A REVIEW OF THE BIOLOGY AND FISHERIES FOR LONGTAIL TUNA (THUNNUS TONGGOL) IN THE INDO-PACIFIC REGION

Mitsuo Yesaki
Indo-Pacific Tuna Development and Management Programme
Colombo, Sri Lanka

1. INTRODUCTION

This review is abridged from the species synopsis compiled by Yesaki (1987). Information published subsequent to 1987 has been added.

2. CLASSIFICATION

Kingdom Animalia
    Phylum Chordata
        Subphylum Vertebrata
            Superclass Gnathostomata
                Class Osteichthyes
                    Division Teleostei
                        Cohort Acanthopterygii
                            Order Perciformes
                                Suborder Scomberoidei
                                    Family Scombridae
                                        Subfamily Scombrinae
                                            Tribe Thunnini
                                                Genus Thunnus
                                                    Species tonggol

Thunnus tonggol is included in the subgenus Neothunnus with two other tropical species of this genus, T. albacares and T. atlanticus (Collette, 1978).

3. EARLY LIFE HISTORY

There is no information on the fertilized eggs on longtail tuna. Rao (1964) collected ripe residual ova from a spent ovary of an 81-cm fish captured in September in India. These ova were translucent, with average diameter of 1.09 mm and an oil globule varying in diameter from 0.31 to 0.33 mm in fresh condition.

Chayakul and Chamchang (1988) described the morphological development of longtail tuna larvae captured in the Gulf of Thailand. Larvae of this species have been collected in the Gulf of Thailand in January, February, April, May, and June (Chamchang and Chayakul, 1988) and off the west coast of Thailand in February, March, and April (Boonragsa, 1987). Larvae have also been reported from Malacca Straits (Wilson, 1981b) and South China Sea (Chen and Wei, 1981). There are relatively few reported collections of longtail tuna larvae; the paucity of larval and postlarval specimens of this species may be due either to the fact that its distribution is restricted to the neritic regime or to misidentification as a more common, better-known species.

Preadult longtail tuna, as small as 20 cm, are captured by luring purse seines off the west coast of Thailand. Preadults were also captured during exploratory fishing cruises by pole and line and troll line off this coast. The smallest fish captured by these gears were 24 and 28 cm, respectively. Preadults with mode at 30 cm were captured during exploratory fishing cruises in March 1980 and February 1981 (Yesaki, 1982). Many schools of preadults were sighted during these months, but were fished only for samples because of the small size of the fish (Lee, 1982).

Twenty-two-cm preadults are recruited to the purse-seine fishery in the Gulf of Thailand (IPTP, 1985) and slightly larger fish (26 cm) enter the troll fishery off the east coast of Peninsular Malaysia (IPTP, 1986). Preadults are captured in drift gillnets at Cochin in February (Silas, et al., 1986a) and at Mangalore in October (Muthiah, 1986).

4. AGE AND GROWTH

Several growth studies based on length data have been completed for longtail tuna. These include studies where modal lengths have been assigned ages (Serventy, 1956; Chiampreecha, 1978; Klinmuang, 1978; Yesaki, 1982), modal progressions were followed by the “pen and paper” method (Wilson, 1981b; Supongpan an Saikliang, 1987; Yesaki, 1989) and modal progressions were determined by the ELEFAN (Electronic Length-Frequency Analysis) software package (Silas et al., 1986b; Prabhakar and Dudley, 1989). Only one study based on hard parts has been completed to date for longtail tuna. Wilson (1981b) described the growth of this species by counting increments on otoliths of fish captured in Papua New Guinea. He did not validate increments as daily events in this study. Growth parameters and/or lengths-at-age derived by the various authors are summarized in Table 1 and Figure 1.

