A REVIEW OF THE BIOLOGY AND FISHERIES FOR NORTHERN BLUEFIN TUNA, THUNNUS THYNNUS, IN THE PACIFIC OCEAN

William H. Bayliff
Inter-American Tropical Tuna Commission
La Jolla, California, USA

1. INTRODUCTION

Parts of this report are similar to parts of a species synopsis prepared by Bayliff (1980). The material in Sections 6.2 and 7 is similar to that in sections entitled LIFE HISTORY AND STOCK STRUCTURE and ATTRITION in Bayliff et al. (1991). The material in Section 12.2 is similar to part of the section on stock assessment of northern bluefin in Bayliff (1993b), and that in Section 12.3 is similar to the section entitled Cohort analysis in Bayliff (1993a).

2. CLASSIFICATION

Phylum Chordata
    Class Osteichthyes
        Order Perciformes
            Family Scombridae
                Subfamily Scombrinae
                    Tribe Thunnini
                        Genus Thunnus
                            Species thynnus
                                Subspecies orientalis

The classification of northern bluefin and other tunas is discussed by Gibbs and Collette (1967), Sharp and Pirages (1978), Collette and Nauen (1983), and Collette et al. (1984).

3. EARLY LIFE HISTORY

The larvae of northern bluefin have been described by Matsumoto (1962), Yabe and Ueyanagi (1962), Matsumoto et al. (1972), and Richards and Pottoff (1973). The Kuroshio Current probably plays an important role in transporting the larvae, postlarvae, and juveniles northward from the spawning grounds between Japan and the Philippines and southeast of Japan to waters off Japan. Age-0 fish about 15 to 60 cm in length are caught in the vicinity of Japan during the second half of the year (Yabe et al., 1966; Yukinawa and Yabuta, 1967). (In this report fish in their first year of life are referred to as 0-year-olds, age-0 fish, or fish 0 years of age, and so on.)

4. AGE AND GROWTH

The maximum lengths and weights recorded for northern bluefin in the Pacific Ocean, from Foreman and Ishizuka (1990), are as follows:

The parameters of the von Bertalanffy growth equation for northern bluefin in the Pacific Ocean have been estimated by various workers from analyses of hard part, length-frequency, and tagging data. These estimates, and estimates of the average lengths at various ages, are shown in Table 1. The estimates of the average lengths at ages 1–6 are fairly close to one another, but the estimates of L_∞ and K of Bayliff (1986) are considerably greater and less, respectively, than those of the other workers.

Bayliff et al. (1991) studied the growth of northern bluefin from tagging experiments. They had 721 fish with data for the dates and lengths at both release and recapture. Because the data included many fish less than 30 cm in length at release and many fish at liberty less than 1 year, and relatively few larger fish or fish at liberty more than 1 year, the data were combined into 84 groups. The times at liberty and lengths at release selected were 1–30, 31–60, 61–90, … days and 151–200, 201–250, 251–300, … mm, respectively, and the means of each group were calculated and substituted for the individual values. Bayliff et al. (1991) found that the growth of bluefin in the length range of about 16 to 153 cm is best described by a two-stage model, the growth of the fish less than or equal to 564 mm following the Gompertz model and that of the fish equal to or greater than 564 mm being linear. The relationship between length in millimeters and time in days is expressed by the following equations:

fish less than or equal to 564 mm

x_t² = x_∞ + (x_t² - x_∞)e^{-K(t₂ - t₁)}

where

x_t = (L_t^λ - 1)/λ

in which, with their estimate of λ, x was essentially a logarithmic transformation,

L_t = length at time t₁ (t₂ is a time prior to t),
λ = 0.001,
L_∞ = 581 mm, and
K = coefficient of growth = 0.01184 per day.

fish equal to or greater than 564 mm

L_t = 564 + 0.709Δt

TABLE 1. Age and growth data for bluefin in the Pacific Ocean. The lengths are given in centimeters. Schultze and Collins' (1977) estimates are attributed to an unpublished thesis (their Table 1). The sample of Bell (1963b) consisted of 247 fish, of which approximately half had legible scales.

where

Δt = time, in days, elapsed since L_t = 564 mm.

Bayliff (1993a) used length-frequency data to obtain an estimate of 0.675 mm per day for the growth of bluefin in the eastern Pacific Ocean. He found the growth to be more rapid in the summer than in the winter. Such has also been found to be the case for northern bluefin in the Atlantic Ocean (Cort, 1990: Figure 68) and for southern bluefin (Burgess et al., 1991). Monthly estimates of the lengths are shown in Table 2.

TABLE 2. Estimated growth of northern bluefin in the eastern Pacific Ocean (from Bayliff, 1993a).

5. MATURATION AND SPAWNING

Fragmentary data on the sex ratios of northern bluefin are given in Table 3.

