pacific which indicated that larvae are likely to be transported southwards from the spawning grounds. The distribution of post-larvae and juveniles smaller than 29 cm length to caudal fork (LCF) is unknown but presumed to be in tropical oceanic waters.
4. DISTRIBUTION, MIGRATION, AND STOCK STRUCTURE
4.1 Distribution of Larvae, Juveniles and Adults
Nishikawa et al. (1985) summarise albacore larval distribution patterns in the South Pacific Ocean. They indicate larvae occur from northeastern Australia eastward through the French Polynesia EEZ (about 140°W) between 5°S and 25°S. Most larvae have been caught during the austral spring months of October to December (although larvae were also caught in small numbers in all other months except December, January, March, and April).
Catches of juvenile albacore by commercial fishers and research surveys suggest a zonal distribution from about 30° to 45°S across the South Pacific in surface waters where austral summer temperatures are generally 16° to 21°C. Research survey information suggests that juveniles are largely distributed in the upper 100–120 m and that fish tend to stay deeper during daylight hours.
Juvenile albacore usually range in size from about 45 to 85 cm LCF but fish as small as 34 cm are occasionally caught by trolling (Roberts and James, 1974). Fish smaller than 45 cm LCF represent only a small fraction of commercial catches, primarily along the New Zealand continental shelf north of 39°S. Roberts (1980) reports that juveniles are equally abundant in subtropical and Subtropical Convergence Zone (STCZ) waters in the area east of New Zealand but only rarely caught in surface Subantarctic Water. In high seas areas east of the area surveyed by Roberts (1980), juveniles have been caught primarily within the STCZ rather than in subtropical waters (Laurs, 1986, Laurs et al., 1987). There is no information on the distribution of juvenile albacore other than in austral summer months in the South Pacific.
Adult albacore occur throughout the South Pacific from the equator to at least 49°S (Wang, 1988; Bailey and Ross, 1987). Although caught in surface Subantarctic Water, albacore are most common in subtropical and tropical waters. Using commercial longline catch data, Wang (1988) portrays a seasonal pattern where adult albacore are concentrated in the area 15°–25°S in austral summer and 30°–40°S in austral winter months from Australia eastward to 100°W. Adult albacore are reported to be caught most often in depths of 170–220 m off Chile, 100–220 m off Easter Island (Ichikawa and Shirasawa, 1980), and 150–300 m in the area west of Fiji (Saito, 1973). In temperate waters off New Zealand adult albacore are caught shallower than 150 m.
The general migration pattern of albacore in the South Pacific is described by Jones (1991). He uses tag returns and seasonal patterns of fishing effort to explain patterns observed in his study of parasite fauna. These data support the hypothesis that juveniles move from the tropics into temperate waters at about 35 cm LCF and then generally eastward along the STCZ. Based on the loss of tropical endemic trematodes (didymozoids of several species) with increasing fish size and their absence in intermediate-sized fish, he infers that albacore do not return to the tropics until they mature at about 85 cm LCF. The presence of didymozoids in sexually mature albacore in temperate waters suggests that these fish return to temperate waters from spawning grounds in tropical waters.
The movements of juveniles comprising the troll catch are unknown other than in austral summer months. Didymozoid trematodes are not known to occur south of about 30°S and no evidence of reinfection of albacore is evident in fish 50–85 cm LCF (Jones, pers. commun.), suggesting that juveniles do not return to tropical waters north of 30°S. Research trolling surveys targeting albacore in austral winter months as far north as 26°S in the South Fiji Basin and in the Tasman Sea south of 30°S in the austral spring have failed to catch albacore. Longline vessels which fish deeper primarily catch juveniles in the austral autumn in temperate and subtropical waters. Based on these observations, albacore appear to move from temperate to subtropical waters during the austral autumn. As they move into subtropical waters they are presumed to move deeper where they are less vulnerable to trolling. Based on commercial trolling catches, juveniles move southwards to temperate waters in late austral spring or early summer months across the South Pacific. Adults occur from the tropics to temperate waters of the South Pacific throughout the year.
