A REVIEW OF THE AUXIS FISHERIES OF THE PHILIPPINES AND SOME ASPECTS OF THE BIOLOGY OF FRIGATE (A. THAZARD) AND BULLET (A. ROCHEI) TUNAS IN THE INDO-PACIFIC REGION

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

and

Flerida Arce
Bureau of Fisheries and Aquatic Resources
Quezon City, Philippines

1. INTRODUCTION

The “Synopsis of biological data on frigate tuna, Auxis thazard, and bullet tuna, A. rochei,” was published in 1981 (Uchida, 1981). This synopsis summarizes all available information concerning these species throughout the tropical and subtropical oceans and seas. There has been little new information published on the biology of these species since this date.

The world catch of Auxis species has increased 62% since publication of the above synopsis and in 1988 was 176,000 mt, of which 107,000 mt (61%) was caught in the Philippines (FAO, 1988). The only other country reporting catches exceeding 10,000 mt was Japan with 25,000 mt (14%). The total world catch is low considering it is generally acknowledged that Auxis is the most abundant tuna, in numerical terms, in the world's oceans (FAO, 1976). There are several reasons for the present low catch of Auxis including the following:

1. Low demand because there is no international market for these species.

2. Under-reporting of catches, especially by the tuna purse-seine fleets operating in the Pacific, Atlantic, and Indian Oceans.

3. Confinement of fisheries in many developing countries to near-shore areas of low Auxis abundance.

4. Primitive level of development of purse-seine fisheries in most developing countries.

The present review of Auxis consists of two parts. The first examines in detail the fisheries of the Philippines. The Philippines is the only country in the world with highly developed fisheries for Auxis, so a review of its fisheries will cover many essential aspects of Auxis fisheries. The second part summarizes current information for the Indo-Pacific region on the biology of Auxis species, especially on reproduction and population characteristics which are essential for the assessment of stocks.

2. THE AUXIS FISHERIES OF THE PHILIPPINES

2.1 Source of Information

Information presented in this review of the Auxis fisheries in the Philippines was obtained primarily from three sources. These were:

1. Fisheries Statistics of the Philippines, Volumes 28–37 published by the Bureau of Fisheries and Aquatic Resources (BFAR), Ministry of National Resources, Metro Manila, Philippines.

2. The BFAR, under the supervision of the South China Sea Fisheries Programme and with funding from Norway through the Programme of Assistance to Developing Countries in Planning the Utilization of the Living Resources in the Expanded Economic Zones (EEZs), initiated a 1-year tuna-sampling programme in late 1979. The objectives of this programme were to obtain data on species and length compositions, and seasonality of tuna and associated species landed at four sites on Mindanao Island (White and Yesaki, 1982). These included General Santos (Moro Gulf), Santa Cruz (Davao Gulf), Labuan (East Sulu Sea) and Opol (Bohol Sea). The BFAR continued the tuna sampling programme at these and other sites after 1980 with assistance from the Japan Trust Fund Project through the Indo-Pacific Tuna Development and Management Programme (IPTP). Sampling effort was directed primarily towards monitoring the catches of ringnets and handlines and secondarily for purse seine, bagnet, gillnet, and troll lines. However, only information for ringnets at General Santos, Santa Cruz, and Opol and for purse seine at General Santos was summarized in the present analysis, as Auxis catches by the other gears were either insignificant or erratic. Also, other landing sites besides the above mentioned four were monitored for varying periods of time. Information from some of these sites were included with the major sampling sites if the sites were close to each other and the vessels were similar in size. In the present analysis, Malita was included with Santa Cruz and Initao with Opol.

3. The BFAR initiated a small-pelagic species sampling programme at various locations throughout the Philippines in 1983. These included Wawa and Lemery in Batangas Coast, Danao in Camotes Sea, Daliao, and Talomo in Davao Gulf. The bagnet fishery at Wawa and the ringnet fishery at Lemery were monitored for 19 and 21 months respectively, whereas, the ringnet fisheries at the other sites were monitored until the end of 1989. Auxis constituted an important component of the landings at all these sites.

