A REVIEW OF THE BIOLOGY AND FISHERIES FOR YELLOWFIN TUNA (THUNNUS ALBACARES) IN THE WESTERN AND CENTRAL PACIFIC OCEAN

Ziro Suzuki
National Research Institute of Far Seas Fisheries
Shimizu-shi, Japan

1. INTRODUCTION

The species synopsis of yellowfin tuna by Cole (1980) was referred to extensively for preparation of this review paper. The layout of this paper follows a similar review paper on the yellowfin tuna in the eastern Pacific (Wild, 1993).

2. IDENTITY

The species name for the yellowfin tuna is Thunnus albacares (Bonnaterre, 1788) (Collette and Nauen, 1983).

3. EARLY LIFE HISTORY

The morphological description of larval yellowfin tuna has been reported by Wade (1951), Yabe and Ueyanagi (1961), Matsumoto (1962) and Mori et al. (1971) etc. Detailed observations on rearing and morphological change of artificially fertilized yellowfin were made available by Mori et al. (1971), Harada et al. (1971), Far Seas Fisheries Research Laboratory (FSFRL, 1973), and Harada et al. (1980a). Harada et al. (1980a) reported that they could rear yellowfin from eggs to a maximum of 51 mm in total length (1.35 g in body weight) 38 days after hatching. It appears from these artificial fertilization experiments that there are two critical periods for larval mortality, one at 4 to 5 days and the other about 11 days after hatching (FSFRL, 1973). In this regard, Nishikawa (in FSFRL, 1973) pointed out that the second critical period, at about 7–10 mm in total length, corresponded to the period of changing prey foods from crustaceans to fish larvae which was observed in the stomach contents of the wild yellowfin larvae.

General aspects of geographical distribution of larval yellowfin tuna in the western and central Pacific are found in several works (e.g., Wade, 1951; Matsumoto, 1958; Strasburg, 1960; Ueyanagi, 1969; and Nishikawa et al., 1985). Among them the studies by Ueyanagi (1969) and Nishikawa et al. (1985) are comprehensive papers covering extensive areas. Figure 1 shows the occurrence of larval yellowfin tuna. Ueyanagi (1969) stated that larval yellowfin tuna occurred all the year round in tropical waters and the range of their distribution expanded to the higher latitudes in the respective summer months in the northern and southern hemispheres. He further mentioned that the lower limiting temperature of larval distribution of yellowfin tuna was presumed to be about 26°C although some larvae occurred in waters lower than 26°C, as low as 24°C according to his data. Mori (1970) reported that the minimum sea-surface temperature where larval yellowfin were found was about 24°C in the waters adjacent to Japan. Harada et al. (1980b) showed in the experiments on artificially-fertilized yellowfin eggs that the rates of occurrence of normally hatched larvae were highest for an average water temperature range from 26.4°C to 27.8°C, but no normal larvae were found in the temperature range below 18.7°C or over 31.9°C.

Figure 1. Distribution of larval yellowfin tuna by plankton net survey; after Nishikawa et al. (1985).

Comparisons of occurrence of yellowfin larvae at the surface and the subsurface layer (FSFRL, 1978) appear to indicate that the larvae in the western and central Pacific were more abundant at the surface, especially within the layer 50 meters from the surface, than in the subsurface layers, and more abundant at night than during the day. However, a detailed statistical analysis with more precise vertical-sampling methods is required to define the vertical distribution of the yellowfin larvae since the studies by Strasburg (1960), Klawe (1963), and Ueyanagi (1969) indicated no diurnal difference while Richards and Simmons (1971) indicated that yellowfin larvae in the equatorial eastern Atlantic migrate to the surface during the day.

4. FEEDING AND FOOD

The feeding of yellowfin appears to take place mainly during day time and they accept a great variety of prey species, including fish, crustaceans, and cephalopods (Reintjes and King, 1953; Watanabe, 1958). Areal variation is noted in occurrence of empty stomachs and volume of stomach contents (Watanabe, 1960). There was a significant difference in prey species composition for the specimens between those around fish aggregating devices (FAD) and non-FAD associated yellowfin in the Hawaiian waters (Brock, 1985).

Yesaki (1983) and Barut (1988) reported the food and feeding habits of yellowfin tuna caught by handline around payaos in the Moro Gulf, Philippines. They found significantly higher daily-meal consumption, about twice that estimated for the eastern Pacific counterpart and this was assumed to be attributable to forced feeding (chumming) specific to the payao-associated fishery. It was also reported that there was no significant difference in consumption rates between male and female yellowfin tuna. Their preliminary estimates indicated that the amount of juvenile yellowfin cannibalized was high, potentially twice the amount caught by the handline fishery.

5. LENGTH AND WEIGHT RELATIONSHIP

Length and weight relationships of yellowfin tuna are available for Philippine waters (Ronquillo, 1963), Hawaiian waters (Tester and Nakamura, 1957), central Pacific (Nakamura and Uchiyama, 1966) and western and central Pacific (Kamimura and Honma, 1959; Morita, 1973). Ronquillo (1963) gave relationships between total length and weight by sex but there was only a very small difference in the sex-specific relationships. Some of these relationships are shown in Table 1.

Table 1. Length and weight relationships (W=aL_b) of yellowfin tuna in the western and central Pacific Ocean. Fork length (L) is in cm and weight (W)1¹ in kg.

1) Whole weight except Kamimura and Honma (1966) who measured gilled-and gutted weight.

Kamimura and Honma (1959) estimated the length-weight relationship between the fork length and gilled-and-gutted weight for yellowfin tuna taken by the Japanese longline boats in the western and central tropical Pacific.