Hirota et al. (1976) reported that a male northern bluefin reared in captivity matured at 3 years of age and that a female of the same age appeared to be approaching maturity. The lengths of these fish (male, 1190 mm; female, 1353 mm) were considerably in excess of the average lengths of 3-year-old fish given in Table 1. Nakamura (1943; cited by Hirota et al., 1976) stated that bluefin mature at 3 years of age in nature. Harada (1980), however, stated that spawning of bluefin occurs at about 5 years of age, and that the lengths and weights of these fish are about 150 cm and 60 kg, respectively. The estimate of 5 years is in agreement with estimates for northern bluefin in the Atlantic Ocean (Clay, 1991) and for southern bluefin (Davis, 1991).

Fish of 270 to 300 kg have about 10 million eggs (Yamanaka and staff, 1963). No information is available on the frequency of spawning.

TABLE 3. Data on the sex ratios of bluefin tuna in the Pacific Ocean. The length range for 1988 is the length range for the 987 fish in the catch, rather than for the 45 fish in the sample.

Spawning occurs between Japan and the Philippines in April, May, and June and off southern Honshu in July (Yamanaka and staff, 1963; Yabe et al., 1966; Nishikawa et al., 1985). Okiyama (1974 and 1979) found larvae in the Sea of Japan in August, and considered them to have resulted from spawning in that area. In addition, Miller (1979) reported larvae of this species from the Hawaiian Islands. Bruce C. Mundy, U.S. National Marine Fisheries Service, Honolulu, Hawaii (pers. commun.), however, reported that no bluefin larvae were found in 460 plankton samples collected 1 to 15 nautical miles off Oahu during 1985 and 1986.

6. DISTRIBUTION, MIGRATION, AND STOCK STRUCTURE

6.1 Distribution

6.1.1 Eastern Pacific Ocean

In the eastern Pacific, northern bluefin are caught mostly between Cabo San Lucas, Baja California, Mexico and Point Conception, California, USA (Figure 1). Within the area between Cabo San Lucas and Point Conception the fish tend to be distributed further to the north in years when the sea-surface temperatures are above normal and further to the south in years when those temperatures are below normal (Hester, 1961). Squire (1983) presents some maps showing distributions of schools of bluefin sighted from aircraft off North America north of 30°N during 1962–1978. Statistics for small amounts of bluefin landed at California ports north of Los Angeles during 1977–1986 are given by Oliphant et al. (1990) and during years previous to 1977 in earlier versions of the same series. On September 24, 1960, a purse seiner caught 50 mt of bluefin about 50 miles west of Cape Mendocino, more than 300 miles north of Point Conception. This species has also been caught off Oregon and Washington (Brock, 1938; Oregon Fish Commission, 1948; Radovich, 1961), off British Columbia (Neave, 1959), and in Shelikoff Strait, Alaska (Radovich, 1961). These occurrences far to the north of the usual range of this species have been attributed to greater-than-normal sea-surface temperatures (Radovich, 1961). Bluefin have been recorded in the vicinity of the Galapagos Islands (Snodgrass and Heller, 1905; Herre, 1936), “but there is no supporting evidence which would eliminate T. maccoyii [southern bluefin] or any other species from consideration” (Gibbs and Collette, 1967). Two large (1553- and 1640-mm) bluefin were caught by a purse seiner at 3°23' S–81°09'W on June 14, 1969. Their identity as bluefin was confirmed by the striations on the livers, but it was not determined if they were northern or southern bluefin. De Buen (1953 and 1958) recorded northern bluefin off the central coast of Chile, and Nakamura and Warashina (1965) recorded a specimen caught at 37°11'S–114°41'W. Also Kume (1974) published data for fish caught off the west coast of South America between 80°W and 115°W during 1968–1970, and Fisheries Agency of Japan (1975, 1976, 1977a, and 1978) recorded this species in the same area during the 1972–1976 period (Figure 2).

FIGURE 1. Annual distributions of northern bluefin catches in the eastern Pacific Ocean, 1970–1989.

6.1.2 Central Pacific Ocean

Jordan and Jordan (1922), Fowler (1928), and June (1952) recorded northern bluefin caught near Hawaii, and Kume (1974) and Fisheries Agency of Japan (1974, 1975, 1976, 1977a, and 1978) recorded other catches of this species made in the central Pacific both north and south of the equator (Figure 2). Fowler's (1928 and 1932) records of a 355-mm bluefin from the Society Islands and of bluefin from the Marquesas Islands could be based upon misidentifications, however, as Klawe (1959) showed that Fowler (1944) had identified juvenile yellowfin, Thunnus albacares, caught in the eastern Pacific, as bluefin.

6.1.3 Western Pacific Ocean and Indian Ocean

In the western Pacific, northern bluefin occur from Sakhalin Island to southwestern Australia and New Zealand (Shimada, 1951; Okachi, 1963; Yamanaka and staff, 1963; Kume, 1974; Shingu et al., 1974; Fisheries Agency of Japan, 1974, 1975, 1976, 1977a, and 1978; Collette and Smith, 1981), but are most abundant in the vicinity of Japan (Figure 2). They are much more plentiful off Japan in years when the sea-surface temperatures are above normal than when they are below normal (Uda, 1962 and 1973). According to Yamanaka and staff (1963), “Fishes having body weight over 8.3 kg live far out at sea in summertime.”

In the Indian Ocean, northern bluefin have been caught west of Australia (Nakamura and Warashina, 1965).