4.3 Stock Structure
Lewis (1990) reviews available information relating to likely stock boundaries and exchange between the South Pacific and other albacore stocks. The seemingly discrete spawning areas based on Nishikawa's et al. (1985) larval distribution for South Pacific, North Pacific, and Indian Oceans suggest distinct stocks. Further support for considering these to be separate stocks is found in the very low longline catch rates in equatorial waters (Lewis, 1990) and the age-dependent environmental requirements of albacore compiled by Pianet (in Anon., 1991). These indicate that the minimum temperature and dissolved oxygen requirements for albacore larvae and juveniles are not met in surface waters of the equatorial Pacific in any season although adult requirements are met. This suggests that an effective physiological barrier exists for larvae and juveniles which prohibits exchange between the North and South Pacific Oceans of all life history stages other than of adults.
Low longline catch rates of adult albacore in equatorial waters suggest that exchange of adults is limited between oceans. The lack of tag recoveries of adults tagged as juveniles in the North Pacific but recovered in the South Pacific Ocean (and vice versa) suggests that exchange between albacore stocks, while possible, is not appreciable. Exchange of adult albacore between the South Pacific and Indian Ocean stocks is also possible south of Australia but is believed to be limited.
These observations and the distribution of longline catch rates support the hypothesis that albacore in the South Pacific constitute a discrete stock. Jones (1991) offers two explanations for observed parasite faunal patterns, one of which suggests separate New Zealand and central STCZ sub-stocks of juvenile albacore. While some degree of separation may exist among juveniles, the recovery of several albacore in the New Zealand area, tagged as juveniles in the central STCZ, suggests a single South Pacific stock.
5. AGE AND GROWTH
Growth rate has been estimated from caudal vertebrae (Murray and Bailey, 1989), otolith increments (Wetherall, et al., 1989), and length frequency data (Hampton et al., 1990). Labelle (1991) compared these approaches by reexamining the data of Murray and Bailey's (1989) and Hampton et al., (1990) in light of additional tag recoveries. He suggests that ages determined from otolith daily increments are likely to be incorrect given the correspondence between growth rates estimated using caudal vertebrae and length frequency and their similarity to growth increments observed in tagged albacore. Relative growth rate based on tag returns, caudal vertebrae, and length frequency analysis suggests rates on the order of 0.5 cm per month.
6. MATURATION AND SPAWNING
Isii and Inoue (1956) report female albacore caught in the Coral Sea in January with ripe ovaries weighing 140–149 gm and containing 812,000 to 870,000 ova. They further report that egg diameters from anterior, central, and posterior sections varied from 0.397 to 0.431 to 0.445 mm, respectively. Egg diameters did not vary from the centre out to the edge of the ovary in any section. Saito (1973) shows that gonad weight increases with fish length in females but not in males in the area west of Fiji. He further indicates that females larger than 87 cm LCF were all mature and had gonads larger than 200 gm, while males with gonads larger than 150 gm were mature (corresponding to fish he sampled larger than 97 cm LCF).
Ratty et al. (1989) studied testes morphology and spermatogenesis in troll-caught albacore from the STCZ and found a marked asymmetry in male gonads with the right gonad larger than the left in 72% of fish examined. They further found that 7% of males 50–70 cm LCF were sexually mature and that a substantial portion of 70–80 cm LCF males in the STCZ were mature. However, females 55–95 cm LCF were not in an advanced state of maturity. Bailey (1991) has confirmed the gonad asymmetry for males and found that in longline-caught females from New Caledonia and Tonga the right gonad is also usually larger than the left. He further indicates that high gonad indices (>2.0) were found in males and females 85 cm LCF and larger.
Maturity has been assessed using gonad weight, with female gonads <140 gm reported to be immature (Saito, 1973). Ramon (1991) reports the highest female gonad weights in November and February. Although the gonad weights she reports are considerably lower than Saito's (1973) maturity criterion, throughout the November–February period she found larger egg diameters than those considered to be ripe by Isii and Inoue (1956).
Despite the differences among authors, female albacore appear to mature at about 85 cm LCF while males can mature at much smaller sizes. Females with late-developing ovaries capable of spawning have been found in the South Pacific north of 20°S from May through September (Otsu and Hansen, 1962). However, 90% of females had late-developing ovaries from November through February. Based on these data and the peak in egg diameter and gonad weight reported by Ramon (1991) spawning appears to take place primarily in the November–February period north of 20°S.
7. NATURAL MORTALITY
Natural mortality rates are unknown for the South Pacific albacore stock. However, Hampton (1990) suggests that a value of M of 0.3 to 0.4 per year may be appropriate given the likely longevity of albacore where few age 9+ fish appear in the longline catch. His suggestion assumes that growth rates for South Pacific albacore are similar to those of the Atlantic albacore stock and uses size composition data from the early years of longline fishery in the South Pacific.