Length and live-weight relationship for the western and central Pacific for yellowfin tuna taken mainly by the Japanese longline boats was formulated by Morita (1973). Morita (1973) also gave a conversion factor from the gilled-and-gutted (GG) weight to the live weight in the following expression:

log r = 0.5707-0.2445 log L

where r = conversion factor at the fork length L(cm).

The r value is multiplied by the GG weight to obtain the corresponding live weight. The samples used cover fish larger than about 70 cm in fork length caught by the longline boats in the western tropical Pacific with a small number of samples from the tropical Atlantic.

At present, the value of 1.15 is being used by the Japanese scientists in the conversion from the GG to the live weight regardless of the area and size of fish for the yellowfin taken by the Japanese longline boats. However, as is obvious in the conversion equation shown previously, the factor changes with the length of the fish, smaller fish have larger values than larger fish (e.g., r=1.32 for 70–80 cm class, 1.18 for 110–120 cm class and 1.09 for 150–160 cm class). Length and weight values in selected sizes among the three papers [W(r) for Kamimura and Honma (1959)] in Table 1 are relatively similar.

White (1982) shows a length-weight relationship which is apparently derived from the Philippine fisheries. However, the range of the data and the method used to fit the curve were not mentioned.

It is noted that there appears to be no length and weight relationship available for yellowfin caught by the purse seine fishery in the equatorial western Pacific. Since the purse seine fishery in that area is the most dominant yellowfin fishery, the relationship specific to the fishery should be established urgently.

6. AGE AND GROWTH

Age and growth of this species in the western and central Pacific were studied through hardpart analysis (Aikawa and Kato, 1938; Nose et al., 1957; Yabuta et al., 1960; Yokota et al., 1961 (including length composition data); Tan et al., 1965; Yang et al., 1969; Uchiyama and Struhsaker, 1981; Yamanaka, 1990), weight frequency analysis (Kimura, 1932; Moore, 1951) and length frequency analysis (Yabuta and Yukinawa, 1957, 1959; Wankowski, 1981; White, 1982; Yesaki, 1983; Ingles and Pauly, 1984; Yamanaka, 1990). Suzuki (1971) reviewed the age and growth studies in the Pacific and suggested the superiority of the methods using the hard parts relative to those using length or weight frequencies. The growth parameters in the major works and sizes at age in the selected papers for the western and central Pacific are shown in Table 2 and Table 3, respectively. Except for the early studies based on small numbers of samples, there are some significant differences in age and growth among the works (Table 3).

Recent work by Yamanaka (1990) based on counting the daily rings on otoliths of juvenile yellowfin tuna in the Philippine waters showed that the growth was best described by two liner stanzas. The two stanzas cover the range from 15 to 35 cm and 35 to 79 cm in fork length, and from 50 to 2,350 g and 2,350 to 9,200 g in whole weight. The Philippine yellowfin tuna attains 57 cm at age one according to the otolith study which is close to the age one estimate of previous studies. However, the size reached in one year derived by Yamanaka (1990) for Philippine yellowfin was the largest found among the three daily-increment studies, i.e., 53 cm and 49 cm for Hawaiian (Uchiyama and Struhsaker, 1981) and eastern Pacific yellowfin tuna (Wild, 1986). It is not clear whether the largest size at one year in the Philippine waters is due to temperature or other factors, such as the method of otolith preparation or differences in the method of counting increments. Yamanaka (1990) infers from these observations that a significant physiological and ecological change occurs when the fish attains a critical size. This size lies between approximately 35 and 50 cm.

Table 2. Comparison of growth equation parameters (von Bertalanffy) for yellowfin tuna in the western and central Pacific Ocean.

After Suzuki (1971) for the works published till 1969 except Yokota et al. (1961)

Table 3. Comparison of size (Fork length in cm) at age in selected growth studies for yellowfin tuna in the western and central Pacific.

* Estimates within the size range of the sample.

Age and growth studies by sex through the length-frequency analysis of yellowfin tuna in the Philippine waters shows that the growth rate of males is higher than that of females (Yesaki, 1983; Yamanaka, 1990). On the other hand, Wild (1986), using daily ring methods for yellowfin tuna in the eastern Pacific shows that young females are initially larger than males of the same age, the growth curves cross one another at around age 2.0 (about 95 cm in fork length) and thereafter males are larger than females. Wild (1986) assumes that differences in mortality between the sexes may be more attributable to the apparent smaller maximum size reached by females, rather than differences in growth rate or availability to the fisheries.

7. MATURATION, SPAWNING, FECUNDITY AND SEX RATIO

7.1 Maturation

The size of yellowfin at first spawning (maturation) in the western and central Pacific has been studied through gonadosomatic indices (GI) (Yuen and June, 1957; Kikawa, 1962), external features of the ovaries (Wade, 1950) and microscopic examination of egg diameters (Buñag, 1956). Results ranged from 53 cm for males in the Philippine waters (Wade, 1950), 57 cm for females also in the Philippine waters (Buñag, 1956), 70–80 cm for females in the central Pacific and 80–110 cm for females in the western and central Pacific (Kikawa, 1962). It appears difficult to judge the size at first spawning without a detailed histological observation of ovaries. The histological examination of ovaries of yellowfin in the eastern tropical Pacific showed that the smallest female found with mature ovaries was 84 cm (IATTC, 1990).