6.2 Migration and Stock Structure

A model of the life history and stock structure of northern bluefin in the Pacific Ocean has been formulated. This model was first described by Bayliff (1980), but it has been modified considerably since then due to the acquisition of additional data for returns of tagged fish.

Figure 3 is a diagram of the model. The migrations shown by dashed lines are more speculative than those shown by solid lines. The diagram is intended to show the general areas where the migrations are believed to begin and end, but not the precise routes, as space limitations would make this impractical even if the routes were known. For example, it appears that the route of migration of juveniles bound for the eastern Pacific is south of the route of migration of maturing fish bound in the opposite direction, but such is not necessarily the case.

FIGURE 2. Average annual distribution of catches of northern bluefin in the Pacific Ocean by Japanese longliners during 1972–1976 (from Bayliff, 1980).

FIGURE 3. A model for northern bluefin migration in the Pacific Ocean (from Bayliff, 1980).

The spawning of northern bluefin occurs between Japan and the Philippines in April, May, and June, off southern Honshu in July, and in the Sea of Japan in August (Yamanaka and staff, 1963; Yabe et al., 1966; Okiyama, 1974 and 1979; Nishikawa et al., 1985). The larvae, postlarvae, and juveniles produced south of Japan are transported northward by the Kuroshio Current toward Japan. Fish of age 0 about 15 to 60 cm in length are caught in the vicinity of Japan during the summer, fall, and winter of their first year of life (Yabe et al., 1966; Yukinawa and Yabuta, 1967). The results of tagging experiments indicate that some of these remain in the western Pacific Ocean and others depart for the eastern Pacific during the fall or winter of their first year of life or the summer, fall, or winter of their second year of life. If the fish are restricted to the temperature range given in Section 8.1 when crossing the ocean, they probably occur mostly in the region shown in Figure 4, which more or less coincides with the North Pacific Subarctic-Subtropical Transition Zone (Roden, 1991). The journey from the western to the eastern Pacific takes as little as 7 months, or perhaps even less. Of 121 such migrants for which dates of recapture were obtained, 23 had been at liberty 215 to 358 days and 98 had been free for 368 to 999 days. It is possible that other fish migrate from the western to the eastern Pacific later in life, but there is no information concerning this because few tagged fish greater than about 1 year of age have been released in the western Pacific. It is also possible that some fish travel part of the way from the western to the eastern Pacific, and then turn back.

The fish which migrate from the western to the eastern Pacific form the basis for the fishery in the eastern Pacific, which takes place principally during May through October. In Figure 3 it appears that the fish in the eastern Pacific occur further and further offshore and have an increasingly restricted north-south distribution as they grow older. It was necessary for the sake of clarity to make the diagram this way, but actually such is not necessarily the case. Fish less than about 100 cm in length, which make up the bulk of the eastern Pacific catch, may or may not leave the eastern Pacific Ocean each fall or winter. Northern bluefin of that size are seldom caught in the eastern Pacific during November-April, which might indicate that they have left that region. If so, they probably do not go all the way to the western Pacific, however, as the minimum time at liberty observed for a tagged fish released in the eastern Pacific and recaptured in the western Pacific was 674 days (Table 4); if most of them migrated to the western Pacific each fall or winter and back to the eastern Pacific each spring some would probably have been recaptured in the western Pacific in the winter or early spring after less than about 100 to 150 days at liberty. Also, the energy costs of making such a long migration are so great that it would probably not be feasible for a fish to make two such migrations each year for several years.

The length of the sojourn in the eastern Pacific appears to be variable. Tagged fish believed to have been 1 year of age at the time of release in the eastern Pacific have been recaptured 2 years later in the western Pacific, but other tagged fish released in the eastern Pacific have been recaptured in the eastern Pacific after as long as 2 years at liberty. Fish of at least 6 or 7 age groups are caught in the eastern Pacific (Bell, 1963b; Schultze and Collins, 1977), so it is possible that some fish stay in that region for at least 5 or 6 years.

After a sojourn in the eastern Pacific, which may or may not be interrupted by visits to the central or western Pacific, the survivors return to the western Pacific, where they eventually spawn. Nakano and Bayliff (1992: Figure 16) show catches of bluefin by longlines between 25°N and 35°N and 120°W and 150°W during the first and fourth quarters of the 1981–1987 period. These were most likely migrating from the eastern to the western Pacific, but they might have been arriving in the eastern Pacific after a trip from the western Pacific. If the fish are restricted to the temperature range given in Section 8.1 when crossing the ocean, they probably occur mostly in the regions shown in Figure 4, which more or less coincide with the North Pacific Subarctic-Subtropical Transition Zone (Roden, 1991). The return journey from the eastern to the western Pacific may take nearly 2 years, as 674 days is the minimum time recorded between release and recapture of a tagged fish making this migration (Table 4).

FIGURE 4. Area of the north Pacific Ocean bounded by the 17° and 23°C surface isotherms (after Robinson, 1976).

TABLE 4. Data on releases and recaptures of tagged bluefin which crossed the Pacific Ocean from east to west (from Bayliff et al., 1991).