The growth curves derived by Wilson (1981b) from otolith increment counts and length frequencies and by Yesaki (1989) from length frequencies grouped by 10-day intervals are similar up to age 3. These growth curves differ from the remaining curves in having larger lengths-at-age. Derivation of similar results by two different methods gives credence to the fast-growth curve as best describing the growth of longtail tuna. The results of tagging experiments lend further support for the fast-growth curve as the most appropriate. Release and recapture lengths of three longtail tuna recovered after relatively long intervals at liberty were superimposed on the growth curves derived by Wilson (1981b) and Yesaki (1989) (Figure 2). Increases in length of tagged fish during the interval at liberty correspond closely with the growth predicted by these curves.

5. MATURATION, SPAWNING, AND SEX RATIO

5.1 Maturation

Yesaki (1982) determined sexual maturity of longtail tuna by visual examination of ovaries and assigned maturity stages according to a 5-point scale. The characters used to define maturity stages were size of gonad, colour, degree of softness, and turgidity. Of 796 females examined, the smallest mature female (stage-IV) was 43 cm. Cheunpan (1984) examined 939 females for ovary weight and used gonad index to determine sexual maturity with size. Fifty percent of the females were sexually mature at 396 mm.

Table 1. Summary of age and growth studies on longtail tuna by length-frequency distributions and hard parts.

¹ - relative age

Figure 1. Estimates of growth and relative age (yr) of longtail tuna from length-frequency analyses (crosses: manual method; triangles: computer programmes; squares: hard parts) (from Yesaki, 1989).

Observations of fish captured in Australia prompted Serventy (1956) to conclude that fish in the size group with mode at 38 cm were immature and in the group with mode at 51 cm were maturing, as these had enlarged, developing gonads. Wilson (1981b) found maturing ovaries first appeared in fish of about 60 cm in Papua New Guinea. These studies suggest that longtail tuna from Southeast Asia mature at a smaller size than do fish from Australia-Papua New Guinea.

5.2 Spawning

Klinmuang (1978) estimated numbers of eggs in ovary pairs of four fish. Fecundity of longtail tuna ranging in size from 43.8 to 49.1 cm varied from 1.2 to 1.9 million eggs and averaged 1.4 million eggs. Wilson (1981b) determined fecundity by counting oocytes in the most advanced mode of six maturing females (ranging in size from 75.5 to 98.0 cm) captured in Papua New Guinea. Oocyte counts ranged from 0.8 to 1.9 million per female.

Figure 2. Growth curves derived from otolith increment counts (squares) and length-frequency distributions (crosses) with growth increments of long-term tag recoveries of longtail tuna.

Longtail tuna, like other species of tuna, probably spawn more than once a year.

Mature (stage-IV) and spent (stage-V) female longtail tuna were captured in the outer-neritic and restricted inner-neritic regime north of Phuket Island during the northeast monsoon (Yesaki, 1982). Mature females were also captured in the outerneritic regime during the southwest monsoon. It was postulated that longtail tuna spawn principally in the outer-neritic regime along the entire west coast of Thailand. Stage of maturity indices collected during a 2-year period off the west coast of Thailand were grouped by month to show development of maturity through the year and to define spawning seasons. The proportion of maturing females (stage-III) was high in January-March, decreased to lowest levels in May-August and thereafter increased in highest level in December. Mature females were found in January-April and again in August-September. Spent females were observed in January and March. There appear to be two distinct spawning seasons for longtail tuna; a major spawning during the northeast monsoon from January through April, with probable peak in March, and a minor spawning during the southwest monsoon in August-September off the west coast of Thailand.

Cheunpan (1984) also suspected two spawning seasons for longtail tuna in the Gulf of Thailand from peaks in gonad indices in March-May and July–December.

Wilson (1981a) surmised from gonadal development of fish captured in Papua New Guinea and the absence of adult fish during October-April that longtail tuna spawned during the austral summer. Serventy (1956) observed fish with definitive ova and others with milt flowing in spring (September, October) and examined fish with spent gonads in autumn (April) off New South Wales. He assumed longtail tuna spawned during the southern hemisphere summer in Australia.

5.3 Sex Ratio

There are few reports in the literature on sex ratios of longtail tuna. Klinmuang (1978) reports a 1:1 ratio for 141 fish from the South China Sea. Yesaki (1982) found no significant differences in sex ratio for fish captured by pole and line (1:1 and troll line (0.97:1) off the west coast of Thailand. Numbers of males and females by 10-cm intervals and for total numbers were not significantly different for longtail tuna captured in Papua New Guinea (Wilson, 1981b).

