Bulletin of the Japanese Society of Scientific Fisheries Vol. 36, No. 1, 1970 9

Little Tuna Euthynnus affinis in the Hong Kong area*

Gordon R. WILLIAMSON** (Received September 10, 1969)

The Little Tuna Euthynnus affinis CANTOR(Fig . 1) is distributed from the east coast of Africa in the Indian Ocean to Indonesia and Japan across the equatorial Pacific Ocean to Hawaii (Fig. 2). KIKAWA et al.1) and WILLIAMS2) have summarised data on the species in the Pacific

and Indian Oceans respectively , TESTER and NAKAMURA3) give additional information from Hawaii, ABE4) gives a good colour illustration of the species and NAKAMURA Fig. 1. Euthynnus affinis CANTOR. and MAGNUSON5) describe periodic changes From NAKAMURAand MAGNUSON5)

in intensity of the fish's black spots . The taxonomy of the species, which was formerly called E. yaito by some biologists, is discussed by FRASER-BRUNNER6), COLLETTEand GIBBS7)and NAKAMURA8). A general account of fishery resources around Hong Kong is given by WILLIAMSON9). Fishermen's reports indicate that E. affinis is the commonest tuna in the Hong Kong area. Auxis thazard (LACEPEDE)is the only species with which it can be confused. E. affinis and A. thazard can be separated by the following characters:

Fig. 2. Distribution of Euthynnus affinis CANTOR. After KIKAWA et a1.1) and WILLIAMS2)and with the Kwangtung coast added to the distribution range. One specimen of E. affinis has been recorded from California.

* Contribution No . 36 from the Fisherier Research Station, Hong Kong. ** Agriculture and Fisheries Department , Fisheries Research Station, Aberdeen, Hong Kong. 10

E. affinis 15-16 dorsal fin rays, transient black spots under pectoral fins A. thazard 10-11 dorsal fin rays, no black spots under pectoral fins. Few E. affinis are landed because Hong Kong Chinese people do not like to eat tuna and in USA the species is not on the USA Food and Drug Administration list of species that are permitted to be sold under the label of "Tuna". In an attempt to develop a tuna fishery in Hong Kong, the Hong Kong Fish Market ing Organization during 1967 and 1968 offered to buy E. affinis at a special high price. Two boats switched to tuna fishing and used a modified purse seine method to catch schools of E. affinis which approach very close to the east coast of Hong Kong during June-August each year. About 16 metric tons of E. affinis were caught each year, but could not be sold at a profit: the tuna fishing development project has been abandoned for the present time. This paper describes the biology of E. affinis in the Hong Kong area based on exami nation of fish sampled from the summer purse seine fishery during its 2 years of operation and from lesser catches taken at other times of the year.

Materials and Methods

Data relating to the purse seine fishery were obtained by visiting fish markets, ac companying fishermen catching tuna and discussing the fishery with them, helicopter surveys, recording the weights and numbers of tuna caught each week and by sampling accumulated catches derived from June-August fishing operations stored in a cold store at the end of each year's season. A random sample of 1000 fish was measured for length and subsamples examined for weight (226 fish) and sex, maturity and stomach contents (158 fish). Other samples of tuna were examined, all caught at the surface south of Hong Kong: 72 fish caught September 1968 by hook ('Mustad' size 14 with Decapterus bait) over water 55 metres (30fm) deep, 64 fish caught November 1968 by gillnet, 42 fish caught December 1968 by hook ('Mustad' size 14), 46 fish caught January 1969 by hook over water 100 metres (54fm) deep and 64 fish caught April 1969 by hook. Fork lengths of fish were measured to the nearest centimetre below in order to give 2-cm length groups whose average is a whole number. Weights of individual fish were recorded to the nearest 50g and weights of whole catches to the nearest kilogram. The maturity of fish was determined according to the following system: Immature: gonads very small, translucent and thin walled, no gametes visible. Mature, spawning: gonads large, swollen with ripe eggs or sperm which are easily extruded when their body wall is pressed. 11

Mature, spent: gonads reduced in size, but still containing some eggs or sperm. Mature, resting: gonads of small size, walls of moderate thickness, no gametes visible. Mature, preparing: gonads swelling, ova visible to naked eye. The degree of fullness of stomachs and the proportion by volume of the food organisms in the stomach contents were estimated by eye. Hydrographic data were obtained from records of the Hong Kong Fisheries Research Station and Royal Observatory.

Results Environment The Chinese coast around Hong Kong (Fig. 3) is composed of steep rocky headlands and islands. The 50 metre (28 fathom) contour lies 25 miles offshore.