Large fish are occasionally caught in the eastern Pacific Ocean, especially in the vicinity of Guadalupe Island, Mexico, and the Channel Islands, off Southern California (Calkins, 1982: Figures 5 and 6; Foreman and Ishizuka, 1990; Bayliff, 1993b: Figure 19). The largest of these are probably over 10 years old. These have not necessarily resided in the eastern Pacific Ocean since they were about 1 to 2 years old. They may have arrived for the first time shortly before they were caught, or they may have made more than one round trip across the Pacific. It seems unlikely that all the large fish could spawn in the western Pacific each year, as some have been caught during or shortly before or after the spawning season, e.g. July 1978 (Calkins, 1982: Figure 6).

Many of the fish caught by longliners in the mid-Pacific (Figure 2) are presumably en route from the eastern to the western Pacific. Upon arriving in the western Pacific they presumably proceed to one or more of the spawning areas to spawn, either immediately or eventually.

Northern bluefin are also caught by longline vessels east of the Philippines, northeast of Papua New Guinea, and southeast of Australia, especially in the vicinity of New Zealand (Figure 2), and by sport gear in the Gulf of Papua (Collette and Smith, 1981). The question arises as to whether these fish come from juveniles which went south from one of the spawning areas, from immature fish which migrated south from Japan after a brief sojourn there, or from older fish which migrated south from the spawning area after spawning. If either the first or second possibility is the case there should be small bluefin south of 20°N. Small numbers of baitboat-caught northern bluefin have been recorded north of Papua New Guinea and in the vicinity of the Solomon and Marshall Islands (Fisheries Agency of Japan, 1977b, 1977c, 1977d, 1977e, and 1977f; this report: Figure 5), so it appears that at least some of the northern bluefin caught south of the spawning grounds by longline vessels are the result of movement of juveniles from the spawning area or the result of migration of immature fish south from Japan. This does not mean, however, that none of them are the result of movement of adult fish south from the spawning areas after spawning.

Larvae or postlarvae of northern bluefin have not been found in the Australia-New Zealand area, so it does not appear that the fish which were caught there by longlines would have spawned there. Rather, they would have to migrate back to the spawning area or not spawn at all. It seems unlikely that all the large fish found in the southwestern Pacific spawn in the northwestern Pacific each year, as the large bluefin reported by Collette and Smith (1981) off Papua New Guinea was caught in April. Also, the bluefin reported by de Buen (1958) in the southeastern Pacific were caught in April and May, during the spawning season.

After spawning, the fish probably disperse from the spawning areas to other areas of the western Pacific. Some may even migrate to the eastern Pacific, as large fish are found there (see above). The following year, if they have not travelled too far, they presumably return to the spawning areas to spawn again.

7. NATURAL MORTALITY

There are no data available for estimating directly the coefficient of natural mortality of northern bluefin in the Pacific Ocean. Pauly (1980) used data on the growth, mortality, and mean environmental temperatures of 175 stocks of fish to calculate the following equation:

FIGURE 5. Five-degree areas south of 20°N in the Pacific Ocean in which northern bluefin were caught by surface gear during 1972–1976 (from Bayliff and Calkins, 1979).

InM = -0.0066 - 0.279InL_∞ + 0.6543lnK+ 0.4634InT

where

M = coefficient of natural mortality
L_∞ = asymptotic length in von Bertalanffy equation,
K = coefficient of growth in von Bertalanffy equation, and
T = mean environmental temperature.

Values of 223 cm and 0.18 for L_∞ and K, respectively, are given by Bayliff et al. (1991: Table 5, line 1). (It should be noted, however, that the best fit to the growth data was not a von Bertalanffy curve, but a Gompertz curve for fish between 160 and 564 mm and a straight line for fish from 564 to 1530 mm.) Pauly (1980) states that, “the values of L_∞ used [to derive the equation] pertain mostly to total length,” so the value of 223 cm for the fork length was converted to 257 cm for the total length by the formula

TL = -17.42 + 1.158FL

where TL = total length and FL = fork length, both in millimeters. Bell (1963a) states that bluefin are found in water with surface temperatures of 17° to 23°C, so 20 was substituted for T. This gives an estimate of 0.276 for M for fish in the range of about 16 to 153 cm. The standard deviation for InM is 0.245 (Pauly, 1980), so the 90-percent confidence limits of the estimate are about 0.161 and 0.471.

Northern bluefin of the Pacific Ocean have lived as long as 16 years in captivity (INFOFISH, 1991), but the life span of wild fish may be more or less than that.

8. OCEANOGRAPHIC FEATURES ASSOCIATED WITH THE SPECIES

8.1 Eastern Pacific Ocean

During their sojourn in the eastern Pacific Ocean, northern bluefin are residents of the California Current, a region of upwelling off California and Baja California. The California Current is described by Reid, Roden, and Wyllie (1958) and Reid (1988). According to Bell (1963a), bluefin are found most often in the eastern Pacific Ocean in waters with surface temperatures between 17° and 23°C (Figure 4). Flittner (1966) stated that 95 percent of the temperatures recorded for 777 purse-seine sets in which bluefin were caught during 1959–1963 were between 17° and 22°C. Hester (1961) noted that bluefin are distributed further to the north in years when the sea-surface temperatures are above normal and further to the south in years when those temperatures are below normal.