2.2 Tuna Landings

The Philippine landings of tuna increased from 184,000 mt in 1978 to 302,000 mt in 1989 (Figure 1). Auxis is the most important tuna, in terms of landed weight, accounting for 39% of the 1989 total. Skipjack (Katsuwonus pelamis), yellowfin (Thunnus albacares), and kawakawa (Euthynnus affinis) each accounted for approximately 20% of the remainder. Auxis and kawakawa landings increased by 131% and 59%, respectively from 1978 to 1989. On the other hand, yellowfin and skipjack landings increased by only 32% and 30%, respectively, during this 11-year interval. Landings of yellowfin peaked in 1985 and those of skipjack in 1986. Landings statistics for these two species in recent years may be overestimated by inclusion of fish captured in extra-territorial waters. Tuna purse seiners from at least three Philippine companies started fishing in the western Pacific Ocean in 1985. Yellowfin and skipjack captured by these vessels may be included in the Philippine landing statistics.

Fig. 1. Tuna landings by species in the Philippines from 1978 to 1989 (FRZ includes frigate and bullet tunas; YFT includes yellowfin and bigeye tunas).

2.3 Landings by Gear Type

Philippine landings statistics are classified into commercial and municipal fisheries on the basis of fishing craft size. Crafts less than 3 gross tons (GT) are considered as municipal and larger crafts as commercial. In 1978, Auxis landings by the municipal sector exceeded that by the commercial sector and in the following year landings by these two sectors were about equal (Table 1). In all subsequent years except 1983, the landings by the commercial sector exceeded those of the municipal sector.

Twelve municipal fishing gears are listed in Philippine statistics as capturing Auxis (Table 1). The most important is the hook and line (including troll line, and pole and line), which accounted for 32–57% of the municipal landings during the 1978–87 interval. Other important gears are gillnet (8–29%) and purse seine/ringnet (5–40%).

Table 1. Landings of Auxis by gear type in the Philippines from 1978 to 1987 (metric tonnes).

¹ - includes beach and round-haul seines
² - includes handline and troll-line and pole-and-line
³ - includes others and driven-in net
⁴ - includes round-haul seine

Eight commercial fishing gears are identified as capturing Auxis (Table 1). Purse seine/ringnet is the most important gear accounting for 62–90% of the commercial landings from 1978 to 1987. The other important gear is the bagnet with catches making up 8–36% of the commercial landings of Auxis.

2.4 Landings by Fishing Areas

Twenty-four fishing areas are identified for the Philippine landing statistics (Figure 2). The bulk of the Auxis landings is made in archipelagic waters, with negligible catches from the fishing areas along the exposed Pacific Ocean and South China Sea coasts (Table 2).

Fig. 2. Map of the Philippines showing territorial limits and fishing areas.

Table 2. Landings of Auxis by fishing areas of the Philippines from 1978 to 1987 (metric tonnes).

Fishing areas in archipelagic waters were classified as either shallow-water or deep-water, depending upon whether the extent of the continental shelf was more or less than half, respectively, of the entire area. Munro (1986) gives the areas of neritic and oceanic zones of those fishing areas bounded by land masses. Yesaki (1983) estimated the extent of the Moro Gulf and Davao Gulf fishing areas.

The catch trends of Auxis and kawakawa from 1978 to 1987 for shallow-water and deep-water fishing areas are shown in Figures 3 and 4, respectively. Kawakawa landings are generally higher in the shallow-water than in the deep-water fishing areas. Auxis landings in shallow-water fishing areas show marked annual fluctuations, especially in Guimaras Strait, Ragay Gulf, Samar Sea and, to a lesser extent, in Tayabas Bay and Visayan Sea. On the other hand, Auxis landings in the deep-water fishing areas are more stable with a slight increase during the decade under review. The most productive shallow-water and deep-water fishing areas for Auxis are Visayan Sea and Moro Gulf, respectively.

Fig.3. Auxis (triangles) and kawakawa (squares) landings in the shallow-water fishing areas from 1978 to 1987.

Fig. 4. Auxis (triangles) and kawakawa (squares) landings in the deep-water fishing areas from 1978 to 1987.

