Bycatch in the purse seine tuna fisheries in the western Indian Ocean.

Abstract

The yield of associated and dependent species taken as bycatch by the purse seine tuna fishery from the Indian Ocean pelagic ecosystem is estimated from data collected by scientific observers aboard Soviet purse seiners in the Western Indian Ocean (WIO), 1986-1992. A total of 494 sets on free swimming schools, whale shark associated schools, whale associated schools, and log associated schools were analyzed. More than 40 fish species and other marine animals were registered. Among them only two species, yellowfin and skipjack tunas, are target species. Average levels of non-tuna bycatch equal 0.518 t per set, 27.2 t for 1,000 t of target species. By the author’s estimates, the annual catch of yellowfin and skipjack tunas in 1990-1995 of the total international WIO purse seine fishery was within the range 215,000-285,000 t. The annual non-registered catch included 1,700-2,300 t of pelagic oceanic sharks, 1,300-1,700 t rainbow runners, 1,250-1,650 t dolphinfish, 900-1,200 t triggerfish, 200-270 t wahoo, 190-250 t billfishes, 100-130 t of mobulas and mantas, 60-80 t of mackerel scad, 15-25 t of barracudas, and 120-160 t of other fish; bycatch of turtles and whale mortality were possible. Absence of bycatch records in purse seining does not make it possible to assess, to the full extent, the impact of the fisheries on the pelagic ecosystem in the Indian Ocean. The first step to the solution of this problem is for the Indian Ocean Tuna Commission to develop a system of scientific control over tuna purse seine and longline fisheries by means of a wide network of Commission scientific observers on board fishing vessels in the area.