Au: Polyspecific nature of Thunnus albacares in eastern tropical Pacific 



345 



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a) Log-fish Sets 



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c) Porpoise-fish Sets 



I I 



Figure 1 



Locations of (a) log-fish sets, (b) school-fish sets, and (c) porpoise-fish sets inspected. Data were examined separately to the east ("off 

 Central America") and west ("off Mexico") of longitude 100 C W. 



Related data gleaned from the records were sum- 

 marized. These included information on size, abun- 

 dance, and actual species composition within the 

 species groups. 



The species composition of bird flocks occurring with 

 the tuna was described after ordering the (n) reported 

 species in each flock according to (1 to n) decreasing 

 ranks of abundance (again, the species were actually 

 grouped species). This was done to circumvent dif- 

 ferences in the accuracy with which different observers 

 estimated species' flock sizes. The fraction of total 

 flocks in which a bird species ranked at least 2 (i.e., 

 was at least second most-abundant) was regarded as 

 that species' importance value. When that value was 

 50% or more of the flocks in a category, that species 

 was considered dominant for that set-type category. 



Several species could be "dominant" according to this 

 criterion. 



Both yellowfin, and skipjack Katsuwonus pelamis, 

 tuna are sought by purse seiners, although yellowfin 

 is usually preferred. Overall, yellowfin was captured 

 in 78% of the log-fish sets, 57% of school-fish sets, 

 and 99% of porpoise-fish sets examined (including sets 

 capturing both tunas). The mean tonnages of yellowfin 

 caught per set, based on "successful sets," are given 

 in Table 1. Successful sets are those catching more than 

 1 ton of yellowfin and/or skipjack tuna. 



Tunas from the different set types are likely to be- 

 have differently, since younger yellowfin tend to asso- 

 ciate with floating logs in nearshore waters while older, 

 larger fish tend to be with dolphins farther offshore 

 (Greenblatt 1979). Therefore, the species associations 