The high landings of Auxis in 1980 resulted from exceptional catches in Ragay Gulf and Visayan Sea. Since this date, Auxis landings from the shallow-water fishing areas have remained more or less constant between 12,000 and 18,000 mt. the increase in Auxis landings in recent years has occurred in the deep-water fishing areas; from 35,000 mt in 1978 to 76,000 mt in 1987 (Figure 5).

Fig. 5. Auxis landings in the shallow-water and deep-water fishing areas from 1978 to 1987.

2.5 Landings by Sites

2.5.1 Species composition of landings

Table 3 shows the percentage species composition of catches by various gears at nine landing sites in the Philippines. The percentage of Auxis was high (43–72%) for enclosed archipelagic fishing areas (Batangas Coast, Camotes Sea, Bohol Sea) and low (15–22%) for the exposed archipelagic fishing areas (East Sulu Sea, Davao Gulf, Moro Gulf). Tunas comprised 60–65% of the catch in the East Sulu Sea and Moro Gulf, a considerably higher percentage than in the other fishing areas.

Table 3. Percentage species composition of catches at various landing sites in the Philippines.

¹ - Arce, 1987
² - Jabat and Dalzell, 1988

2.5.2 Species composition of Auxis catches

Frigate and bullet tuna are similar in appearance and difficult to distinguish from external characters when less than 15 cm (Arce, 1987). The percentage species composition of Auxis catches at 9 landing sites are shown in Figure 6. Frigate and bullet tuna were not distinguished during the first 2 years of the tuna sampling programme at Opol, Labuan, Santa Cruz and General Santos. Identification of Auxis species at these sites should still be taken with caution. Species identification of Auxis catches at Wawa, Lemery, Danao, Talomo, and Daliao are probably more reliable.

Bullet tuna accounts for almost the entire Auxis catch by bagnet at Wawa (Batangas Coast) and ringnet at Danao (Camotes Sea). The percentage of frigate tuna is higher at the other landing sites. The ratio of bullet to frigate tuna can reverse from year to year; for example at Lemery, Talomo, and Daliao. In general, bullet tuna accounts for a high percentage of the Auxis catch in the enclosed archipelagic fishing areas (Batangas Coast, Camotes Sea, and Bohol Sea) and about half of the Auxis catch in the exposed archipelagic fishing ground (Moro Gulf).

2.5.3 Auxis catch rates by landing sites

Catch rates by ringnet, purse seine and bagnet at seven landing sites in the Philippines were examined to detect changes in relative abundance. These three gears account for over 50% of the Philippine catch of Auxis. Ringnetters, purse seiners, and bagnetters focus their effort on a complex of pelagic species, including tuna (skipjack, yellowfin, and kawakawa), Auxis species and small-pelagic species [big-eye scads (Selar spp.), round scads (Decapterus spp.), Indian mackerels (Rastrelliger spp.), sardines (Sardinella spp.), etc.]. These species are either captured under payaos (Fish Aggregating Devices) or in free-swimming schools. As these gears catch a complex of species, it is necessary when reviewing catch rates for a specific species, to do so in relation to the total catch.

Fig. 6. Percentage species composition of Auxis catches by various fishing gears at nine landing sites in the Philippines (filled bar-frigate tuna, clear bar-bullet tuna).

General Santos (Moro Gulf)

Purse-seine catch rates of Auxis and other species were low in 1980–81, but have since increased (Figure 7). Annual catch rates of tuna have increased during the 10-year interval (Table 4).

Ringnet catch rates of all three species groups were low till mid-1982, but have since increased markedly (Figure 8). Annual catch rates of Auxis and tuna show a generally increasing trend since mid-1982 (Figure 8). Larger vessels and greater efficiency probably accounts for the pronounced increases in ringnet catch rates.

Fig. 7. Monthly catch rates of Auxis, tuna and other species by purse seines at General Santos, Moro Gulf from 1980 to 1989.