The size at 50 % maturity of yellowfin tuna is estimated to be about 110–120 cm (Yuen and June, 1957; Kikawa, 1962) in the western and central Pacific. However, as in the case of size at first spawning, these studies did not include histological examinations. There is some indication that yellowfin caught by the surface fisheries tend to be more mature than those taken by the longline fishery (Hisada, 1973; Koido and Suzuki, 1989). Hisada (1973) hypothesized that this observation indicates that the fish move to the surface layer when they mature. The review in this section shows the necessity of a detailed histological study on maturity covering both surface and longline data.

7.2 Spawning

Yellowfin tuna in the Pacific spawn over vast areas, throughout the year in the tropical waters (e.g., Kikawa, 1966), and in the respective summers in the higher latitudes where the water temperature is over 24°C. However, there are peak seasons for spawning in specific areas. Kikawa (1966) reported, using the longline data that the peak spawning potential (product of fecundity, rate of group maturity, sex ratio and abundance index) is in December–January in the western tropical Pacific (120°E–180°) and April–May in the central tropical Pacific (140°W–180°). Two spawning peaks in a year were indicated for the yellowfin tuna in the Philippine waters. The major peak, according to GI analysis by Yesaki (1983) covers the period from March–May and a lesser peak during November–December. Yamanaka (1990) showed a similar peak, one in April and the other in October through GI examination.

Some scientists have suggested that yellowfin is a multiple spawner, based on the observation of multiple modes in ova diameter frequency distributions (e.g., June, 1953; Buñag, 1956). Recent preliminary work by Nikaido (1988) based on the observation of post-ovulatory follicles in the ovaries demonstrates that yellowfin caught by purse seine in the western Pacific spawn every 1.7 days. Similar analysis in the eastern Pacific indicates a spawning interval of about 1.3 days (Schaefer, 1988). Schaefer (1988) estimates that spawning occurs between 2000–2400 h.

Yesaki(1983) shows no correlation between the lunar cycle and spawning activity inferred from GI, but Yamanaka (1990) reports that four otoliths back-dated to spawning indicate that spawning occurs during the new moon. Yamanaka (1990) also suggests that the monsoon seasons significantly affect spawning, growth and recruitment of yellowfin tuna in the Philippine waters.

7.3 Fecundity

June (1953) calculated the relationship between weight and batch fecundity of yellowfin tuna taken by the Hawaiian longline fishery. He fitted a straight line to a sample of 11 yellowfin ranging from 47 to 88 kg and number of eggs from 2.4 to 8.6 millions:

Y = 125,200 X -2,853,000

where Y and X denote number of eggs and weight of fish in kg. Kikawa (1966) proposed a relationship between fork length and ovary weight. However, it should be noted that Kikawa's relationship is derived from data on bigeye tuna, does not specifically apply to yellowfin tuna.

7.4 Sex Ratio

It is generally agreed that the sex ratio (ratio of female to male) is about 1 until a length of about 120 cm is reached. Subsequently, the ratio of females decreases steadily for the larger fish in the western and central Pacific (e.g., Kikawa, 1966; Yesaki, 1983; Yamanaka, 1990). However, in the eastern Pacific, the decrease in number of females is rapid, after reaching the length approximately 140 cm (Wild, 1993).

8. STOCK STRUCTURE, DISTRIBUTION AND MIGRATION

8.1 Stock Structure

There are many studies on stock structure of yellowfin in the western and central Pacific. They derive from morphometric methods (Godsil and Greenhood, 1951; Schaefer, 1955; Kurogane and Hiyama, 1957; Royce, 1964; Schaefer, 1991), circumstantial evidence from fisheries-related information (Yabuta et al. 1958; Kamimura and Honma, 1963; Honma et al., 1971; Suzuki et al. 1978) and from immunological and biochemical methods (Suzuki, 1962; Sprague, 1967; Barrett and Tsuyuki, 1967; Fujino and Kang, 1968).

The morphometric studies tend to show heterogeneity in several areas in the Pacific ranging from very limited areal extent (e.g., Royce, 1964) to ocean-scale such as western, central and eastern Pacific (e.g., Kurogane and Hiyama, 1957; Schaefer, 1991). The inference from the fisheries related information suggests, in most cases, ocean-scale stocks (e.g., Suzuki et al., 1978). The studies using immunological and biochemical methods indicate no difference in the incidence of serum esterase and transferrin systems between fish from the waters of Hawaii, Line Islands and the eastern Pacific Ocean (Barrett and Tsuyuki, 1967; Fujino and Kang, 1968).

With respect to the stock structure in the western Pacific, it was noticed in the Philippine fisheries, one of the most diverse and substantial yellowfin fisheries in the world, that both very small fish from 15 cm to 60 cm and large fish over 110 cm are common, but that middle-sized fish between 60 cm to 110 cm are relatively scarce (e.g., Yesaki, 1983). Since the middle-sized fish have been commonly captured in the offshore oceanic western Pacific by the Japanese longline and to some extent by the industrial purse seine fisheries, the observed apparent size segregation by areas deserves to be analyzed quantitatively from the viewpoint of the stock structure.

As described previously for stock structure in the western and central Pacific, there is no commonly-accepted hypothesis at present. However, for practical reasons due to the existence of large fisheries in the region, some preliminary stock assessments were conducted over the whole western Pacific west of 180° including the Philippine fisheries (e.g., Suzuki et al., 1989).

8.2 Distribution

Higgins (1967) reviewed the available information on the distribution of juvenile yellowfin tuna (12 mm to 300 mm). Juveniles appear to be distributed in the higher latitudes in the western Pacific as far north as the coastal areas of southern Japan (about 30°N) and as far south as 23°S along the Australian coast. In the central Pacific, their distribution band seems to be a little bit narrower, especially to the north, than in the western Pacific.

