Romanov: Bycatch in the tuna purse-seine fisheries of the western Indian Ocean 



103 



Spain, and I'SSRV-''' In the WIO ranged between 

 115,000 and 242.000 t in 1985-94 (Anonymous'"). 

 Japanese and Mauritian catches varied from 3000 to 

 about 51,000 t. Based on these vakies, the estimated 

 bycatcli was 3257 to 8246 t of various fishes during 

 the same period (Table 7), These fishes could serve 

 as food for the coastal countries of the area. Estimat- 

 ed bycatch in numbers is presented in Table 8. 



Turtle bycatch and whale mortality in purse seines 

 are also possible in the WIO, but the probability of 

 the latter is very low. No instances of whale mortal 

 ity have been recorded earlier for tuna purse-seine 

 fisheries in other areas (Northridge, 1984, 1991a. 

 1991b: Medina-Gaertner and Gaertner, 1991; San- 

 tana et al., 1991; Cort, 1992; Cayre et al.. 1993; Bai- 

 ley et al.. 1996). No avian mortality by the Soviet 

 tuna purse-seine fishery has been noted by observ- 

 ers. A similar fact was reported for the western Pa- 

 cific (Bailey et al.. 1996). 



Target fishing for rainbow runner, dolphinfish, 

 triggerfishes, wahoo, mackerel scad, and barracuda 

 is not conducted in the WIO, and these fish are taken 

 only as bycatch. Their bycatch levels, estimated in 

 this study, do not seem to endanger the populations 

 of these species. 



Estimated bycatch of billfishes ( 104-251 1 annual- 

 ly) was less than I'Ti of the total catch for these spe- 

 cies (14,000-33,000 t during 1985-94) in the WIO 

 (Anonymous^"). The bycatch by the purse-seine fish- 

 ery was unlikely to substantially affect the billfish 

 stocks. 



Many pelagic sharks are taken as bycatch by the 

 longline, trawl, coastal driftnet, and other fisheries, but 

 are not recorded. The total shark catch by all fisheries may 

 be considerable. Many shark species are characterized by 

 low abundance, low fecundity, long life span, and conse- 

 quently, by high vulnerability to overfishing. Underesti- 

 mation of the removal through fisheries of a number of pe- 

 lagic shark species, and the impact of the fisheries on their 

 populations, may lead to a reduction in their abundance to 

 critical levels, diminishing the biodiversity of the pelagic 

 ecosystem of the Indian Ocean. 



Some part of the bycatch is released into the ocean 

 alive, although subsequent survival rates are unknown. 

 The lack of bycatch and discard records and estimates of 

 survival rates of discarded animals prevents assessment 

 of the impact of the fishery on the Indian Ocean pelagic 

 ecosystem. 



Fishing tactics in the WIO have changed considerably 

 by all principal purse seine fleets toward the extensive 

 use of FADs in recent years (generally from 1995). The 

 majority of Japanese vessels have left the area and have 

 moved to the eastern Indian Ocean. Therefore estimates 

 presented here for total WIO purse-seine fisheries are ap- 



plicable for a limited time span only (pre- 1995). Recent de- 

 velopment of the WIO fisheries warrants further investi- 

 gation of bycatches through extensive observer sampling 

 by time-area strata. 



Establishing a scientific program by the Indian Ocean 

 Tuna Commission to monitor the principal tuna fisheries in 

 the region, by placing international scientific observers on 

 purse-seine and longline vessels, might be the first step to- 

 ward a more accurate assessment of the impact of bycatch- 

 es on the epipelagic ecosystem of the Indian Ocean. This 

 program might also lead to developing technical and man- 

 agement measures to reduce the bycatches or to use them. 



The solution to the bycatch problem should take two di- 

 rections: 1 1 an effort to reduce or eliminate bycatches of un- 

 desired species; or 2) to use bycatch animals to make them 

 target species. The former involves developing gear modi- 

 fications or changes in fishing tactics. The latter involves 

 management regulation of the fishery so that bycatch spe- 

 cies are treated in the same way as other target species. 



Acknowledgments 



'^ Including vessels flying flags of convenience. 



■"' Anonymous. 1998. Indian Ocean tuna fisheries data sum- 

 mary, 1986-1996. Indian Ocean Tuna Commission (lOTC) 

 data summary 18, 180 p. lOTC, P.O. Box 1011, Victoria, 

 Seychelles. 



I am giateful to AtlantNIRO scientists V. F Bashmakov, G. 

 A. Budylenko, V. Z. Gaikov, M. E. Grudtsev, to TINRO sci- 

 entist K. A. Karyakin for their data made available to the 

 author and to V. F. Bashinakov and G. A. Budylenko for their 

 personal sampling efforts. I sincerely thank masters of the 