6. STOCK STRUCTURE, DISTRIBUTION, AND MIGRATION

6.1 Subpopulations and Stocks

Several authors have noted differences in characters of longtail tuna from various areas throughout its range. Serventy (1956) found body depth and distance from tip of snout to origin of first dorsal fin of fish from western Australia to differ significantly from those measurements for fish of northern and eastern Australia. Wilson (1981b) concluded from a study of 13 morphometric measurements that fish from western Australia had larger head, deeper body, and longer pectoral fin than fish from Papua New Guinea. Gibbs and Collette (1967) found total gillraker counts for fish from Southeast Asia and Australia to be similar to one another and less than counts of fish from India and Red Sea. Silas (1967) found significant differences in the number of gillrakers, especially on the lower limb, for fish from western and northern Australia versus fish from India. Significant differences were also found in the number of gillrakers for fish from Oman, as compared to those from the Gulf of Mannar in the southeastern part of India (Abdulhaleem, 1989). Lewis (1981) found fixed and large differences in enzyme frequencies for longtail tuna from Malaysia and Australia. These differences in morphometric, meristic, and electrophoretic characters may reflect distinct stocks. Longtail tuna is essentially a neritic species with most phases of its life cycle confined to the continental shelf. Therefore, it is highly probable that there are numerous stocks (self-sustaining units) throughout the distributional range of the species.

6.2 Distribution

The general distribution of longtail tuna extends from the coast of Somalia, Gulf of Aden, Red Sea of Japan, Papua New Guinea, Australia, and New Zealand (Figure 3). Longtail tuna is basically confined to the neritic regime (Anon., 1975).

Figure 3. Distribution of longtail tuna.

In the Indian Ocean, the southernmost distribution of longtail tuna off the east coast of Somalia is at about Ras Hafun. It occurs in the Gulf of Aden along the north coast of Somalia (Losse, 1970) and coast of PDR Yemen and in the Gulf of Oman off Oman and United Arab Emirates. In the Red Sea, this species has been reported from around the Dahlak Archipelago of Ethiopia (Ben Yami, 1968) and from Jizen, Saudi Arabia (Peacock and Alam, 1977), but probably extends further north along both coasts. The distribution of longtail in the Persian Gulf extends along the Iranian coast to Bushehr and along the Arabian side to Bahrain. It has a continuous distribution along the continent of Asia from Iran to mainland China. The northeastern extent of its range off China is unknown, but longtail tuna have been reported from Wakasa Bay on the Japan Sea coast of Japan (Nakamura, 1969) and off Taiwan (Chi and Yang, 1971).

Longtail tuna do not occur in the Lakshadweep Islands of India (Jones and Kumaran, 1980) and have not been found in the Maldives. The species is, however, found in Sri Lanka (Munro, 1955), Nicobar, and Andaman Islands. The latter island groups are nearer to the Asian continent and large island (Sumatra) than the former groups, though the difference in distances is slight. Distance does not appear to be the limiting factor, but rather the extent of the continental shelf. Lakshadweep and Maldives are comprised of very small islands, with restricted continental shelves so the waters over them would be essentially oceanic in character and unsuitable for longtail tuna.

This species occurs throughout the northwest and south coasts of Irian Jaya and south coast of Papua New Guinea (Munro, 1967). Its distribution probably does not extend further eastward. Longtail tuna have not been reported in the Solomon Islands chain and its presence off the north coast of Papua New Guinea and in New Britain (Serventy, 1956) has been questioned (A. Lewis, person. commun.). Longtail tuna have also been reported from New Zealand (Wilson, 1981a), but this has to be confirmed as there may be misidentification with bigeye tuna (Thunnus obesus) (Gibson¹).