8.2 Central Pacific Ocean

During their trans-Pacific migrations northern bluefin probably inhabit the North Pacific Subarctic-Subtropical Transition Zone (Roden, 1991), which more or less coincides with the zones with sea-surface temperatures of 17° to 23° C shown in Figure 4.

8.3 Western Pacific Ocean

In the western Pacific Ocean most of the northern bluefin inhabit the Kuroshio Current (Marr, 1970; Stommel and Yoshida, 1972; Sugawara, 1972; Takenouti, 1980). This current probably plays an important role in transporting the larvae and postlarvae northward from the spawning grounds between Japan and the Philippines and southeast of Japan to waters off Japan. According to Uda (1957), bluefin are found most often in the western Pacific Ocean in waters with surface temperatures between 14° and 19°C. Kida (1936) gives this range as 15° to 17° C for waters off Hokkaido. He said that the temperature range increases with increasing size of the fish. Bluefin are much more plentiful off Japan in years when the sea-surface temperatures are above normal than when they are below normal (Uda, 1962 and 1973). Data on the sea-surface temperatures and lengths of fish tagged during the Inter-American Tropical Tuna Commission's tagging experiments of 1980–1982 in the western Pacific Ocean are given in Table 5. Fish 15 to 31 cm in length were caught in the Kochi summer fishery at temperatures of about 24° to 29°C, fish 31 to 60 cm in length were caught by the Kagoshima winter and spring troll and trap fishery at temperatures of about 15° to 21°C, and fish 44 to 52 cm in length were caught by the Toyama Bay spring and summer trap fishery at temperatures of about 20° to 22°C.

Ogawa and Ishida (1989a and 1989b) reported that peak catches of adult northern bluefin occur off northeastern Honshu during May-July and peak catches of juvenile bluefin occur in the same area during June-August and October–December. They stated that the declines in the catches of both adults and juveniles since the mid- 1960s have coincided with decreasing latitudes of the southern limit of the cold waters of the First Oyashio Intrusion.

9. INTERACTIONS WITH OTHER SPECIES

9.1 Feeding and Food

Uotani et al. (1990) found that bluefin larvae collected near the Nansei Islands, Japan, had fed on small zooplankton, mainly copepods. Those less than 5 mm long consumed copepod nauplii, while those more than 5 mm long ate larger copepods, especially Corycaeus.

Blunt (1958), referring to surface-caught fish, stated: “Of 168 bluefin taken off California [during 1957], 70 percent had been feeding on anchovies [Engraulis mordax] … Bluefin sometimes feed close inshore near the surf. Tuna captured in shallow water had eaten starfish, kelp, sandabs [Citharichthys], surf perches [Embiotocidae] and white croakers [Genyonemus lineatus].” Bell (1963a) listed 14 species of fish found in northern bluefin stomachs. Pinkas (1971) reported that surface-caught bluefin sampled in 1968 and 1969 consumed, in order of importance, anchovies, red crabs (Pleurocodes planipes), sauries (Cololabis saira), squid (Loligo opalescens), and hake (Merluccius productus). Anchovies made up 80 percent of the stomach contents by volume. In addition, other species of fish, cephalopods, and crustaceans, Atlanta (a gastropod), and pieces of kelp were found in the stomachs. He remarked that bluefin occurred further offshore in 1968 and 1969 than in 1957, which probably accounted at least partly for the differences in his results and those of Blunt (1958). Scott and Flittner (1972) found that red crabs are a significant part of the diet only south of 29°N, and that boiling and jumping schools of fish are much more common north of that latitude, where fish are the principal item of the diet. They stated that the differences in behavior in the two areas could be due to differences in the food, i.e. “modified filter feeding” might be employed while feeding on red crabs, while “vigorous pursuit” would be required for feeding on fish.

TABLE 5. Lengths (cm) and sea-surface temperatures (°C) of fish tagged during the IATTC tagging experiments of 1980–1982 in the western Pacific Ocean.

Yamanaka and staff (1963) summarized the available information on the feeding and food of northern bluefin in the western Pacific. Fish 20 to 65 cm in length consume anchovies and other fish, plus crustaceans and squid, while longline-caught (larger) fish eat fish and squid. Fish of unspecified size were reported to consume both pelagic and demersal fish, Calamarius (probably a squid), pteropods, Pyrosoma (a tunicate), and crustaceans. In addition to the above, Doi (1960) reported that bluefin eat round herring (Etrumeus teres), pampanos (Carangidae), and mackerel (Scomber japonicus). Yokota et al. (1961) stated that bluefin consume frigate mackerel (Auxis spp.), spotted mackerel (Scomber tapeinocephalus), jack mackerel (Trachurus japonicus), scad (Decapterus muroadsi), sauries, skipjack tuna (Katsuwonus pelamis), anchovies (Engraulis japonica and Stolephorus zollingeri), beaked salmon (Gonorhynchus abbreviatus), Acinacea notha, argentines (Argentina semifasciata), sprat (Spratelloides japonicus), lanternfishes (Myctophidae), and common squid (Todarodes pacificus). Mori (1972) also found skipjack in the stomachs of bluefin. Kida (1936) said that a “certain connexion seems to exist between the incoming schools of sardine [Sardinops melanosticta] … on one side and the [bluefin] fishery on the other,” and, according to Yamanaka and staff (1963), “Nakamura (1949) reported that … the route of the northward migration of this species coincides with that of the migration of the sardine; … when sardine suddenly decreased, the fishing of [bluefin] simultaneously declined. Uda (1960) reported that (i) the resources of both this species and of sardine were large from 1933 to 1940, but on and after 1941 decreased simultaneously; (ii) with the decrease in resources, the northern limitation of the area of occurrence for both kinds of fishes shrank southward.” The catches of sardines in Japan increased greatly during the 1976–1985 period (Yamanaka et al., 1988: Table 4); the catches of bluefin in the western Pacific increased from 1976 to 1981, but then decreased from 1981 to 1985 (Table 6).