Santa Cruz (Davao Gulf)

Total annual catch rates of ringnetters fluctuated between 240 and 400 kg/day from 1980 to 1985 (Figure 9). Fishing effort decreased appreciably after 1984 with relocation of fishing vessels to landing sites closer to fishing grounds outside the Davao Gulf (Table 4).

Daliao (Davao Gulf)

Total catch rates of ringnetters at this landing site increased from 1983 to 1989 with higher catches of Auxis and other species. Tuna catch rates did not change appreciably during this interval (Figure 10).

Talomo (Davao Gulf)

Catch rates of Auxis and other species varied markedly from year to year with lowest rates in 1987 for both groups. Tuna catch rates were more stable at low levels during the 6-year interval (Figure 10).

Total catch rates at Talomo were the lowest in 1987 and were also low at Daliao during this year. Low abundance of Auxis, tuna and other species in the Davao Gulf in 1987 may have contributed to the abandonment of Santa Cruz as a landing site by ringnetters.

Table 4. Average annual catch rates (kg/day) by species groups of purse seiners and ringnetters at General Santos, Santa Cruz and Opol.

Fig. 8. Monthly catch rates of Auxis, tuna and other species by ringnets at General Santos, Moro Gulf from 1980 to 1989.

Fig. 9. Monthly catch rates of Auxis, tuna and other species by ringnets at Santa Cruz, Davao Gulf from 1980 to 1989.

Fig. 10. Annual catch rates of Auxis (FRZ), tuna and other species by ringnets at three landing sites from 1983 to 1989 (1989 catch rates for Daliao and Talomo includes information only from June to December).

Labuan (East Sulu Sea)

Catch rates of Auxis increased markedly in late 1982 and remained high to the end of 1988. Auxis catches were not reported after February 1989. Tuna catch rates also increased markedly in 1984 and remained high to the end of 1988 (Figure 11). The exceptional total catch rates for 1984 to 1988 resulted primarily from high tuna catches and to a lesser extend to the Auxis catches (Table 4). Annual catch rates of other species were more constant and fluctuated between 120 and 390 kg/day (Table 4).

Opol (Bohol Sea)

Auxis catch rates of ringnetters were highest during the 1983–85 interval. Catch rates of other species increased, while those of tuna declined from 1980 to 1989 (Figure 12 and Table 4).

Fig. 11. Monthly catch rates of Auxis, tuna and other species by ringnets at Labuan, East Sulu Sea from 1980 to 1989.

Fig. 12. Monthly catch rates of Auxis, tuna and other species by ringnets at Opol, Bohol Sea from 1980 to 1989.

Danao (Camotes Sea)

Ringnet catch rates of Auxis were most variable, with prominent peaks in 1983 and 1987. Catch rates of other species and tuna were relatively stable at moderate and low levels, respectively (Figure 10).

Auxis catch rates show wide annual fluctuations at all landing sites. High annual catch rates of Auxis occurred during 1983 at all sites, except Daliao and Talomo in the Davao Gulf (Table 4 and Figure 10), suggesting an exceptional year-class during this year throughout much of the Philippines. There is no evidence of declining Auxis catch rates at any of the landing sites.

2.6 Size Composition of Landings

Length-frequency distributions grouped by 1-year intervals of frigate and bullet tuna captured in ringnets at General Santos, Labuan, and Opol are given in Figures 13–15, respectively. Figure 13 shows little variation in size composition of frigate and bullet tuna at General Santos from 1980 to 1988. Frigate ranged from 16 to 38 cm and bullet from 16 to 30 cm. Size compositions of these two species were most varied at Labuan, especially in the early eighties, when frigate ranged from 20 to 54 cm and bullet from 14 to 50 cm (Figure 14). Bullet length-frequency distributions probably also include some frigate tuna.

Annual length-frequency distributions of bullet tuna capture by ringnets at Danao (Camotes Sea) show the dominant mode to be in the 20–23 cm interval for all years (Figure 16). A minor mode in the 13–16 cm interval was also observed from 1985 to 1987.

3. REVIEW OF THE BIOLOGY OF FRIGATE AND BULLET TUNAS IN THE INDO-PACIFIC REGION

This review summarizes information from the Indo-Pacific region which for the most part has become available since publication of the synopsis by Uchida (1981) on some aspects of the biology of frigate and bullet tunas. The topics covered in this brief review include reproduction and population characteristics of these species.

