ASSOCIATION OF YELLOWFIN TUNA AND DOLPHINS IN MALDIVIAN WATERS

Large yellowfin tuna (Thunnus albacares) are regularly found in association with dolphins in Maldivian waters. The species involved are the spotted dolphin (Stenella attenuata) and the spinner dolphin (Stenella longirostris). Maldivian fishermen targeting large yellowfin use the presence of dolphin schools to locate the tunas. The yellowfin are caught using simple handlines, and are mostly within the length range 70-160 cm FL. No dolphins are caught. 1 Marine Research Centre, Ministry of Fisheries Agriculture and Marine Resources, Malé, Republic of Maldives Introduction Yellowfin tuna (Thunnus albacares) is a major target of tropical pelagic fisheries around the world. The total world catch in 1993 amounted to some 1.2 million metric tonnes (t). In some areas of their range, most notably in the eastern tropical Pacific (ETP), large yellowfin tunas frequently associate with dolphins. The main dolphin species involved are the pantropical spotted dolphin Stenella attenuata, the spinner dolphin Stenella longirostris, and common dolphins Delphinus spp. (Evans, 1994; Norris et al., 1994; Perrin and Gilpatrick, 1994; Perrin and Hohn, 1994), although other species are also involved. The targeting of dolphin-associated yellowfin tuna schools by tuna purse seiners in the ETP and consequent mortality of the dolphins has been a major issue since the early 1970s (Joseph and Greenough, 1979; Joseph, 1994; Hall, 1998). In the Maldives, fishermen have traditionally used livebait pole and line, and targeted small surface-swimming tunas including skipjack and juvenile yellowfin tuna (Anderson, 1985; Adam and Anderson, 1996). Schools of these small tunas are located mainly by the presence of sea birds. Although large yellowfin are present in Maldivian waters, most fishermen did not traditionally target them. This was because the fishermen could achieve higher catch rates using pole and line and because there was no specific market for such large fish. Within the last few years, however, markets (both domestic and export) have developed for large yellowfin. As a result, some Maldivian tuna fishermen are now targeting these fish, and it has become apparent that large yellowfin do associate with dolphins in Maldivian waters. The aim of this report is to document this association and the Maldivian fishery. Materials and Methods A questionnaire on the yellowfin tuna fishery was sent by the Marine Research Centre (MRC, formerly the Marine Research Section) to all inhabited islands in the Maldives in September 1995. A reminder was sent in April 1996 to islands that had not responded. Responses were received from 188 out of the 200 islands (94 %). As part of this questionnaire, information was requested on the extent and seasonality of large yellowfin fishing and the association of dolphins and yellowfin. Length frequency data were obtained from Malé fish market, during the regular sampling programme of MRC/MRS. During the period April to December 1997, samplers (including both authors) specifically asked fishermen from what type of school their large yellowfin catches were made. Yellowfin caught in association with dolphins were recorded separately from all other yellowfin. Sampling was not continued in 1998 due to shortage of staff. Fork lengths (FL) were measured with tapes. Tape lengths were converted to board lengths using the formula of Anderson, Adam and Nadheeh (1996). The authors have participated in many tuna fishing trips and some cetacean survey trips in Maldivian waters. Field notes have been kept by the senior author of all dolphin sightings since 1990. Details of sightings involving dolphins associated with tunas are listed in Table 2.