9.2 Competitors

Bell (1963a) stated that albacore (Thunnus alalunga), yellowtail (Seriola dorsalis), barracuda (Sphyraena argentea), and mackerel (Scomber japonicus) compete with northern bluefin for food. According to Yamanaka and staff (1963), “it can be supposed that other kinds of tunas, spearfish [Tetrapturus angustirostris], Spanish mackerel [Scomberomorus spp.], and some others are competitors.”

9.3 Predators

Yokota et al. (1961) mentioned the presence of juvenile northern bluefin in the stomachs of skipjack. Yamanaka and staff (1963) stated that “immature forms … have many enemies--seals, dolphins, spear-fishes, sword-fishes [Xiphias gladius], sharks, and large forms of their own or allied species. Tunas are devoured by killer-whales which are said to catch them at the nape and kill them immediately. They fear killers so greatly that they are frightened several miles away from the spots where these ferocious enemies are found. Thus catches made by pound-nets [traps] vary greatly according to the favorable or unfavorable proximity of killers. Sometimes tunas leap recklessly on to beaches to escape their enemies.”

TABLE 6. Catches, in metric tons, of northern bluefin in the western Pacific Ocean. ROC and ROK stand for Republic of China and Republic of Korea, respectively.

* data not available

9.4 Parasites

Information on parasites of northern bluefin is given by Bell (1963a), Silas (1967), Silas and Ummerkutty (1967), Love and Moser (1977) and Cressey and Cressey (1980).

10. GENERAL DESCRIPTION OF THE FISHERIES

10.1 Eastern Pacific Ocean

Sport fishing for northern bluefin began in California in 1898 (Bell, 1970). Prior to World War I, before the advent of the commercial fishery, many large fish were taken, particularly by vessels based at Catalina Island. The largest of these fish weighed 251 pounds (114 kg). More recently, the average size of the sport-caught fish has been roughly 25 pounds (11 kg), although large fish are still taken (Dotson and Graves, 1984). Many of the sport-caught fish are taken by fishermen who are directing their efforts primarily toward albacore.

The purse-seine fishery for northern bluefin began in 1918 (Whitehead, 1931), and since then the catches by purse seiners have far exceeded those by any other type of gear. From 1918 until about 1959 or 1960 most of the vessels were relatively small, with capacities less than about 200 short tons (181 mt). None of them fished exclusively for bluefin. The smaller ones fished chiefly for sardines, mackerel, and other pelagic fish other than tunas, and the larger ones fished mostly for yellowfin and skipjack. During 1959 and 1960 most of the larger tuna baitboats were converted to purse seiners, and during the ensuing years many new purse seiners were built. During the 1960s, 1970s, and 1980s many of the smaller, older vessels sank or dropped out of the fishery, and the new vessels which replaced them tended to be larger. As a result, there are now more larger purse seiners and fewer smaller ones than had been the case during the early 1960s. Bluefin are now taken by vessels of all sizes, but the smaller ones account for a proportionally greater share of the catch. The proportion of the catch made by vessels which fish primarily for pelagic fish other than tunas is less now than it was during the early years of the fishery, as the centre of abundance of the fish appears to have shifted southward beyond the area where those vessels normally fish. Most of the fish caught by purse seiners weigh less than about 50 pounds (23 kg), but larger ones are sometimes caught, including one weighing 1,009 pounds (458 kg) (Section 4).

Most of the information regarding the distribution of the catches by tuna purse seiners after 1947 has been obtained from the logbook records of these vessels. The vessel captains or navigators usually record the catch of each set in short tons, the location of each set to the nearest degree and minute of latitude and longitude, the date, and the times each set was initiated and completed. An abstract of the vessel logbook is prepared at the end of each trip. These abstracts are incorporated into the Inter-American Tropical Tuna Commission's computer records, from which summaries of the northern bluefin catch, stratified by any combination of area, time, vessel size class, etc., can be prepared. During the 1961–1991 period the annual logged catches ranged from 69 to 100 percent of the corresponding weighed-out catches.