The geographical distribution of adult yellowfin tuna, studied by using longline data, covers a wide area centering on the tropical waters and extending to the temperate waters (e.g., Suzuki et al., 1978). Adults have a much wider distribution than juveniles, covering the whole area between 40°N and 40°S in the western Pacific and becoming narrower latitudinally toward the central Pacific (Figure 2).

It has been inferred from analysis of fisheries-related information (Suda and Schaefer, 1965; Suzuki et al., 1977) that yellowfin tuna inhabit a relatively shallow swimming layer mainly above the thermocline. Direct studies using sonic tags later verified the previous inference but indicated more dynamic aspects of vertical distribution (Yonemori, 1982; Suzuki, 1984; Holland et al., 1990; Koido and Miyabe, 1990). According to the results of sonic tagging (all experiments so far made on juvenile fish less than about 80 cm) the yellowfin show diurnal, vertical movement shallow at night and deep in the daytime. The study around Hawaii by Holland et al. (1990) indicates that the fish tend to move away from the FAD at night and return to the same FAD the next morning and stay around the FAD during the day. However, studies by Yonemori (1982), Koido and Miyabe (1990) and Suzuki (1984) do not show such a consistent pattern of diurnal, horizontal movement associated with the FAD.

8.3 Migration

The migratory pattern of yellowfin has been inferred from the change in seasonal fishing grounds together with information on size of fish in the catch in the western and central Pacific. There are several tagging experiments pertinent to the migratory aspects, but they are far less extensive than those conducted in the eastern Pacific. Recent, ongoing large-scale tagging by the SPC is expected to provide valuable information on this subject.

The fisheries-related information indicates a fairly clear seasonal movement along the Kuroshio and the East Australian Currents, moving to the higher latitudes in the warmer seasons and returning to the lower latitudes in the colder seasons (Suzuki et al., 1978). There are several instances of juvenile yellowfin tuna tagged in the equatorial Pacific and recovered in the temperate waters of Japan. These indicate that yellowfin has potential for long-distance migration.

Figure 2. Average quarterly distribution, January–March and July–September, of yellowfin tuna longline catch rates in the Pacific Ocean, 1966–1975. Areas of darkest shading denote highest catch rates. (After FAO, 1980).

However, little is known about the east-west migration in the western and central tropical Pacific. As shown by Kamimura and Honma (1963), yellowfin tuna may migrate extensively from the western to central Pacific, however, Royce (1964) suggests that most of them may stay within a range of several hundred miles throughout their life span. It should be mentioned that preliminary results from large scale-tagging programs in the western equatorial Pacific for juvenile yellowfin (mostly between 30 cm and 60 cm in fork length) tend to indicate more east-west migrations than north-south migrations (SPC, 1991a). The net distances traveled by yellowfin tagged within certain intervals of days at liberty are sometimes cited as an indicator of the extent of the migration (e.g., Miyabe and Bayliff, 1987). However, the interpretation of such data requires extreme care since there are many biases and constraints attached to inferring the movement of tunas from tagging experiments (Hunter et al., 1986). One such example is that of adult yellowfin crossing the Atlantic Ocean in significant numbers. Until a large number of large fish were tagged, no such transatlantic migration was discovered.

In summary, the extent of migration of yellowfin tuna remains unknown in the western and central Pacific.

9. NATURAL MORTALITY

Natural mortality coefficients of yellowfin tuna (hereafter shown as annual values) have been estimated through the analysis of the catch curve of the Japanese longline data in the equatorial western and central Pacific (Ishii, 1968; Honma et al., 1971) and by the use of Pauly's equation (Pauly, 1980) for yellowfin in the Philippine waters (White, 1982). The study by Ishii (1968), using a sequential-recruitment model with the catch-at-age data, gives an estimate of 0.9 for fish older than 3 years. This might be an overestimate given the possibility of dispersion of fish from the tropical areas to the higher latitudes. The estimates by Honma et al., (1971), which include emigration, are 2.5 for the western Pacific (west of 180°) and 1.1 for the central Pacific. The difference between the two areas is considered by them to represent fish movement from the western to the central Pacific as they grow. Honma et al. (1971) also calculated a natural mortality coefficient of 0.3 from an empirically-obtained relationship between longevity and natural mortality coefficient for various fishes (Tanaka, 1960).

The estimate by White (1982) for Philippine yellowfin is 0.5 given a mean annual sea surface temperature of 27°C in the southern Philippines.

Natural mortality coefficient estimates in the eastern Pacific ranged from about 0.6 to 1.0 (Cole, 1980).

10. OCEANOGRAPHIC FEATURES ASSOCIATED WITH THE SPECIES

There are a number of studies on relationships between distribution of yellowfin tuna and various oceanographic factors such as surface water temperature (e.g., Broadhead and Barrett, 1964). While the observed relationships work fairly well in specific areas or for specific developmental (e.g., growth) stages of tunas, they are often not applicable on a global scale. One exception to this is the relatively good-agreement between areas with high basic productivity and those with high abundance of yellowfin tuna (Suda et al., 1969 etc.). Nakamura (1965) developed a working hypothesis that each tuna species has its own specific distribution in a specific ocean current with inter-current habitat change depending on the developmental stage. His working hypothesis was based on the observation of longline fishing grounds where the species-specific distribution areas were segregated by longitudinal bands which roughly corresponded to different currents. Yamanaka et al. (1969) analyzed water type (T-S diagram) and distribution of tunas in a similar manner to Nakamura (1965). However, as Nakamura (1965) recognized, there were several later findings that could not be explained by his hypothesis.

