90 



Abstract— Bycatch taken by the tuna 

 purse-seine fishery from the Indian 

 Ocean pelagic ecosystem was estimated 

 from data collected by scientific observ- 

 ers aboard Soviet purse seiners in the 

 western Indian Ocean (WIO) during 

 1986-92. A total of 494 sets on free- 

 swimming schools, whale-shark-associ- 

 ated schools, whale-associated schools. 

 and log-associated schools were ana- 

 lyzed. More than 40 fish species and 

 other marine animals were recorded. 

 Among them only two species, yellow- 

 fin and skipjack tunas, were target spe- 

 cies. Average levels of bycatch were 

 0.518 metric tons (t) per set, and 27.1 

 t per 1000 t of target species. The total 

 annual purse-seine catch of yellowfin 

 and skipjack tunas by principal fishing 

 nations in the WIO during 1985-94 

 was 118.000-277,000 t. Nonrecorded 

 annual bycatch for this period was 

 estimated at 944-2270 t of pelagic oce- 

 anic sharks, 720-1877 t of rainbow 

 runners, 705-1836 t of dolphinfishes, 

 507-1322 1 of triggerfishes, 1 13-294 t of 

 wahoo, 104-251 t of billfishes, .53-112 

 t of mobulas and manias, 35-89 t of 

 mackerel scad, 9-24 t of barracudas, 

 and 67-174 t of other fishes. In addi- 

 tion, turtle bycatch and whale mortal- 

 ities may have occurred. Because the 

 bycatches were not recorded by some 

 purse-seine vessels, it was not possible 

 to assess the full impact of the fish- 

 eries on the pelagic ecosystem of the 

 Indian Ocean. The first step to solving 

 this problem is for the Indian Ocean 

 Tuna Commission to establish a pro- 

 gram in which scientific observers are 

 placed on board tuna purse-seine and 

 longline vessels fishing in the WIO. 



Bycatch in the tuna purse-seine fisheries 

 of the western Indian Ocean 



Evgeny V. Romanov 



Southern Scientific Research Institute of Marine Fisheries and Oceanography (YugNIRO) 



2, Sverdlov St 



98300, Kerch, Crimea, Ukraine 



E mail address islande'cnmeacom 



Manuscript accepted 20 March 2001. 

 Fish. Bull. 100(1): 90-105 (2002). 



One of the most inipoitaiit require- 

 ments of the UN Convention on the Law 

 of the Sea of 1982, which determines 

 strategies for exploitation of marine 

 living resources (Article 119, b), is to 

 take into account the impact of fish- 

 eries on ". . . species associated with 

 or dependent upon harvested species 

 with a view to maintaining or restor- 

 ing populations of such associated or 

 dependent species above levels at which 

 their reproduction may become seri- 

 ously threatened. . ." (United Nations, 

 1983). Estimating the magnitude of 

 bycatch is one of the first steps to deter- 

 mine the impact of fisheries on associ- 

 ated species. 



Tuna purse-seine fisheries probably 

 apply the most intensive direct htnnan 

 impact on the tropical epipelagic eccsys- 

 tems in all oceans. Because of the world- 

 wide scale of purse-seine fisheries, an 

 assessment of their impact on associat- 

 ed and dependent species is essential. 



Two tunas, yellowfin Thunnus alba- 

 cares (Bonnaterre, 1788) and skipjack 

 Katsiiwonus pe/amis (Linnaeus, 1758), 

 are the target species of most purse- 

 seine fisheries. In this study bycatch is 

 defined as the fraction of the catch that 

 consists of nontarget species (including 

 other species of tuna) that are encircled 

 by the fishing gear and are unable to 

 escape by themselves. Bycatch of asso- 

 ciated and nonassociated species dur- 

 ing purse-seine fishing for tropical tu- 

 nas may be rather high, and generally 

 depends on fishing tactics. 



The species composition of bycatch 

 in purse-seine fisheries depends on the 

 structure, behavior, and spatial organi- 

 zation of siu'face multispecies aggrega- 

 tions. Schools of different tuna species 

 and other pelagic fishes, marine mam- 

 mals, and other marine animals have 

 aggregated distributions. From our ob- 



servations and in the opinion of other 

 researchers (Au and Ferryman, 1985; 

 Au and Pitman, 1986; Au, 1991; Cort, 

 1992), marine birds arc also an inte- 

 gral component of the majority of these 

 multispecies groups. 



The tunas, as a rule, prevail by bio- 

 mass and abundance in such groups. 

 Tuna schools are traditionally classi- 

 fied by the visually distinctive part of 

 the group or by whether they associate 

 with floating objects or marine mam- 

 mals (Scott, 1969; Petit and Stretta, 

 1989). "Free-swimming schools" may 

 include associations between different 

 species of tuna. For each type of school, 

 its various components occur in differ- 

 ent ratios. 



Some epipelagic species that occur in 

 the purse-seine bycatches are not mem- 

 bers of multispecies aggregations. They, 

 instead, may comprise members of the 

 flotsam community or are tuna forage. 

 Several associated components, such as 

 whales and birds, usually escape or 

 avoid the nets and do not become by- 

 catch. Therefore, the composition of the 

 catch often does not represent the actu- 

 al species composition of the multispe- 

 cies associations. 



Assessments of bycatches have been 

 made for the eastern Pacific Ocean 

 purse-seine tuna fishery (Joseph, 1994; 

 Garcia and Hall, 1995; Hall, 1996, 1998; 

 Anonymous, 1997, 1998, 1999). where 

 the bycatch problem attracted attention 

 because of dolphin mortality during 

 sets on dolphin-associated tuna schools. 

 The economic, political, and ecological 

 implications of this problem produced 

 wide international attention (Charat- 

 Levy, 1991; Jcseph, 1991, 1994; Hall, 

 1998). Bycatch estimates for the west- 

 ern Pacific purse-seine tuna fisheries 

 have been published also (Bailey et al., 

 1996). 