Northern bluefin are rarely encountered south of Cabo San Lucas, Baja California, or north of Point Conception, California. Within this area a considerable change has taken place during the 20th century (Bell, 1970). Fishing was conducted only off California prior to 1930. During that year bluefin were discovered off Isla Guadalupe, Baja California, located at about 29°N–118°W, and about 40 percent of the catch in 1930 was made in that area (Whitehead, 1931). From 1930 through 1947, fishing was conducted off both California and Baja California, but in most years the majority of the catch came from off California. From 1948 to the present, however, most of the catch has been made off Baja California (Calkins, 1982: Figure 2; this report: Figure 1). In 1989, for the first time in many years, no catches were made south of 30°N (Figure 1). The average annual catches made off California during the 1960s, 1970s, and 1980s have been considerably less than the average annual catches made in the same area during the 1918–1929 period. Part of the reason for this may be that during more recent years the larger vessels fishing off Baja California have tended to intercept the fish before they reach the waters off California.

The distributions of the catches by months are shown in Figure 3 of Calkins (1982), Figure 4 of Hanan (1983), and Figure 6 of this report. During January through April there are typically only light and sporadic catches. Most of these are made off the coast of Baja California between 24°N and 26°N and in the vicinity of Isla Guadalupe. In May and June the catches increase, and most of them are made between 24°N and 27°N. During July the fishing area spreads to the north and is at its widest extent of the year; most of the catch is made between 25°N and 33°N. In August there are usually only light catches at the southern end of the fishing area, most of the catch being made between 28°N and 33°N. During September most of the catch is made in the same area as in August, but the amount of catch is usually considerably less. In October the catches continue to decline, and most of them are made north of 30°N. During November and December, as in the first months of the year, the catches are light and sporadic. The distribution of the catches was a little more northerly during the 1980s than during the 1970s.

FIGURE 6. Average monthly distributions of northern bluefin catches in the eastern Pacific Ocean, 1970–1979 and 1980–1989.

Northern bluefin are caught in the vicinity of Isla Guadalupe during all months of the year, but not during every month of every year.

Small amounts of northern bluefin are caught by longline vessels in the eastern Pacific (Fisheries Agency of Japan, 1974, 1975, 1976, 1977a, and 1978; Nakano and Bayliff, 1992; Figure 16; this report: Figure 2).

10.2 Western Pacific Ocean

Information on the fisheries which exploit northern bluefin in the western Pacific Ocean is given by Yamanaka (1958 and 1982), Yamanaka and staff (1963), Tatsuki et al. (1963), Yukinawa and Yabuta (1967), Shingu et al. (1974), Honma and Suzuki (1978), and Bayliff (1980). Bluefin are exploited by various gears in the western Pacific Ocean from Taiwan to Hokkaido. Age-0 fish about 15 to 50 cm in length are caught by trolling during July-October south of Shikoku and south of Shizuoka Prefecture. During November-April age-O fish about 35 to 60 cm in length are taken by trolling south and west of Kyushu. Age-1 and older fish are caught by purse-seining, mostly during May-September between about 30°N–42°N and 140°E–152°E. Bluefin of various sizes are also caught nearshore by traps, gillnets, and other gear, especially in the Sea of Japan. Small amounts of bluefin are also caught near the southeastern coast of Japan and offshore by longlining.

10.3 Central Pacific Ocean

Gillnet fisheries for salmon, Oncorhynchus spp.; flying squid, Ommastrephes bartrami; and albacore, Thunnus alalunga, have recently existed in the north Pacific Ocean, but almost nothing has been published about the catches of northern bluefin by these fisheries. The distributions of these fisheries are shown in Figure 7. In the eastern Pacific, bluefin occur mostly in waters with sea-surface temperatures of 17° to 23°C (Bell, 1963a). If their distribution with regard to temperature is the same in the central and western Pacific it would be approximately as shown in Figure 4. Since this distribution coincides with the distributions of the gillnet fisheries, it seems likely that bluefin have been taken by gillnets in the central Pacific Ocean. Bayliff et al. (1991: Table 6) list data for five tagged bluefin released in the western Pacific and recaptured by gillnet vessels in the central Pacific between 34°N and 40°N and 161°E and 161°W. International North Pacific Fisheries Commission (1986:55) reported that “a single day's catch [of a squid gillnet] ranged from 871 to 8,256 squid and 5 to 530 tuna.” International North Pacific Fisheries Commission (1991: Table 4) stated that 107 bluefin were recorded by observers aboard Japanese squid gillnet vessels in 1990. According to San Diego Union (1991), observers were aboard about 10 percent of the vessels of that fleet, so the total catch, if the sampling had been random, would have been roughly 1,000 fish. It may be possible, eventually, to stratify the data so that a better estimate can be calculated.

FIGURE 7. Areas in which offshore gillnet fishing by Japanese vessels takes place (after Suzuki, 1990).

The Japanese baitboat fishery for albacore extends as far to the east as 175°W (Majors et al., 1984: Figure 2). Bayliff et al. (1991: Table 6) reported that one tagged northern bluefin released in the western Pacific was recaptured by a baitboat at approximately 38°N–167°E.