3.1 Reproduction

3.1.1 Frigate tuna

Various authors have used the maturity-stage method based on the general appearance of the gonads (size, colour, ova development, firmness) to determine the maturity of frigate tuna. Chiampreecha (1978) determined maturity of 79 frigate tuna from the east coast of Peninsular Malaysia with a 5-stage scale and found more than 50% of the fish around 37 cm to be mature (stage-IV).

Klinmuang (1978) used a similar maturity scale for 100 fish from the Gulf of Thailand and east coast of Peninsular Malaysia and concluded that this species attains maturity between 37 and 41 cm.

Fig. 13. Annual length-frequency distributions of frigate tuna (left) and bullet tuna (right) captured by ringnets at General Santos, Moro Gulf.

A 5-stage scale was also used by Yesaki (1982) to determine maturity of 120 frigate tuna off the west coast of Thailand. The smallest maturing female (stage-III) was 33 cm and 79% of the fish in this maturity stage were over 38 cm.

Gonad indices of 767 fish from the Gulf of Thailand were determined by Cheunpan (1984) over a 4-year period. She found the minimum size of maturity to be 310 mm and size at 50% maturity to be 341 mm (Table 5).

Fig. 14. Annual length-frequency distributions of frigate tuna (left) and bullet tuna (right) captured by ringnets at Labuan, East Sulu Sea.

Table 5. Number of immature and mature female frigate tuna by length class (from Cheunpan, 1984).

Fig. 15. Annual length-frequency distributions of frigate tuna (left) and bullet tuna (right) captured by ringnets at Opol, Bohol Sea.

Muthiah (1985) used a 7-point scale for determining maturity of fish from off Mangalore, India. He examined 336 males and 410 females and found the size at 50% maturity to be 30.0 and 30.5 cm for these respective sexes. However, fish less than 28 cm were not examined in this study.

The sizes at first and 50% maturity for frigate tuna are probably about 31.0 cm and 34.0 cm, respectively.

Fig. 16. Annual length-frequency distributions of bullet tuna captured by ringnet at Danao, Camotes Sea (from Jabat and Dalzell, 1988)

3.1.2 Bullet tuna

Muthiah (1985) determined maturity stages of 292 males and 354 females from off Mangalore, India, and found the sizes at 50% maturity for these respective sexes were 24.0 cm and 23,8 cm. Fish in the 20–32 cm interval were examined in this study.

Arce (1987) examined fish from the Batangas coast of the Philippines and found the size at first maturity was 17.0 cm and size at 50% maturity was 18.8 cm.

3.2 Spawning

3.2.1 Fecundity

Muthiah (1985) examined ovaries of 69 frigate and 85 bullet tunas for fecundity. His estimates of fecundity ranged from 78,000 to 717,900 ova for 31.5–39.0 cm frigate tuna and 52,600 to 162,800 ova for 25.2–33.7 cm bullet tuna.

3.2.2 Frequency

Diameters of ova in the various maturity-stage ovaries of frigate and bullet tunas were measured by Muthiah (1985). Frequency polygons show a progression of ova diameters from 4–8 micrometer units in immature ovaries (stage-I and -II) to 44–64 micrometer units in ripe ovaries (stage-VI) for both species. There are several size groups of ova in maturing (stage-III), mature (stage-IV and -V) and ripe ovaries. Multimodal ova frequency polygons are characteristic of multiple-spawning fish that release successive batches of ova throughout the spawning season. Fecundity estimates of multiple-spawning fish based only on ova counts will greatly underestimate total ova production.

3.2.3 Areas and seasons

Maturity-stage and gonad-index studies to define spawning seasons of frigate and bullet tunas have not been conclusive (Table 6). Some studies suggest two spawning seasons and others only one spawning season in a year. The latter studies have been based on limited samples and samples for only part of the year. Length-frequency distributions of bullet tuna at Danao (Figure 17), Philippines show two recruitment periods in a year. These occur in the first and third quarters of the year. There are two spawning seasons for bullet tuna, and most probably frigate tuna, at least in the equatorial regions of their distributions.