Fig. 3. Map of Hong Kong area showing location where schools of spawning Euthynnus affinis occur during June-August. (Data from the coast of the Peoples Republic of China are not available.)

Surface water temperatures range from 14-29•Ž annually. The estuary of the West

River greatly influences the west side of Hong Kong, causing the bottom to be muddy and

the water turbid and brackish (surface salinity 33•ñ, in February, 6•ñ in July). The waters on the east side of Hong Kong are more oceanic, the bottom is sandy or muddy sand and surface salinities range from 34•ñ in February to 26•ñ at the height of the summer rainy season around July.

Distribution and Behaviour Fishermen reported concentrations of E. affinis close inshore during June-August on the east side of Hong Kong (Fig. 3) and scattered tuna over deeper water south of Hong Kong all year round. Tuna were not seen inshore during the coldest months nor in the brackish waters on the west side of Hong Kong.

On the east coast schools first appeared in June, maximum numbers were present 12 in July and by the end of August the schools had disappeared. These schools swam close to the surface. One day in July 1967 I saw three separate schools of tuna along a 2-mile stretch of coast near fishing site No. 1 (Fig. 3). The fish in each school were in a close mass thrashing the water white with their tails, and milled about for at least 2 hours without moving in any direction. These schools were located about 400 metres (1300 feet) from the base of the cliffs over water about 20 metres (12fm) deep. More often, no tuna were breaking surface, although schools were present. On such occasions,, provided the weather is calm, experienced fishermen climb about 40 metres (132ft) up the cliffs and detect submerged fish by looking down into the water. Apart from schools of tuna that swam several hundred metres offshore, other schools swam along the edge of the shore, only about 10 metres (33ft) off the rocks over about 11 metres (6fm) of water. These schools were the ones caught by the present fishing methods. The schools were quite discrete and in one day's fishing of 7 hours (0900-1600 hours) usually only 3-6 schools were detected passing along the rock's edge. Fishermen's catches showed that the smallest schools were of 10 fish, average schools contained about 60 fish and occasionally schools contained 200 fish. No other species of fish swam with E. affinis at this season. A fisherman told me that he often saw what he reckoned were fish in the act of spawn ing: a sudden flash of silver as two fish turned onto their sides and swam along together for a brief period. Fishermen say that during June-August the schools of tuna they catch are moving along the rocks edge in an east to west direction. They believe that in June the tuna approach along the coast from the east, but do not know where the fish go in August. The presence of nonmoving schools further off the cliffs indicate that fish may not be engaged in a continuous migration in these months. Fishing Method The present summer fishery is carried out by a rather specialised method, which makes use of underwater gulches which occur at a few places along the cliffs (Fig. 4). Underwater, the rocks slope steeply to a level bottom about 11 metres (6fm) deep in these places. The fishermen arrive at one of the gulches about 0900 hours, in full daylight , and tie one end of their net to the western side of the gulch. A man climbs the cliff and looks down into the water. When he sees a school of fish swimming along the edge of the rocks towards the gulch he whistles. Guided by the lookout's signals, a sampan sets the net from surface to bottom in an arc around the tuna, then takes the second end ashore . The net is then tightened thus penning the tuna alive in the gulch. A second net is now prepared to catch a second school in the same way . By partially raising the first net, a second and subsequent schools can be admitted to the gulch pen . 13

Fig. 4. Modified purse seine method used to catch tuna during June-August.

At about 1600 hours the second net is set inside the pen, and the fish taken on board the boat for transport to market.

The main limitations of this method are that it does not catch the larger schools which are further offshore, and it is only possible when the surface is calm enough for the lookout man to see the approaching fish.

Catch in relation to Month, Temperature and Salinity During the summer purse

seine fishery in 1967 one boat operated and caught 3,497 tuna weighing 13.8 metric tons

(average fish weight 3.4kg) and in 1968 2 boats operated catching 5,403 fish weighing

19.4 metric tons (average fish weight 3.6kg). The catches were made during June-

August with heaviest landings in July (Fig. 5). Surface waters during these months had temperature 25-29•Ž and salinity 26-29•ñ. During the whole year small numbers of E. affinis were caught by hook (incidental to Nemipterus long lining: the tuna take the bait as the line is cast out from the surface)

Fig. 5. Purse seine catches of tuna and sea surface temperature and salinity at Hong Kong during 1967-68. 14

or gillnet south of Hong Kong. Tuna were caught by these methods at the surface from the coast to over water 150 metres (82fm) deep during April-December, but not within

30 miles of the coast during the coldest months of January-March. It appears that the tuna moved away from the coast in winter in order to remain in offshore waters which are always temperature 19•Ž or more.