Kawai (1969) emphasized the importance of all aspects of the water-temperature structure including not only the water temperature itself but also the several other vital factors such as salinity, oxygen content and productivity. Thus, Kawai (1969) defined the conditions which characterized the main fishing ground of Atlantic yellowfin tuna taken by the longliners. Suzuki et al. (1977) compared the fishing efficiency of the regular and deep longline gear in the western and central Pacific and hypothesized that the main habitat of yellowfin tuna is above the thermocline.

A series of sonic tag experiments has been made for juvenile yellowfin tuna in the western (Yonemori, 1982) and central Pacific around Hawaii (Holland et al., 1990). These results support the hypothesis that swimming depth of yellowfin tuna is mainly above or in the upper part of the thermocline with a tendency for deep swimming in the daytime and shallower swimming at night. The sonic tag experiments by Holland et al. (1990) reveal several behavioural characteristics including diurnal movement pattern away from the FAD at night and towards the FAD in the day. Information was also gained on the effective range of the payao in terms of space, the swimming speed and possible fly-glide behaviour. Further, as mentioned previously, Holland et al. (1990) state that “… different phenomena may underlie the association of tuna with natural debris and their association with the FADs,” i.e., the tunas associated with natural logs and debris in the western equatorial Pacific tended to aggregate around drifting objects during the night and to leave these in daytime.

Recent studies of El Niño events indicate that these occur on a far more global scale than previously thought and that the perturbation of oceanographic conditions in the western equatorial Pacific is as great as that in the eastern equatorial Pacific. Suzuki (1988) compared the hook rate of yellowfin caught by the Japanese longline boats in the western and eastern Pacific with regard to El Niño events. While it was suggested that the hook rates of yellowfin during or one year after El Niño years tends to be higher than those in other years in the eastern Pacific, there was no apparent relationship between the two factors in the western Pacific. Although Suzuki (1988) did not find any changes in annual hook rates of longline-caught yellowfin related to El Niño and non-El Niño years, analysis based on more detailed time-area stratum appears to indicate a slight inverse relationship in the western equatorial Pacific between the depth of mixed layer (shallower in El Niño years) and the hook rates of yellowfin caught by the longline gear (FSFRL, 1987). In the eastern Pacific, there is an indication that the increases in recruitment follow two years after El Niño conditions since the mid-1960s (IATTC, 1989).

11. INTERACTIONS WITH OTHER SPECIES

There is no confirmed information as far as the Japanese purse-seine fishery is concerned that yellowfin associate with dolphins in the western and central Pacific. It is generally known in the purse-seine fishery of the western and central tropical Pacific that juvenile yellowfin tuna, usually when associated with drifting objects, are caught mixed with skipjack and bigeye tunas of similar size, whereas adult yellowfin tuna tend to be taken as a pure school. There are several by-catches of marlins, rainbow runners, triggerfish, etc., caught mixed with tunas in the purse-seine fishery of the western Pacific. Yellowfin also associate with whales and sharks (although not so frequently as with drifting objects) in the purse-seine fishery of the western and central equatorial Pacific (Suzuki 1981). In the purse-seine fishery of the Philippine waters, small juvenile yellowfin are frequently caught mixed with a substantial amount of small tunas and small pelagic fishes such as kawakawa, bullet tunas, and round scad.

Juvenile yellowfin and bigeye are often caught together in the purse-seine fishery; bigeye under 1.5 kg are not separately classified in the Japanese unloading sites. This may have a significant implication for estimates of juvenile bigeye catch since a preliminary study by the National Research Institute of Far Seas Fisheries (NRIFSF, unpublished) shows that about 15% of bigeye were included in the same category as small juvenile yellowfin.

12. GENERAL DESCRIPTION OF THE FISHERIES

The yellowfin fisheries of the central and western Pacific are probably the most substantial and diverse in the world. The fisheries are roughly categorized as purse seine, longline, baitboat, and coastal. The coastal fishery includes various fishing gears such as handline, trap net, trolling, and gillnet. In addition, there are artisanal fisheries throughout the Southeast Asian countries and island countries in the South Pacific. A detailed description of general trends in the tuna fisheries of the western and central Pacific is not available but rough information can be found in a series of publications by the South Pacific Commission (SPC, 1991b etc.). The following is a summary description of yellowfin fisheries by country and gear in the western and central Pacific from various sources.

12.1 Longline Fishery

The yellowfin catch by the longline fishery was the largest in the western and central Pacific until the mid 1970s. Suzuki (1988) describes the development of the longline fishery in the Pacific as follows: “The operation of the (Japanese) longline was confined to the northwestern Pacific and did not last throughout the year in and before the 1940s. In that older period, the fishing boats were used as skipjack baitboats in the warmer months and as longline boats in the colder months of the year. After 1952, the longline fishery developed remarkably equipping with advanced navigation and freezing techniques, and expanded the fishing ground, having covered most of the tropical waters by the mid 1950s. In the late 1960s, the longline fishing grounds expanded toward the higher latitudes reflecting the shift of target species from yellowfin tuna and albacore to bigeye and southern bluefin tunas. After Japanese, Taiwanese and Korean tuna fleets started distant water longlining in this area from 1954 and 1964, respectively. Their major fishing grounds are areas south of the equator.” For distant-water longlining, the Korean fleet sets its targets on bigeye and yellowfin and the Taiwanese fleet on albacore.

Both Japan and Taiwan have coastal and offshore fishing grounds (approximately corresponding to FAO area 61) for longlining, mostly fished by smaller boats (less than 100 GT) than the distant water boats. A significant amount of yellowfin, about 30 to 40 thousand tons combined for the two countries, has been caught by these fisheries.