Troll-line catches off the west coast of Thailand averaged 0.31 kg/line-hour in the neritic regime and only 11 of a total of 604 troll-hooked longtail tuna were captured in the oceanic regime. However, these fish were all captured in grid areas which included small segments of the continental edge (Yesaki, 1982). This species was not found under a FAD (fish aggregating device) deployed in 432 m west of Ranch Yai Island. On the other hand, kawakawa (Euthynnus affinis) were captured around the FAD after 2 months soaking and kawakawa and skipjack tuna (Katsuwonus pelamis) after 3 months soaking (Lee, 1982). Longtail tuna have been reported occurring on the edge of shelf areas of eastern Indonesia and in the Philippines (A. Lewis, pers. commun.).

There are few reported catches on longlines, which support the contention that longtail tuna is a neritic species. One longtail tuna was captured in the Andaman Sea during cruises conducted by the Exploratory Fishing Division of the Department of Fisheries, Thailand (Poreeyanond and Kambud, 1985). Exploratory fishing trials with modified longlines on the north coast of Somalia captured yellowfin tuna (Thunnus albacares) and a few kawakawa, but no longtail tuna (Losse, 1975).

¹ Gibson, D. (pers. comm.) Marine Resources Assessment Group, Imperial College of Science and Technology, 48, Princess Gardens, London, SW7 ILU, U.K.

6.3 Migrations and Local Movements

In Australia, longtail tuna are found in the tropical province along the northern coast throughout the year. The species migrates into the subtropical province along the east coast during the austral summer and autumn. No definite seasonal movement is apparent on the coast of Western Australia (Serventy, 1956). There are differences in the geographic distribution of size groups; fish in the 30- to 70-cm interval are found off northern Australia, whereas fish with a mean length of 94 cm are encountered at the southern extremity of its range off the east and west coasts. Longtail tuna captured during summer in the northern extremity of its range off Japan, however, are smaller. Fish ranging in size from 28 to 30 cm have been reported from Tsushima Island (Fukusho and Fujita, 1972) and 49 to 51 cm from Wakasa Bay (Nakamura, 1969) in September.

Pole-and-line fishing explorations off the west coast of Thailand delineated juvenile longtail tuna in the inner-neritic regime southwest of Phuket (Yesaki, 1982). These fish move offshore with increasing size and have essentially all departed from the inner-neritic by 40 cm. Longtail tuna in the 40- to 49-cm size range are distributed principally in the outer-neritic and is the dominant species in this regime. Larger fish (more than 50 cm) were rarely captured during the investigations and it was speculated that these fish emigrate out from the coast.

7. NATURAL MORTALITY

Pauly's (1980) empirical equation based on growth parameters and mean water temperatures have been used by various authors to estimate natural mortality coefficients of tropical species. Prabhakar and Dudley (1989) obtained M of 0.429 for longtail tuna (sizes ranging from 22 to 116 cm, fork length) in Oman with input values for K of 0.228, L of 133.6 cm and mean water temperatures of 25.5°C.

8. OCEANOGRAPHIC FEATURES ASSOCIATED WITH THE SPECIES

Longtail tuna were captured with pole and line and /or troll line throughout the year during exploratory fishing cruises off the west coast of Thailand. Sea-surface temperatures during these cruises ranged from 28.0° to 31.3°C (Yesaki and Jantarapagdee, 1981). Pole-and-line catches of longtail tuna were highest from January to April, with peak in March (Lee, 1982). During this period, mean sea-surface temperatures were rising from 28.6°C in December-January to a high of 30.2°C in June. Water over the mid and outer continental shelf was strongly stratified with a thin veneer of warm water and rapidly decreasing temperatures with depth. The 25°C isotherm was stationary at 50 m from December to March. During the southwest monsoon, when longtail tuna catches were poor, temperatures were essentially uniform from surface to bottom over the mid and outer shelf. Temperatures during September were 29°C at the surface and 25°C at 110 m.