Length-Weight Relation Analysis of measurements of 226 fish indicate that in summer the length-weight relation of tuna may be represented by:

W = 0.008853 L2.5649 where W = whole weight in kg and L = fork length in cm. Size Composition Length frequencies of random samples of fish (Fig. 6) show that tuna of 24-73cm occur in the Hong Kong area. The fish caught during the June-August purse seine fishery were of two size groups: many fish 50-73cm long (weight 2.5-7.0kg) and a few fish 38-49cm long (weight 1.2-2.5kg). Average weight of all fish caught during June-August was 3.5kg or 7.7 pounds. Sex Composition June-August purse seine catches. A stratified subsample of 158 fish was examined for sex, the proportion of males in each 2-cm length group plotted and a curve fitted by eye. Values obtained from the smoothed curve were applied to the random sample of 1000 fish to separate male and female fish. The results show that at all lengths both male and female fish occurred, but that among fish above 57cm long the proportion of females was much reduced (Fig. 6). Among the larger size- group of tuna 50-73 cm long, 67 % of the fish appeared to be males. Among the smaller 38-49cm long size-group of fish. the sexes appeared to be present in equal numbers. Maturity The great majority of tuna caught in June-August were mature spawning fish; the larger size group (50-73cm long) exclusivelyso. Running eggs and milt were easily squeezed out from freshly caught fish. Among the smaller size group of fish (38-49 cm long) 10 % of fish were immature. Fig. 6. Lengths of Euthynnus affinis caught Fish 24-49cm long caught during Sep- in the vicinity of Hong Kong 1967-69. tember-April were immature In the upper diagram only the sexes are , except for a separated; males black, females white. few individuals among 40-49cm long fish Table 1. Degree of fullness of stomachs and food of Euthynnus affinis caught in vicinity of Hong Kong 1967-69 ! Degreeof fullness Contributionof FoodOrganisms to Stomachcontents by Volume Capture details Length of Stomachs Fi Range No. sh Crustacea lopodaCepha_ Fish Empty 1-25% 26-50% Unre- Trichi- Ather- Sardin- Decap- Euph- Squilla Date Gear cm full full cogm- urus inidae ella terus ausiidae larvae L oligo sable

VI-VIII 67 & 68 purse seine 50-71 897 80 20 - 55 - 30 15 - - - -

11 36 -49 103 71 29 - 70 - 10 10 - - 5 5

IX 68 hook 48-53 2 no data 11 24 -41 70 83 17 - 25 - - - - 70 - 5 X1 68 gill net 36-47 64 89 11 - 80 ------20 X11 68 hook 54-69 12 50 17 33 30 70 ------" 32-45 30 87 13 - 70 ------30 I 69 53-67 31 70 25 5 85 - - - 15 - - - i " 30-47 15 no data IV 69 50-63 19 75 20 1 80 - - - 20 - - - " 29-49 44 no data 15 " 16

caught in April which were preparing to spawn. Fish over 50cm long caught during September-January were mature in fully resting condition, the gonads empty of gametes and much reduced in size compared with spawning fish. Fish over 50cm long caught in April were mature preparing to spawn. Food Results of examination of tuna stomachs are given in Table 1. Among different samples of tuna 11-50% of individuals had food in their stomachs. The quan tity of food in stomachs seldom exceeded 20% of the stomach volume. Perhaps tuna digest food fast, causing stomach contents to disappear quickly. Food in stomachs consisted chiefly of fish, most of which were too digested to be identified but among which were several pelagic forms: Trichiurus, Atherinidae, Sar dinella and Decapterus. Loligo squid were found in some stomachs. Some small tuna of 24-49cm long had been eating large numbers of Euphausiids.

Discussion Data on E. affinis around Hong Kong generally agree with findings of other workers and provide some new information. It appears that a stock of E. affinis inhabits the south coast of Kwangtung, China. During most of the year the fish live dispersed over the continental shelf area but move close inshore during June-August to spawn. The fish spawn in waters of reduced salinity. More data are needed to clarify the apparent dominance of males among the spawn ing fish. Possibly larger females tend to remain in the middle of the bays and thus are seldom caught by the present fishery which only traps fish that approach close to the rocks. The spawning fish are composed of two distinct size-groups which presumably re present consecutive year-classes whose ages are whole numbers of years. The younger year-class is only weakly represented. Per haps the majority of its members are imma ture and swim elsewhere at this season . The average size of fish in the younger year-class is about 44cm (1.9kg) and in the dominant older year-class about 62cm (4.5kg) . It is possible that these year-classes are 1 and 2 years-old respectively and that the growth rate in the stock of tuna which visits Hong Kong is as shown in Fig. 7. If the suggested growth rate is correct , Fig. 7. Possible growth rate of Euthynnus progeny of June-August spawning will grow affinis in Kwangtung area. to about 25cm long in December . Since no 17