8. OCEANOGRAPHIC FEATURES ASSOCIATED WITH THIS SPECIES
Albacore are with few exceptions reported to be caught in oceanic waters within a relatively narrow temperature range, which differs for larvae, juveniles, and adults. Larvae occur shallower than 60 m where temperatures are higher than 24°C and the thermocline is at about 250 m (Ueyanagi, 1969).
Laurs et al. (1977) have shown that juvenile albacore tend to accumulate near coastal upwelling fronts, dispersing when the fronts break up. They also show that albacore movements are related to sea surface temperature, with fish spending little time in waters cooler than 15°C. Roberts (1980) also reports that albacore in the South Pacific are seldom caught in STCZ water cooler than 15°C.
The sea surface temperature range where juveniles can be caught differs between areas, ranging from 18.5° to 21.3°C in New Zealand continental shelf waters (Roberts, 1974) 14° to 19.5°C in oceanic and slope waters of the eastern Tasman Sea off New Zealand (Roberts, 1975), and 15.5° to 20.6°C in the STCZ (Laurs, et al. (1987). Highest catch rates occur in a narrower sea surface temperature range within each of these reported ranges, typically 16.1° to 18.3°C (Roberts, 1974; Laurs et al., 1987). Depth of the thermocline also affects juvenile albacore catches, and few fish are caught by trolling when the thermocline is deeper than 95 m. Most albacore catches by trolling occur when the thermocline is shallower than 50 m (Murray and Bailey, 1986).
Adult albacore are primarily caught at depths of 100 to 380 m where in situ temperatures range from 9° to 20°C (Saito, 1973; Ichikawa and Shirasawa, 1980). Saito (1973) reports that in the area west of Fiji, albacore are most abundant at depths of 200–300 m where a high salinity, warm water mass intrudes into a low salinity, cold water mass. He suggests that albacore concentrate within a water mass boundary layer where food is particularly abundant.
9. INTERACTIONS WITH OTHER SPECIES
9.1 Food and Feeding
Albacore in the South Pacific are opportunistic carnivores which feed on a wide variety of small fish, planktonic crustaceans, and squid. Stomach contents of juvenile albacore caught incidental to the New Zealand purse-seine fishery and by trolling in the STCZ east of New Zealand have been analyzed by Bailey and Habib (1982) and Bailey (1983, 1986).
Bailey and Habib (1982) present data suggesting that diet changes with albacore size. In juveniles smaller than 50 cm LCF, diet is almost exclusively planktonic crustaceans. For juveniles 50–75 cm LCF, diet is a mixture of crustacea, squid, and small fish. Albacore 76–95 cm LCF feed primarily on small fish and squid. Bailey (1983) demonstrates that diet can differ substantially in different regions and that diet appears to be more varied in continental slope compared to continental shelf waters. He further demonstrates a relationship between time of day and fullness of albacore stomachs, suggesting that juvenile albacore feed primarily during morning hours (0900–1200 hours) and early evening (1800–2100 hours). In oceanic areas east of New Zealand, Bailey (1986) reports that juvenile albacore caught by trolling feed on planktonic crustacea, squid, and small fish, with small fish predominating throughout the STCZ area. He further reports differences in the fish prey near New Zealand, where myctophids and saury predominate, in contrast to the central STCZ, where Peruvian jack mackerel predominate in the diet.
Examination of stomach contents of adult albacore caught by longline indicates that they also feed primarily on crustacea, squid, and fish (Saito, 1973), with significant differences in stomach fullness occurring in fish caught at different depths. He notes that albacore in the area west of Fiji caught at depths of 80–200 m had less in their stomachs on average than those caught at greater depths (200–380 m). He also noted differences in diet between the shallower and deeper layers, with crustacea predominating in the diet at deeper layers.
Many oceanic pelagic species are opportunistic carnivores with a wide dietary spectrum. Species caught incidentally in surface and longline fisheries targeting albacore (see Murray, 1990 and Sharples et al., 1991) demonstrate an ability to recognise and feed on similar prey items. By virtue of their co-occurrence and similar response to a prey item, in this case a particular bait, they are likely to be competitors of albacore. The species which probably compete with albacore vary with area and season and are primarily skipjack, yellowfin, and bigeye tunas, and pomfrets.