11. TRENDS IN CATCH, EFFORT, AND CATCH PER UNIT OF FISHING EFFORT

11.1 Eastern Pacific Ocean

11.1.1 Annual catches

The total annual catches of northern bluefin by commercial and sport vessels in the eastern Pacific Ocean are shown in Table 7. The catches prior to 1918 were negligible. The data for 1918 through 1960 include only the catches landed in California, but it is believed that the catches landed elsewhere prior to 1961 were inconsequential. The eastern Pacific subsurface catches for all years are omitted, but these are also believed to be inconsequential.

TABLE 7. Catches, in metric tons, of bluefin in the eastern Pacific Ocean. The sources of the data are as follows: commercial - 1918–1960, Bell, 1963a; 1961–1992, Bayliff, 1993b; sport - 1936–1946, Bell, 1963a; 1947–1990, Leet et al., 1992.

* data not available

The catches tended to be greater during the 1960s and 1970s than during the previous period. This is probably due to the fact that during 1959 and 1960 most of the tuna baitboats were converted to purse seiners, and during the 1960s and 1970s many new purse seiners were built. During the earlier period the catch was made by smaller purse seiners, most of which fished part-time for tunas and part-time for other pelagic fish, whereas during the later period most of the catch has been made by larger purse seiners which fish full-time for tunas (but not exclusively for northern bluefin, of course).

11.1.2 Monthly catches

Data on the estimated monthly catches of northern bluefin in the eastern Pacific surface fishery, estimated from logged monthly catch and total annual landings data, are given in Table 8. The proportions of the annual catches taken during the various months vary considerably from year to year.

11.1.3 Effort and catch per unit of effort

Calkins (1982) stated that, since the area in which northern bluefin are caught is subjected to intensive searching for tunas and other pelagic fish throughout the year, the “total catch may be the best indicator of bluefin abundance.” Nevertheless, he attempted to ascertain the fishing effort directed toward bluefin during 1961–1980. As stated previously, the vessels which catch bluefin also catch considerable quantities of other species of pelagic fish, so it cannot be considered that all their effort is directed toward bluefin. Accordingly, he adopted the following rules: (1) no effort made south of 23°N was considered to be bluefin effort; (2) no effort during the November-April period was considered to be bluefin effort; (3) no effort in 1-degree area-month strata in which no sets were made on bluefin was considered to be bluefin effort. In the area-time strata in which bluefin effort was assumed to occur during the 1961–1980 period, the catches of bluefin ranged from 19 to 83 percent of the total catch, with an average of 51 percent (Calkins, 1982: Table 5). These effort data are listed in his Table 6. No attempt was made to standardize these data by vessel size class. Hanan's (1983) rules were similar, but not identical, to those of Calkins (1982). He stated that “one boat-day or part of a boat-day of effort is assigned to a seiner for each day or partial day of purse seining or searching for tuna in the bluefin fishing range (north of lat. 22°N) during months in which bluefin were caught.”

Calkins (1982) calculated the catches per unit of effort (CPUEs) by dividing the catches of northern bluefin in the area-time strata in which bluefin effort was assumed to occur by the effort in those strata. These results are also shown in his Table 6. The total annual catches of bluefin during 1961–1989, plus the logged bluefin effort and the CPUEs for those years, are shown by Bayliff (1991: Figure 7). The catches and CPUEs for 1961 through 1984 were highly correlated, but after that the relationship broke down, high CPUEs occurring in years with low to medium catches. This is due to the fact that in the years with high CPUEs the fish tended to occur in only a few area-time strata, with high CPUEs in them. In 1989, for example, no bluefin were caught south of 30°N (Figure 1), and 70 percent of the catch was made during August.

TABLE 8. Estimated catches, in metric tons, of northern bluefin in the eastern Pacific Ocean (after Bayliff, 1993a).

Another index of effort and catch per unit of effort, the “habitat index,” is described by Bayliff (1993b). Bluefin are most often caught by purse-seine vessels in the eastern Pacific Ocean in waters with surface temperatures of 17° to 23°C (Bell, 1963a), so it was assumed that fishing effort exerted by tuna purse seiners off California and Baja California in waters in that temperature range during May through October is bluefin effort, regardless of whether bluefin were caught. This avoids the problem caused by concentration of bluefin in a small part their range, as discussed above for the index of Calkins (1982). The sums of the logged catches of bluefin in the eastern Pacific Ocean during May-October were divided by the sums of the logged purse-seine effort in the 1-degree areas which were suitable bluefin habitat during the same months to get the annual CPUEs. These data are shown in Figure 8. They appear to indicate that both reduced abundance and reduced fishing effort have contributed to the reduced catches of bluefin in the eastern Pacific Ocean during the 1980s and early 1990s. Two years of this period, 1985 and 1986, were apparently years of high abundance of bluefin, but the catches were only average because the fishing effort was so low. Unless the fishing effort increases, it appears unlikely that catches in the eastern Pacific Ocean greater than those of 1985 and 1986 will be taken in the future.

The relative abundance of many pelagic ocean fishes can also be monitored by aerial surveys. Squire (1983 and 1993) gives information on northern bluefin recorded during aerial surveys of pelagic fishes carried out over the eastern Pacific Ocean, but the surveys covered only waters off Southern California and Baja California north of 30°N.