Romanov Bycatch in the tuna piiise seme fishenes of the western Indian Ocean 



95 



schools. I did not attempt, however, to take this factor into 

 consideration. 



The average CPUE (e.g. total catch per set) in the purse- 

 seine tuna fisheries varies greatly by type of vessel. I did 

 not find a strong correlation between the bycatch per set 

 and the total catch, or catch of target species in the same 

 set. Level of bycatch generally depends on the type of as- 

 sociation and the total fishing effort directed at this type 

 of association. 



Species composition and catch by school types 



A total of 50 species (or higher taxa) of fishes and other 

 marine animals were recorded in the catch of the fishing 

 vessels (Table 4). 



Free-swimming schools 



Free-swimming schools are the predominant type of sur- 

 face schools in the WIO. Such schools occurred in the area 

 all year round (Table 2, Fig. 2, A and B). Soviet purse sein- 

 ers set on free-swimming schools generally south of the 

 equator ( including Mozambique Channel )( Fig. IB ). Yellow- 

 fin, skipjack, and bigeye (Thunnus obesus) tunas were the 

 principal components of free-swimming schools, compris- 

 ing 80%, 15%, and 4%, respectively (Table 5). Monospe- 

 cific (nonassociated) tuna schools, consisting completely of 

 yellowfin or skipjack tuna, were found to occur in 47% of 

 free-school sets. Multispecies free schools were observed in 

 53% of all free-school sets and generally consisted of two 

 target species and bycatch. Bycatch occurred in 45%- of the 

 free-school sets, and nontuna bycatch in 22%. 



A total of 19 species (or higher taxa of fishes) were 

 observed in catches on free schools (Table 4). Some spe- 

 cies were considered to be tuna prey (e.g. Exocoetidae) 

 and some to be accidental bycatch (e.g. Gempylus serpens. 

 Canthidermis maculatus, Diodon spp.). Nontuna bycatch 

 in this type of association was on average 0.060 ±0.031 

 t per positive set (Fig. 3) and 3.403 ±2.770 t per 1000 t 



of target species (Fig. 4). The bulk of this bycatch was 

 sharks of the genus Carcliarhiniis (0.023 t/1.296 t), rays of 

 the Mobulidae family (0.020 t/1.128 t), marlins of the ge- 

 nus Makaira, and sailfish Ustiophorus platypterus) (0.016 

 t/0.895 t) (Tables 4 and 6). In the present study, bycatches 

 are presented in parentheses as t per positive set/t per 

 1000 t of target species. 



Whale-shark-associated schools 



Two schools associated with whale sharks were sampled 

 only in the winter season (Table 2, Fig. 2, A and B) south of 

 the equator (Fig. IC). In these sets, the bycatch consisted 

 of the shark itself and a small quantity of albacore (Thun- 

 nus ahilunga) (Table 4). This small sample size prohib- 

 ited reliable bycatch estimates for whale-shark-associated 

 schools and inferences of the species compositions of such 

 associations. 



Whale-associated schools 



In the observers' logbooks, among the 45 sets on whale- 

 associated tuna schools, 13 sets were made on schools 

 associated with sei whales (Balaenoptera borealis Lesson, 

 1828) and one set on a school associated with a fin whale 

 (B. physaliis (Linnaeus, 1758)).'- The remaining sets were 

 made on unidentified baleen whales. According to verbal 

 reports by some observers,'^ tuna schools associated with 

 Brydes whale (B. edeni Anderson, 1878), Minke whales [B. 

 acutorostrata Lacepede, 1804), and pygmy blue whales (B. 

 muscitliis brevicauda Linnaeus, 1758) were also observed 

 in the WIO. From personal observations and those of 

 the observers, sperm whales (Physeter catodon Linnaeus, 

 1758) were found often in the areas of the tuna purse- 

 seine fishery; tunas, on the other hand, were not observed 

 to associate with sperm whales. According to observations 

 made during setting and searching operations, whales 

 associated with tunas generally were found in groups of 

 up to 8 individuals, more often in groups of 2-3 whales. 



Whale-associated schools were most often obsei-ved from 

 January to April. A whale-associated school was obsei^ved 

 in July north of the equator (Table 2, Figs. IC, 2, A and B). 

 Schools of this type were distributed mainly south of the 

 equator at latitudes 4-9°S. Skipjack, yellowfin, and bigeye 

 tunas dominated in whale-associated schools — 59%, 32%, 

 and 6%, respectively (Table 5). The percentage of each spe- 

 cies in different sets varied greatly: 0-100% for skipjack, 

 0-100% for yellowfin, and 0-74% for bigeye tuna. Associa- 

 tions consisting of one tuna species and a whale were en- 

 countered in eight cases (22%). Bycatch in whale-associ- 

 ated schools was found in 68% of the sets, and nontuna 

 bycatch in 43% of the sets. 



During sets on whale-associated schools, the fishermen 

 keep the whale(s) inside the purse seine as long as pos- 



'- Species identification could be erroneous. 

 ' ' Bashmakov, V. F. 1990. Personal commun 



Atlantic Scien- 

 tific Research Institute of Marine Fisheries and Oceanogra- 

 phy (AtlantNIRO), 5 Dmitry Donskoy St., Kaliningrad, 236000, 

 Russia. 