Argue et al. (1983) report juvenile albacore smaller than 12 cm from the stomachs of skipjack tuna and wahoo; other tunas, tuna-like species, and billfish are also likely to prey on albacore as small juveniles. Several apex predators caught incidentally on longlines set for albacore or observed during trolling are likely predators of albacore. They include mako and blue sharks, billfish, and dolphins. Bailey (1983) has confirmed that of three marlin species (blue, black, and striped marlin) whose stomach contents were analyzed, blue marlin occasionally feed on albacore. Also known to prey on albacore is the small pelagic “cookie cutter” shark (Hampton et al. 1991), which inflicts small nonfatal wounds. Hampton et al. (1991) also report substantial wounds from blue sharks.
Jones (1991) lists 24 parasites identified from troll-caught and longline-caught albacore in widely separate geographical areas. The parasite fauna includes 12 trematode species (11 of which are didymozoids), 4 cestode species, 1 acanthocephalan, 3 nematodes, 3 copepods, and 1 coccidian. The parasite fauna of South Pacific albacore is generally widespread throughout the area, with little evidence of regional speciation.
10. DESCRIPTIONS OF FISHERIES
Stable longline and surface troll fisheries for albacore have operated in the South Pacific for several decades. Since 1986, however, high seas surface fisheries including troll and large-mesh pelagic drift gillnet fisheries have expanded. The drift gillnet fishery in the South Pacific ended in June 1991.
10.1 Longline Fisheries
The longline catch of South Pacific albacore is summarised by vessel flag in Table 1. The total catch peaked in 1967 when over 40,000 mt were caught. Since 1967, catches have averaged 29,700 mt (range 20,900 mt to 38,900 mt).
South Pacific albacore fisheries began in 1952 with the expansion of Japan's distant water longline fleet into the South Pacific (Watanabe and Nishikawa, 1990). The Japanese catch rapidly increased to a peak of 34,620 mt by 1962 before declining to less than 7,000 mt in the late 1960s. The Japanese catch since the 1970s has averaged 3,800 mt (range 1,333 to 5,723 mt).
Following the success of the Japanese fleet, Korean longliners began targeting South Pacific albacore in 1958, followed by Taiwan in 1967. Korean catches increased from a few hundred tonnes in the early 1960s to over 10,000 mt in the mid-1960s. The catch by Korean longliners has been variable and except for a few years, when catches have been of the order of 5,000 to 10,000 mt, catches have been 10,000 to 15,000 mt. Catch data since 1987 suggest that catches by this fleet have declined to a few thousand tonnes.
The first reported Taiwanese catch of South Pacific albacore was in 1967 when 11,751 mt were caught. Since 1967, Taiwanese longline catches have consistently been higher (except in 1969 and 1985 when about 9,600 mt were caught). Longline catches, while variable, increased to over 20,000 mt in the late 1970s. Since the peak catch in 1980, catches have averaged 12,600 mt (range 9,600 to 17,100 mt).
Since 1983, South Pacific coastal states have begun longlining for adult albacore on the high seas and within their Exclusive Economic Zones (EEZs). Longline vessels from Tonga and New Caledonia target albacore, while albacore is a significant by-catch in a number of longline and handline fisheries throughout the South Pacific. The combined catch by South Pacific coastal states, while steadily increasing, is only about 2,000 mt.
The combined catch by all fleets targeting adult albacore has been at or below the estimated maximum sustainable yield (MSY) of 31,000 to 33,000 t (Wang et al., 1988) in all years since 1981 except one (1986). This MSY estimate is based on surplus production models using target longline catches and adult size composition, and assumes a small stable surface fishery catch of about 2,000 mt.
Table 1. Longline catches (mt) of South Pacific albacore by vessel flag. South Pacific coastal states includes the catches by Australia, Fiji, French Plynesia, New Caledonia, New Zealand, and Tonga. The symbol “+” denotes small but unknown catches. Data from Anon. (1993).
|Year||Japan||Korea||Taiwan||Pacific Coastal States||Total|
10.2 Surface Fisheries
Trolling for albacore first developed in the South Pacific in the New Zealand EEZ in the late 1960s. The New Zealand troll fishery operates chiefly in a band 40–80 miles offshore of the west coast of the South Island from January through March. Season length varies from 6 weeks to 6 months depending on weather conditions. The fishery also experiences variable landings due to weather conditions and probably due to climatic factors affecting the southern limit of the distribution of juveniles. In good seasons (warm calm summers) over 200 small vessels enter this fishery and can land 2,000 mt to over 4,000 mt of juvenile albacore. In bad seasons (cold windy summers) fewer vessels fish and landings can be less than 1,000 mt. In the 1990/91 season 142 vessels landed 1,626 mt in New Zealand.