10-25cm long tuna were caught around Hong Kong during the autumn, it appears that the numerous progeny of the summer spawning migrated away from the Hong Kong area almost as soon as they were big enough to swim. The larger-than-25cm size of such immature tuna as were caught during September-December suggest that they were the result of spawning elsewhere, earlier in the year. Studies such as tagging are needed to discover the relation of the fish which visit Hong Kong to E. affinis in other areas. It would seem that commercial catches could be considerably increased during June- August if normal purse seine methods were used. A normal purse seine vessel could harvest schools of tuna anywhere in the coastal area, not just the small schools that ap proach close to the rocks. Annual fluctuations may be expected in this fishery since it is based on only one main year-class of fish.

Summary

This is the first description of the biology of Euthynnus affinis in the Hong Kong area

and is based on examination of samples of fish and on other data collected during 1967-

69. A population of E. affinis inhabits the south coast of Kwangtung, China where it is

the commonest tuna species. Adult fish concentrated inshore to spawn each year during

June-August in water of surface temperature 25-29•Ž and salinity 26-29•ñ. Two distinct

size-groups of fish made up the spawning schools; many fish of mean size 62cm long

(weight 4.5kg) and lesser numbers of fish of mean size 44cm long (weight 1.9kg). These

size-groups were probably 2 and 1 years-old respectively. The tuna lived dispersed over the continental shelf during other months of the year. Food eaten consisted chiefly of pelagic fish and cephalopods. The behaviour, length-weight relation and growth rate of the fish are described. Commercially useful catches could probably be taken from the summer spawning concentrations if purse seining methods were used.

Acknowledgments For tuna landing statistics I thank Jim Ying-kwan, Leung Sai-cheong, Tsang Chiu-yan and Siu Ming of the Fish Marketing Organization. Of the Fisheries Research Station staff W. L. Chan, Senior Research Offices, identified stomach contents and read the proofs, A. J. E. Smith, P. J. Gaiger and K. C. Lee criticized the text, my assistant So Lai-hang helped throughout and Chan On-tong drew the figures. Mr. E. L. Nakamura of U.S. Bureau of Commercial Fisheries, Hawaii criticized the text and supplied Fig. 1 and Dr. Shoji Kikawa of Far Seas Fisheries Research Laboratory, Shizuoka, Japan criticised the text. This report is published by permission of Mr. E. H. Nichols, Director of Hong Kong Agriculture and Fisheries Department. 18

References

1) S. KIKAWA and staff of Nankai Regional Fisheries Research Laboratory, Kochi, Japan: Proc. World Sci. Meet. Biol. Tunas and Relat. Species, La Jolla, Calif., U.S.A. 2-14 July 1962, FAO Species Synopsis 7, 28 p. (1963). 2) F. WILLIAMS: Proceedings of the World Scientific Meeting on the Biology of Tunas and Related Species, La Jolla, Calif., U.S.A., 2-14 July 1962, FAO Fish. Rep. No. 6, Vol. 2, 167-179 (1963). 3) A. L. TESTERand E. L. NAKAMURA: Spec. Sci. Rep. U.S. Fish Wildl. Serv. (Fish.), 250, 25 p. (1959). 4) T. ABE: Guide to fishes of Japan, Hokuryu Kan Publishing Co., Ltd., Tokyo, Japan, 358 p. (1963). (In Japanese with colour illustrations). 5) E. L. NAKAMURAand J. J. MAGNUSON: Copeia, 1965(2), 234-235 (1965). 6) A. FRASER-BRUNNER: Ann. Mag. Nat. Hist., 12 (2), 622-627 (1949). 7) B. B. COLLETTEand R. H. GIBBS, Jr.: Proceedings of the World Scientific Meeting on the Biology of Tunas and Related Species, La Jolla, Calif., U.S.A., 2-14 July 1962, FAP Fish. Rep. No. 6, Vol. 1, 23-32 (1963). 8) 1. NAKAMURA: Bull. Misaki Marine Biol. Insit., Kyoto Univ., No. 8, 7-38, 16 figs (1965). 9) G. R. WILLIAMSON: Fish. News. Internat., 7 (7), 26-30 (1968).