9. INTERACTIONS WITH OTHER SPECIES

Longtail tuna are generally found in the neritic regime with kawakawa and frigate tuna (Auxis thazard). Ben Yami (1968) gives the relative importance of pelagic fish around the Dahlak Archipelago, in decreasing order of importance, as kawakawa, longtail tuna, Indian mackerel (Rastrelliger kanagurta), queenfish (Scomberoides lysan), and frigate tuna. The subordinate rank of longtail tuna in the area to kawakawa may be attributed to the western extremity of its geographic range.

The chartered purse-seine vessels Southward Ho and Royal Venture conducted several exploratory fishing cruises for small tuna and other pelagic species in the eastern Andaman Sea and South China Sea from November 1975 to April 1977 (Simpson and Chikuni, 1978). They reported all three species of small tuna (longtail, kawakawa, frigate) were common throughout the coastal waters, with longtail tuna more abundant off Peninsular Malaysia and Thailand than off Sabah and Sarawak. Small tuna schools were common from the coast out to some 40 miles offshore and were occasionally seen more than 60 miles from shore, but were not encountered beyond the continental shelf.

The relative abundance of large pelagic fish off the west coast of Thailand was estimated by Yesaki (1982) from troll catches and/or visual identification of fish schools and the composition of pole-and-line catches. Longtail tuna was the dominant species, accounting for 60–70%, and kawakawa an abundant species, representing 17–19% of all fish schools off this coast. Frigate tuna were not as common, though the relative abundance of this species was probably underestimated because of the selectivity of the pole-and-line for larger fish. Species composition of schools fished with pole and line suggests a non-preferential association of longtail tuna with the other three tuna species (kawakawa, frigate, and skipjack).

Longtail tuna were found with only 5 out of a total of 510 porpoise pods observed and with none of the 6 whales sighted off the west coast of Thailand. However, this species was found under a school of whale sharks (Rhincodon typus) and successfully fished with pole-and-line (Yesaki, 1982).

Seventy-three percent of all fish schools sighted during pole-and-line activities were accompanied with birds. Percentages of longtail, kawakawa, skipjack, and frigate tuna schools accompanied by bird flocks were similar and ranged from 68% to 75% (Yesaki, 1982). Sooty terns were dominant and especially common in the oceanic and outer-neritic regimes. White terns and gulls were more common in the inner-neritic, whereas frigate birds and shearwaters were common throughout the neritic regime (Lee, 1982).

10. GENERAL DESCRIPTION OF THE FISHERIES

There are two major fishing grounds for longtail tuna, one off the South China Sea coast of Thailand and Malaysia and the other off the countries bordering the North Arabian Sea.

10.1 South China Sea

Descriptions of the purse-seine and gillnet fisheries of Thailand and the purseseine, gillnet, and troll fisheries of Malaysia are given in Yesaki (1993).

10.2 North Arabian Sea

Countries exploiting longtail tuna in the North Arabian Sea include Oman, United Arab Emirates, Iran, and Pakistan. Gillnet is the most important gear used to capture longtail tuna in all these countries. Troll lines are also used in the United Arab Emirates and Oman. Two types of vessels are engaged in fisheries for longtail tuna. One type is the traditional design dhow constructed of wood ranging in length from 10 to 25 m. These vessels are powered by inboard diesel engines of 33–240 HP. The other type is the planing hull skiff constructed of glass-reinforced plastic (GRP) ranging in length from 5 to 9 m. These skiffs are powered by outboard engines of 25 to 120 HP. The large wooden dhow is the predominant type used in Pakistan and Iran, whereas the small GRP skiff is more common in the United Arab Emirates and Oman.

Gillnets fished by the small GRP skiffs are 1,000 to 1,700 m long and those used by the large wooden dhows are 2,500 to 9,000 m long. These gillnets are approximately 12 m deep with mesh size between 10 and 16 cm, with the most common being 13 cm. One line, usually with a single hook, is fished off the stern of the small GRP skiffs engaged in the troll fishery.

Pakistani gillnetters fish throughout the North Arabian Sea to the coasts of Oman, P.D.R. Yemen, and Somalia. Gillnetters of Oman and the United Arab Emirates confine fishing operations to continental shelf waters of their respective coasts.