The troll fishery has expanded since 1986 following exploratory fishing along the STCZ east of New Zealand. The STCZ albacore troll fishery has developed from 89 mt and two vessels in 1985/86 to over 4,000 mt and more than 50 vessels since 1988/89. The STCZ troll fishery operates entirely in high seas areas primarily from 39°to 41°S, between 165° and 140°W. Some exploratory fishing in high seas areas of the Tasman Sea was done during the 1989/90 season with limited success. Vessels from the United States, Canada, New Zealand, French Polynesia, and Fiji participate in this fishery. In the 1990/91 season one Japan Marine Fishery Resource Research Center vessel also fished by trolling but reportedly was unsuccessful. Effort in this fishery is not expected to increase much beyond the 1990/91 level.
While the STCZ troll fishery was developing, a large-scale pelagic drift gillnet fishery for juvenile albacore was also developing in high seas areas east and west of New Zealand. This fishery, started by Japan in 1982/83 in high seas areas of the Tasman Sea, expanded to include vessels from Taiwan and Korea fishing most of the high seas temperate waters of the South Pacific. The rapid expansion of the drift gillnet fishery from 10 vessels in 1986/87 to at least 130 in 1988/89 and the magnitude of drift gillnet catches caused considerable concern over the sustainability of continued harvests of juveniles at 1988/89 levels. Concern over the potential for interactions between drift gillnet and troll fleets also arose since both fisheries caught similar sizes of fish in the same areas and in the same months.
The rapid development of the drift gillnet fishery without adequate information on the population dynamics of South Pacific albacore and without collecting catch and effort statistics from the fleets has made impact assessment extremely difficult. However, reductions in drift gillnet fleet size since 1988/89 and the cessation of drift gillnet fishing in the South Pacific since July 1991 has reduced the threat posed by the 1988/89 catch levels (Anon., 1991). Table 2 summarizes surface catches of albacore in the South pacific by country and fishing method by calendar year.
10.3 Geographical and Seasonal Distribution of Fisheries
As mentioned previously, surface fisheries are restricted to austral summer months, primarily December into April. The geographical extent of areas where surface fishing is effective is also limited to mid-temperate latitudes where austral summer sea surface temperatures tend to be 16° to 21°C. In the South Pacific high seas fishery (east of New Zealand and in the Tasman Sea) most fishing takes place between 39° and 41°S. Along the shelf edge of New Zealand, especially in the Tasman Sea, commercial concentrations are more broadly distributed from about 37° to 45°S.
Table 2. Surface fishery catches (mt) of South Pacific albacore since 1967 by area (Australia= western Tasman Sea, NZ = eastern Tasman Sea, STCZ = high seas areas east of New Zealand) and gear. The STCZ includes troll catches by vessels from Canada, Fiji, French Polynesia, New Zealand, and the USA. Drift gillnet catches include catches from the Tasman Sea and the STCZ areas combined. Data from Anon. (1993) except for total drift gillnet catches in 1988/89, which include industry sources.
|Fishing Year||Japan P/L||Total Driftnet||Australia||NZ Troll||STCZ Troll||Total|
1 In the absence of accurate Taiwanese statistics, catch is estimated from number of vessels reported active in the South Pacific (industry sources), season length, and average catch rates per day.
The longline fishery for albacore operates year round, moving from north to south seasonally. These patterns are described by Wang (1988) for the Taiwanese fleet and by Wetherall and Yong (1989) for the Korean fleet. The pattern of movement is assumed to be similar for the Japanese fleet although the fishing area extends further east (Polacheck, 1987). The general pattern is southwards during January to April to subtropical waters and then northwards from July to October. The Taiwanese fleet appears to concentrate fishing from 5° to 45°S while Korean vessels fish from north of the equator to 45°S. Most albacore longlining by Taiwanese and Korean fleets is west of 120°W, with little longline targeting for albacore in the Tasman Sea or in the area adjacent to New Zealand in recent years. Data presented by Wang (1988) and Wetherall and Yong (1989) suggests that the Tasman Sea and New Zealand EEZ were extensively fished prior to the early 1980s, while more recent fishing is predominantly in high seas areas.