12.2 Purse-Seine Fishery

At present, the yellowfin catch by the purse seine fishery is the most dominant in the western and central Pacific. Suzuki (1988) summarizes the development of purse seine fishing in the western and central Pacific, “Before the mid 1970s some Japanese purse seiners fished in the western equatorial Pacific Ocean only during northern winter, offseason for exploiting tuna in the Japanese waters. But there were no extensive operations there. In 1977 the Japanese fishermen discovered it easy to catch tunas associated with such materials as logs, and established year round operations in the western equatorial Pacific Ocean. The discovery resulted in a sharp increase in the amount of fishing effort and catch in waters north of Papua New Guinea (Honma and Suzuki, 1978).” In the 1980s, other countries joined the purse-seine fishery in this area including Korea, Taiwan, USA, Philippines, Indonesia, Solomon Islands, etc. This fishery has been the largest yellowfin fishery since the early 1980s. Skipjack is usually dominant in the purse-seine catch, accounting for about two thirds of the total catch. The ratio, however, is often subject to change by country and season. The remainder of the catches is mostly yellowfin with small amount of bigeye as a by-catch.

12.3 Baitboat Fishery

The yellowfin catch from the baitboat fishery has been far smaller than that from other major fisheries. Japan is the only major distant-water baitboat-fishing country operating in the western and central Pacific. Skipjack is predominant in the catch and yellowfin accounts for only a small percentage of the total catch. The Japanese distant-water baitboat fishery is declining due to economic problems caused by such factors as the labour-intensive nature of the fishery.

There are domestic baitboat fisheries in several South Pacific countries including the Solomon Islands, Fiji, Kiribati, Hawaii, etc., all with a relatively small by-catch of yellowfin.

12.4 Philippines and Indonesia

Other than distant-water fishing nations mentioned previously, the Philippines and Indonesia catch a substantial amount of yellowfin. The combined catch of these two countries is the second largest after that by the entire purse-seine fishery. The magnitude of the Philippine and Indonesian yellowfin catch is uncertain since there exists a species separation problem in their reported landings, i.e., other small tuna species are included with yellowfin. The main characteristic of the tuna fisheries in these two countries is the substantial amount of production made by small-scale and artisanal fisheries with diverse fishing gears. Most of the fisheries in these two countries are operated within their respective EEZs but at least 11 and 3 purse seiners have operated beyond the EEZ in 1990 for the Philippines and Indonesia, respectively (SPC, 1991b). The fisheries relating to yellowfin are briefly described below for the two countries.

12.4.1 Philippines

The tunas are exploited by both municipal fishing vessels (less than 3 gross tons) and by the commercial vessels (over 3 gross tons). Fish aggregating devices (FADs), or payaos, characterize almost all the Philippine tuna fisheries. The most important commercial fishing gears are purse seines and ringnets, and for municipal fisheries, hook and line (handlining). However, there is substantial inter-annual variation in catch by gear types both in the municipal and commercial fisheries.

12.4.2 Indonesia

Comprehensive documentation of the Indonesian tuna fisheries, specifying catch by species, fishing gear and area is scarce. Catch data by species, fishing gear and by FAO area are available but most of the catch of yellowfin tuna is listed as unclassified “UNCL” (e.g., IPTP, 1991).

Merta (1985) describes the tuna fishery of Indonesia from 1976 to 1982 and noted that the species category “tuna” in Indonesia includes yellowfin, bigeye, albacore and southern bluefin tuna, but the amount of yellowfin catch appears dominant. The 1982 statistics for “tuna” in FAO area 71 (mostly Pacific side) indicate that the major fishing methods, in order of importance in production in weight, are handlining, trolling and longlining. As in the Philippine tuna fisheries, usage of FADs has assumed major importance.

Recent information on Indonesian tuna fisheries (IPTP, 1991) indicates that the longline, purse-seine and baitboat fisheries all contributed to the substantial, steady increase of yellowfin tuna catch by Indonesia.

13. TRENDS IN CATCH, FISHING EFFORT, AND CATCH PER UNIT EFFORT

Only approximate trends of overall catch of yellowfin tuna by country are available in the western and central Pacific (e.g., FAO Yearbooks) due to the lack of a coordinating organization responsible for collecting the basic catch and effort statistics, such as those by country, fishing gear and area. Except in the case of some fisheries, effective fishing effort on yellowfin and standardized CPUE have not been estimated.

13.1 Catch

The overall catch of yellowfin tuna in the western and central Pacific is reflected in the statistics of the FAO fishing area 71, and to a lesser degree, area 61. Table 4 shows the catch by country and fishing gear (for Japan) for area 71. As mentioned earlier, the annual catch of yellowfin tuna in area 61 is about 30 to 40 thousand tons, caught predominantly by Japan and Taiwan mostly by longline. The overall catch in area 71 has increased from 24 to 213 thousand tons from 1971 to 1986. Before 1973, most of the yellowfin catch seems to have been made by the Japanese longline fishery. The Japanese catch increased steadily till about 1977 when a catch of 51 thousand tons was landed. After a short period of fluctuating catches, the yellowfin catch remained stable after 1980 at a higher level mostly over 70 thousand tons. The Philippine and Indonesian catches appear in 1973 and 1975 in the statistics, but there must have been substantial yellowfin catch before by those countries and some catch by other island countries in this area as well. The Philippine catch does not show any consistent increasing trend but fluctuated between 40 and 60 thousand tons since 1974. The Indonesian catch has been increasing and reached 34 thousand tons in 1986. The USA catch reached a high level in a short time starting in 1980, and produced about 30 to 50 thousands tons after 1982. Most of the USA catch is from purse seining with small catches by artisanal fisheries mainly around Hawaii by longline, handline and troll gears (Coan, 1993).