3.3 Age and Growth

3.3.1 Longevity

Frigate tuna

The largest frigate tuna measured in the Maldives from 1985 to 1989 was 53 cm (IPTP data files). Sivasubramaniam (1973) reports the presence of this size fish in a length-frequency distribution for this species captured off Sri Lanka. Similarly, Silas et al. (1985a) show occurrence of frigate tuna up to 52 cm in length-frequency distributions of fish captured off Cochin, India. The maximum size of frigate tuna in the tropical regions of its distribution is probably about 53 cm. A first approximation of the longevity of this species is 2 years.

Bullet tuna

Anderson (1988) reports a rare occurrence of 30–35 cm bullet tuna among catches of similar-sized frigate tuna in the Maldives. A 33.7 cm bullet tuna was examined by Muthiah (1985) during his study of fecundity. Silas et al. (1985a) and Sivasubramaniam (1973) show in length-frequency distributions the incidence of this species in the 34–36 cm and 31–35 cm intervals, respectively. The maximum size of bullet tuna in the tropical regions of its distribution is probably about 35 cm. A first approximation of the longevity of bullet tuna is 1 year.

Fig. 17. Length-frequency distributions grouped by three month intervals of bullet tuna captured by ringnet at Danao, Camotes Sea (from Jabat and Dalzell, 1988).

3.3.2 Growth rate

Frigate tuna

Growth of juvenile frigate and bullet tunas has been studied in rearing experiments with small (0.5 tonne capacity) and large (3 and 70 tonne capacity) aquaria at the Marine Laboratory of Kinki University, Shirahama, Japan (Harada et al., 1973). Growth rates differed markedly in these aquaria, with fish attaining 120 mm total length in 33 days in the large aquaria versus 55 mm in 35 days in the small aquaria (Figure 18).

Fig. 18. Growth of juvenile frigate and bullet tunas in small (triangles) and large (squares) aquaria (from Harada et al., 1973).

Growth of adult frigate tuna has been studied by various authors (Table 7). All these studies have been based either on the “pen and paper” method or ELEFAN programme analyses of length-frequency distributions grouped by 1-month intervals, except the analysis by Yesaki (1989). He grouped length-frequencies by 10-day intervals and obtained lengths-at-ages 1 and 2 of 39 and 49 cm, respectively. All the other studies give lower predicted lengths, mostly between 24–27 cm for age 1 and 30–42 cm for age 2 (Figure 19).

Fig. 19. Estimates of growth and relative age(years) of frigate tuna from length-frequency analyses (crosses-manual method; triangles-computer programmes).

Rearing experiments of frigate tuna show rapid growth after hatching, supporting the growth curve giving length-at-age 1 as 39 cm. Maturity studies of frigate tuna indicate size at 50% maturity to be about 34 cm, which also lends credence to the growth curve that gives 39 cm at age 1.

Table 6. Spawning seasons of frigate and bullet tuna at various locations in the Indo-Pacific region.

Table 7. Summary of age and growth studies of frigate tuna by length-frequency analyses (from Yesaki, 1989).

¹ - relative age
² - monthly basis.

Bullet tuna

Harada et al. (1973) found that growth rates of bullet tuna also varied markedly in aquaria of different sizes. Fish reared in small aquaria (0.5 tonne capacity) grew to 92 mm (total length) in 43 days, whereas fish reared in large aquaria (3 and 70 tonne capacity) attained 140 mm in 40 days and 155 mm in 58 days (Figure 18).

There have been no studies on the growth of bullet tuna from equatorial regions of its distribution.

3.4 Population

3.4.1 Sex ratios

Sex ratios of frigate and bullet tunas determined by various authors are given in Table 8.

3.4.2 Length-weight relationships

Length-weight relationships for frigate and bullet tunas are given in Table 9.

Table 8. Sex ratios of frigate and bullet tunas from various locations in the Indo-Pacific region.

Table 9. Length-weight relationships (w = aL^b) of frigate and bullet tunas.

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