Polacheck (1987) indicates that the Japanese longline fishing pattern for albacore differs slightly from those of Korea and Taiwan. Japanese longliners appear to fish a narrower band east of New Zealand from 10° to 35°S and as far south as 50°S in the Tasman Sea and around New Zealand. Japanese vessels also appear to fish further east across the South Pacific than the other fleets.
11. TRENDS IN CATCH, EFFORT, AND CPUE
Before the start of the drift gillnet fishery, surface fishery catches were less than 2,500 mt, most caught by the seasonal and weather-limited New Zealand near-shore troll fishery. Between 1983/84, the start of commercial-scale drift gillnet fishing by Japan, and 1985/86, when the STCZ troll fishery began, total surface fishery catches were less than 5300 t. The historically high total catch of 29,000 to 58,000 mt occurred in 1988/89 due to the rapid expansion of drift gillnet fishing, primarily by Taiwanese vessels. Subsequent reductions in drift gillnet fishing (following the consensus adoption of United Nations Resolution 44/225) resulted in the progressive decline in the total surface fishery catch from 29–58,000 mt in 1988/89 to 14,218 mt in 1989/90 and to 9,419 mt in 1990/91. Since no drift gillnet fishing has taken place since 1991, other surface fishing methods tried thus far have had only limited success, and catches by trolling are not expected to exceed about 10,000 mt; it seems unlikely that the surface fishery will return to 1988/89 levels.
Longline catches increased with expanding effort from 1952 to 1967 to reach its historical peak of 40,572 mt. Since 1967, total longline catches have ranged from about 21,000 to nearly 39,000 mt but have usually been less than 35,000 mt. Most longline catches are by distant water vessels from Taiwan, Korea, and Japan, with increasing but still small catches (about 2,000 mt) by vessels from South Pacific states.
Depending on weather and other factors, a variable number of New Zealand troll vessels, ranging from 25 to over 200, fish for albacore each summer. In most years over 100 vessels fish from January through March in this near-shore fishery. High-seas troll vessels from the USA, Canada, New Zealand, French Polynesia, and Fiji have been slowly increasing from 44 vessels in 1987/88 to about 70 in 1990/91. Further increases in vessel number are not expected. The other surface fishery, composed of drift gillnet vessels primarily from Taiwan and Japan, increased rapidly from 11 in 1986/87, to 28 in 1987/88, to at least 130 in 1988/89. Since 1989 these fleets have been reduced in equally dramatic fashion to 32 vessels in 1989/90 and 7–9 vessels in 1990/91. Based on agreement, all drift gillnet fishing interests promised not to use drift gillnets in the South Pacific after June 1991.
Longline fishing effort since 1970 has been variable and mostly in the range of 200–250 million hooks set per year. Korean and Taiwanese longline vessels are the major fishing interests in recent years, and declining catches by Korea and Japan probably reflect changing economics for tuna longline fisheries with less effort directed at albacore compared to yellowfin and bigeye tunas.
Variation in weather for the New Zealand troll fishery and changes in species targeting by Japanese and Korean longliners mean that the best data for indices of abundance are the STCZ troll and Taiwanese longline CPUE data. However, work is still ongoing to standardise these data and only nominal CPUE estimates are presently available.
Average CPUE for juvenile fish in the STCZ troll fishery has been high compared with other areas fished (New Zealand and the Tasman Sea), ranging from about 100 fish (1.2 mt) per vessel-day to 250 fish (2.6 mt) per vessel-day between 1986/87 and 1990/91. Coan and Rensink (1991) show that STCZ troll fishery CPUE, expressed as mt per day, has been relatively stable since 1988. Figure 1 shows STCZ troll fishery and Japanese drift gillnet CPUE, expressed as number of fish per vessel-day. The CPUE of the STCZ troll fishery was nearly constant from 1986 to 1990, while drift gillnet CPUE generally increased over the period.