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

11.1 Annual Catches

Nominal catches of longtail tuna in the Indo-Pacific region increased from 19,000 mt in 1979 to 116,000 mt in 1988 (Table 2). Thailand is the most important producer of longtail tuna, accounting for 54% of the total catch in 1988, followed by Iran with 15% and Oman with 13%.

11.2 Fishing Effort and Catch Per Unit of Effort

A relatively long time series of catch and effort statistics is available only for the purse-seine and gillnet fisheries of Thailand and the troll fishery of Malaysia. However, catch rates for the Thai purse-seine and gillnet fisheries are not reliable indices of abundance because these fisheries target a variety of species and/or species groups. Also, information of luring, one-boat and tuna purse seines are included in the purse-seine fishery. These purse seines are fished differently and target different species groups. Luring purse seines are fished in conjunction with FADs and night lights to capture small pelagic species, whereas one-boat purse seines are used to capture free-swimming schools of small pelagic species. Small tunas are generally captured only incidentally by these purse seines. Tuna purse seines are used to capture free-swimming schools of large pelagic fish, including small tunas, torpedo scad (Megalaspis cordyla), yellowtail scad (Caranx mate), etc. (Supongpan and Saikliang, 1987).

The Malaysian troll fishery targets small tuna so catch rates are reliable indices of abundance for these species. A tuna sampling programme was initiated by the Terengganu Marine Fisheries Resources Research Center in 1982. This programme was continued at various levels of intensity through 1986. This Center initiated another tuna sampling programme with assistance from the Indo-Pacific Tuna Development and Management Programme (IPTP) in January 1987. Effort, catch. and species composition are available for most months from August 1982 through December 1989, except for all months of 1986.

Catch and effort for the Thai purse seine and gillnet and Malaysia troll fisheries show different trends (Figure 4). The Thai purse-seine fishery shows increasing catch and effort from 1979 to 1988, whereas the Thai gillnet fishery shows increasing effort after 1985 with decreasing catch after 1986. Catch and effort of the Malaysian troll fishery was higher during the 1982–1985 interval than during the 1987–1989 interval.

Catch rates of longtail tuna by the Thai purse-seine fishery were at low levels till 1982, than increased markedly to highest level in 1983 after this fishery started targetting for small tuna and developed a net specifically for large pelagic species (Figure 5). Purse seine catch rates have fluctuated between 200 and 300 kg/day since 1984. Gillnet catch rates of longtail tuna were low till 1980, then increased to highest level in 1982 after this fishery started targetting for small tuna. Gillnet catch rates decreased to moderate levels in the 1983–1986 interval, then decreased to lowest levels in 1987–1988. These low catch rates for the most recent years may reflect a shift back to targetting for king mackerel. Average troll-line catch rates of longtail tuna decreased from 240 kg/trip during the 1982–1985 interval to 115 kg/trip during the 1987–1989 interval.

Table 2. Landings in metric tons of longtail tuna in the Indo-Pacific region by country and year.

Source - IPTP, 1991
¹ - SEAFDEC, 1981; 1982; 1983; 1984; 1985; 1986; 1987; 1988; 1989; 1990

* - estimated - MALAYSIA - 1979/1982 - 49% of TUN (% from 1984 - LOT+KAW+TUN)
-                   - 1989 - 33% of TUN (% from 1988 - LOT+KAW+FRZ+TUN)
- PAKISTAN - 1979/1984 - 32% of KAW+SKJ (% from 1986 - YFT+LOT+KAW+SKJ+FRZ+TUN)
- THAILAND - 1988/1989 - Gulf of Thailand landigs from logbook survey

Figure 4. Catch and effort of longtail tuna for purse-seine and gillnet fisheries of Thailand and troll-line fishery of Malaysia.

12. POPULATION DYNAMICS

Stock assessment studies have not been completed for longtail tuna because of the lack of a long time series of reliable catch, effort, species, and size composition data.

Figure 5. Catch per unit effort of longtail tuna for the purse-seine and gillnet fisheries of Thailand and troll-line fishery of Malaysia.

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