Table 4. Catch of yellowfin tuna by countries from the FAO Area 71. (After Suzuki et al., 1989).

Unit:ton

The overall catch of yellowfin tuna after 1986 remained at a level similar to the early 1980s, except that in 1987 the catch was 260 thousand tons, mostly due to increased USA purse seine catch in this year (FAO, 1990).

It is possible to describe briefly the trend of yellowfin catch by fishing gear type for area 71. The longline catch has been decreasing, especially from the early 1980s both for Japan and Korea. There appears to be a very small yellowfin catch by the Taiwanese longline fishery in area 71 judging from the fishing grounds mostly in the middle latitudes of the South Pacific aimed at albacore. The overall baitboat catch of yellowfin tuna appears to be stable or slightly decreasing although the catch by the Solomon Islands has increased rapidly during the last few years (SPC, 1991b). On the other hand, the purseseine catch has been increasing notably from the early 1980s. Table 5 shows the increase of the USA purse seine catch, and the recent increase in Korean catch (FAO, 1990) which is also due to increased catch by its purse seiners. Taiwanese catch is not specified in the Table 4. However, the Taiwanese purse seine catch of yellowfin tuna has increased remarkably in the late 1980s (SPC, 1991b). As mentioned previously for the artisanal fisheries, the Philippine catch appears to be stable whereas the Indonesian catch has been increasing although it is not known which kinds of fishing gear are contributing to the increase. Another source of statistics by SPC gives approximate yellowfin catch by country and fishing gear for the SPC area (SPC, 1991b). According to SPC estimates, the purse seine catch dominates, followed by artisanal and longline catches (Table 6). The baitboat catch appears to be very small.

Table 5. USA distant-water purse-seine landings (mt) of tropical tunas in the western and central Pacific. (After Coan, 1993).

Remarks:
--- “-” indicates that landings are not available, but may be greater than zero.
--- “*” indicates values less than 10 mt.
--- Yellowfin tuna landings include some landings of bigeye tuna.
--- Since trips that start late in one year may land their catch in the next, landings in each calendar year may contain some catches from the previous year.
--- Landings before 1979 are from Pacific Tuna Development Foundation exploratory fishing charters; landings from other USA vessels fishing in 1976 to 1978 are unknown.
--- Values in this table for 1980 to 1985 are different from those in Doulman (1987) due to inclusion here of USA vessels operating out of Guam and direct exports.

Table 6. Preliminary estimates of catches (mt) of tunas in the western tropical Pacific Ocean in 1989.

13.2 Fishing Effort and Catch per unit Effort

Description of trends in fishing effort and catch per unit effort of yellowfin tuna in the western and central Pacific, is more difficult than describing catch trends due to lesser availability of the fishing effort data. However, in a general sense, a cursory review of available information suggests that the trend of yellowfin catch closely relates to the corresponding trend of fishing effort. These trends in fishing effort and the CPUE are described as follows separated by fisheries by the major countries which capture yellowfin tuna.

Longline fishing for yellowfin is conducted mainly by the Japanese, Korean and Taiwanese boats. Among them the Japanese catch dominates. Suzuki et al. (1989) estimated the effective fishing effort and CPUE for yellowfin tuna caught by the Japanese longline fishery for the area west of 180° and north of 40°S and south of 40°N during 1952 to 1986. The effective effort peaked in 1962 at about 170 million hooks and then declined to a low in 1970 of about 75 million hooks, probably reflecting the change in target species from yellowfin to bigeye. The effective effort again began to increase and attained an historically high level in 1981 of about 230 million hooks. It then decreased to about 105 million hooks in 1986. The up-and-down trends in this period are due to the effects of introducing deep longlining aimed at bigeye tuna which resulted in overall increase in fishing effort in the area. This was followed by a reduction in domestic fishing effort implemented in the early 1980s. The trend in CPUE based on effective fishing effort in the early 1980s is about one half the level of that at the beginning of the fishery in the 1950s.

Major purse seining countries in the western and central Pacific are Japan, USA, Korea and Taiwan. Korean and Taiwanese purse seine catches of yellowfin in recent years has increased remarkably while Japanese and USA catches have leveled off with a high variability noted in the USA catches (SPC, 1991b). Information on the effective effort by purse seine fishing on yellowfin is not available due partly to difficulty in accounting for significant differences in the catch rates by countries as well as differences by school types, difficulties in separating the effort reasonably between skipjack and yellowfin catches, and difficulty in taking account of a possible improvement in gear efficiency (Tsuji, 1990). The number of purse-seine boats operating in the region has been limited for the Japanese fleet since 1982 and for the USA since 1988. Table 7 shows the trend of nominal fishing effort and CPUE for the Japanese, USA, Taiwanese and Korean purse-seine fleets (SPC, 1990). Fishing effort by the Japanese purse seiners increased sharply until 1984 and subsequently leveled off. The Japanese CPUE shows an increasing trend until 1986 with some drop in the 1982–1985 period but a sharp drop in 1988. No long-term trend of fishing effort by the USA purse-seine fishery is available and caution must be advised on the use of the series of effort and CPUE data of the USA fleets before 1988 because only part of the data is included. The number of USA purseseine boats operating peaked in 1983 at 62 then decreased to 34 in 1989 (Coan, in press). Similar to the Japanese CPUE trend, the USA fleet experienced a sharp drop in CPUE in 1988 while no such drop was shown in the catch rate of the Taiwanese and Korean purse seine fisheries.

