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.