Figure 2 indicates that Taiwanese longline CPUE is highest and the most variable south of 20°S. CPUE in the waters 0°–20°S declined slightly from 1967 to 1975 and has been approximately stable since 1975 at 2–3 fish per 100 hooks. The CPUE in the waters 20°–30°S, also declined to 1975 and has since fluctuated approximately between 2.5–5.5 fish per 100 hooks. Highest and most variable CPUE is found in the 30°–50°S band where most fishing is conducted immediately north of the STCZ surface fishery area. CPUE declined in this area from 1967 to 1976 but generally trends upwards after 1976 from 3 to about 9.5 fish per 100 hooks. Estimates from a different source suggest declines in CPUE in subtropical and temperate latitudes, but within the range observed since the mid-1970s.
12. POPULATION DYNAMICS
Since 1986, scientists from South Pacific states and distant water fishing interests have worked to increase the information available to model the population dynamics of South Pacific albacore. The absence of large surface fisheries and the inadequate size composition data and ancillary biological information on age and growth, etc. prior to 1988 resulted in reliance on relatively simple surplus production models (Skillman, 1975, Wetherall and Yong, 1984, and Wang et al., 1988). These authors present essentially the same conclusion: that the average annual yield to longliners could not be increased by increasing effort and that nearly the same yield would result from lower effort. The estimated MSY of the longline fishery operating in the absence of a surface fishery ranges from 31,000 to 33,000 mt (Wang et al., 1988) up to 37,000 mt (Wetherall and Yong, 1984).
Figure 1. CPUE trends in South Pacific albacore surface fisheries operating in the STCZ. Reprinted from Anon (1991).
Using available information on total catches and size composition by fishery and making assumptions about M, average recruitment, stock structure, and growth, Hampton (1990) constructed an age-structured simulation model of South Pacific albacore. Using a range of assumed parameter values in combination, he predicted the consequences of continuing the high 1988/89 combined exploitation levels by all fisheries operating at the time (drift gillnet, troll, and longline). He predicted that continued exploitation at the 1988/89 levels would result at best in parental stock declines over the next 5-year period to 61% of 1988/89 levels (equivalent to 32% of preexploitation levels). He concluded that unless South Pacific albacore behaved more like yellowfin tuna rather than albacore in other areas, the 1988/89 catch levels were very unlikely to be biologically sustainable. No other population dynamics modelling has been completed on the South Pacific albacore stock.
13. INTERACTIONS AMONG FISHERIES
Indications that interactions may occur between surface fisheries (drift gillnet-troll and between troll fisheries) and between surface and longline fisheries come from the spatial and temporal distribution of fleets and observations on drift gillnet damaged fish in different fisheries. Higher incidence of drift gillnet damaged fish in the STCZ troll fishery when drift gillnet effort was high and also when drift gillnet and troll vessels operated in close proximity suggests interactions between surface fisheries, at least during a season. The appearance of fresh drift gillnet damage in the New Zealand troll fishery when drift gillnet fishing was restricted to the western Tasman Sea near Australia also suggests that interactions can occur over distances of several hundred miles during a season. During 1988 and 1989, when drift gillnet fishing effort was reaching its maximum, STCZ troll fishery CPUE declined, as did drift gillnet CPUE in the year of peak effort. The CPUE in both fisheries increased in 1990 when drift gillnet effort was much reduced.
Figure 2. CPUE trends in South Pacific albacore longline fisheries stratified by latitude. Reprinted from Anon (1991).
The appearance of recent drift gillnet damage in longline-caught albacore of intermediate size from New Zealand waters indicates that some interaction also occurs between surface and longline fisheries without a significant time lag. The apparent declines in CPUE in the Taiwanese longline fishery in temperate waters and to a lesser extent in subtropical waters from 1986 to 1988 may also signal an interaction between surface and longline fisheries. Interaction effects over larger areas have not been indicated, and possible interactions between industrial albacore fisheries and small-scale commercial, artisanal, and recreational fisheries have yet to be examined because of a lack of data for these fisheries.
Given the current state of information on South Pacific albacore, it is not possible to better define the extent of interactions between fisheries. In particular the inadequate historical data from drift gillnet fleets, the spatial resolution of size composition data from high seas longline fleets, the lack of opportunity to place observers on high seas longline fleets, and apparent nonreporting of tags all limit inferences which can be drawn about interactions between South Pacific albacore fisheries. While it appears clear that there is potential for interaction between surface fisheries and between surface and longline fisheries, at present there is little evidence which suggests that any existing fishery is appreciably affecting catch rates in another.
14. REFERENCES CITED
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