There are few references on the general trend in fishing effort and CPUE exerted on yellowfin by the various fisheries of the Philippines and Indonesia. In the Philippine tuna fishery, the statistics for the municipal fishery are not precise, but for the commercial fishing boats no trends appear in the number of boats in major fisheries except for an increasing trend of ringnet vessels (Barut and Arce, 1991). In the Indonesian fisheries, the number of longline boats of both domestic and foreign registration started to increase rapidly after the mid 1980s (Naamin and Bahar, 1991). However, most of that longline fleet appears to be operating in the Indian Ocean.

Table 7. Effort (days fishing and searching) and CPUE (tons) for the major purseseine fleets of the western Pacific. (After SPC, 1990).

The USA 1988 and 1989 data were substituted by estimates of Coan (1993).

14. POPULATION DYNAMICS

Kamimura et al. (1966) assessed the Pacific yellowfin tuna stock, mostly covering the western and central tropical Pacific, based on the Japanese longline data from 1955 to 1964. Their analysis indicated that the longline CPUE in the western Pacific west of 180° did not show any appreciable decreasing trend while that in the central (180°–150°W) and eastern (150°W–120°W) Pacific showed a decreasing trend. The CPUE decreased to one half, and then to one fourth of its original level over the period of the study. They found a significant difference in age composition of the catch between the western and central Pacific, i.e., in the west it was composed mostly of young fish around age 3, while in the central Pacific the fish were older than age 3. During the period of the study, the dominant age in the catch in the central Pacific shifted from age 4 to age 3. These changes were interpreted by the authors to mean that the longline exploitation of yellowfin tuna in the western and central Pacific had reduced the adult stock size, but that the recruitment to the longline fishery was not affected by the increased fishing effort. They concluded that the longline fishery operated around the MSY level in the mid 1960s.

A similar analysis was conducted by Honma et al. (1971) using a more comprehensive data series covering the period from 1950 to 1964. Their conclusion was close to that of Kamimura et al. (1966) indicating that the longline fishery was exploiting the stock around the MSY (about 30 thousand tons from the tropical Pacific) in the mid 1960s. Furthermore, Honma et al. (1971) estimated the recruitment in number of fish at age 1.5 assuming M=0.3 and 0.8. It ranged from 1.7 to 3 million and from 4.5 to 7 million for the two values of M. It should be noted that these values are applicable to tropical areas only, as in the case of the MSY previously mentioned. The follow-up studies indicate a much higher MSY for the yellowfin stock available to the longline fishery, from some 60 thousand tons (Honma, 1974) to 70–110 thousand tons (Suzuki et al., 1989).

Preliminary Y/R analysis of data from the purse seine (Japan and USA), longline (Japan, Korea and Taiwan) and Philippine fisheries suggests that increasing purse-seine fishing effort beyond the 1980–1982 level will not increase the total catch due to the offset of increased catch of purse-seine fishery by reduction of catch in the longline fishery (Suzuki, 1986). However, the total catch from the western and central Pacific after 1982 increased sharply although the longline catch decreased. Suzuki et al. (1989) suggested from the catch trend of the major yellowfin fisheries and catch-at-age information that the yellowfin stock in the western and central Pacific was harvested at a sustained level of 200–210 thousand tons in the 1980s.

White (1982) analyzed the Philippine tuna-fishery data and made several attempts to estimate the effects of limiting the fishing mortality of a specific gear type on various other fisheries and on the total fishery. However, as he stated, it was hard to estimate fishing mortality specific to the Philippine fisheries because yellowfin tuna caught in the Philippines are probably a part of the offshore oceanic stock of the western and central Pacific.

15. INTERACTIONS AMONG FISHERIES

One of the major concerns of interactions among the fisheries in the western and central Pacific has been the possible impact of the increasing purse seine fishery on either the longline fishery or the coastal fisheries.

At present, no concrete information regarding interaction is available; this shortcoming is largely due to a lack of basic statistics needed for stock assessment. A theoretical study indicates that the total yield from the yellowfin stock is larger in the case of a coexistence of a surface and subsurface fishery than in the case where either one of the fisheries operated solely in the area (Lenarz and Zweifel, 1979). Hilborn (1989) supported the result by Lenarz and Zweifel (1979) and showed more explicitly that the total yield could be maximized if the yellowfin stock fished by the longline fishery was somewhat discrete from that fished by the surface fisheries. If the two fisheries were fishing a homogeneous common stock, a longline fishery would maximize the yield.

A preliminary study of the CPUE for yellowfin tuna in the western and central Pacific based on the SPC data indicates that there is no evidence that the stock available to the surface fisheries has been significantly affected by fishing activity during 1978 and 1988 (Hampton, 1988). Suzuki et al. (1989) calculated the trend of CPUE for yellowfin taken by the Japanese longline fishery in the western Pacific west of 180°. The recent trend showed a decrease from a high peak in 1978 to a low in 1986 (Figure 3). During this period the yellowfin catch by the purse seine fishery increased sharply, especially after 1982 (Table 4, Japanese purse seine catch plus the USA catch). However, it should not necessarily be assumed that this reveals the impact of the purse seine fishery on the longline fishery as the decline of the longline CPUE appears to have started before the substantial increase in the purse seine catches. In addition, there is a 1–3 years age gap in longline and purse seine caught yellowfin. When the gap is accounted for, the decline of the longline CPUE started too early if the decline is assumed caused by the purse seine fishery.

Figure 3. Trend of CPUE for yellowfin taken by the Japanese longline fishery in the western Pacific. (After Suzuki et al., 1989). The HONMA1, HONMA2(DL-AD), and GLM denote Honma's method without deep and regular longline adjustment, with adjustment, and general linear model, respectively.

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