Au: Polyspecific nature of Thunnus albacares in eastern tropical Pacific 



351 



Off Central America 



Off Mexico 



Log-fish 

 (58 sets) 



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(no sample) 



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Porpoise-fish 



Porpoise-fish 

 (70 sets) 



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Figure 4 



Histogram envelopes of bird flock sizes 

 off Central America and off Mexico 

 according to log-fish, school-fish, and 

 porpoise-fish set types. Each histogram 

 plots frequency {ft SETS) against the 

 species' abundance in the flocks 

 (BIRDS/FLOCK). T.M. Duffy samples 

 only. 



swimmers like tuna, those trips with low occurrence 

 rates may represent separation of this species from 

 tuna during capture. When captured together with 

 tuna, and with other associated fishes as well, dolphin- 

 fish averaged 6 individuals per set, with the range from 

 1 to 200 (Table 4). 



Several other miscellaneous fishes were caught 

 rather persistently, especially in log-fish sets. Notable 

 among these were yellowtail or amberjack (Seriola sp.) 

 and wahoo Acanthocybium solandri (Table 4). Others, 

 such as triggerfish (Balistidae) were common and were 

 probably associated with the flotsam near which the 

 tuna were caught; these small fish are frequently seen 

 under such objects (pers. observ.). 



The fraction of successful (not total) purse seine sets 

 that took both yellowfin and skipjack tuna measures 

 the likelihood of these two tunas being associated. 

 Yellowfin and skipjack were together in nearly 70% 

 of successful log-fish sets. This fraction declined 

 markedly in school-fish sets and even more so in the 

 porpoise-fish sets, i.e., to 3% or less (Table 2). 



The largest category of other animals associated with 

 tuna was seabirds (Figs. 2, 3). Birds have a close rela- 

 tionship to tuna (and tuna fishing to birds), and 

 observers usually record some aspect of their presence. 

 Approximately 80% of log-fish and school-fish sets 

 were in company with birds, and birds were present 

 with virtually all porpoise-fish sets, regardless of area. 

 The dominant bird species, as identified by the impor- 

 tance-ranking criterion, were usually boobies (Sula 

 spp.) or boobies and shearwaters (Puffinus spp.) with 

 log- and porpoise-fish sets; in the school-fish sets, the 

 dominants were shearwaters or shearwaters and terns, 

 Sterna especially (labeled histograms in Figures 2 and 

 3). The distribution of flock sizes of component species 

 from the single-observer TMD data (Fig. 4) shows an 

 increase in flocks and flock sizes of frigatebirds 

 (Fregata spp.) and especially boobies in porpoise-fish 

 sets relative to the other set types. Also, large tern 

 flocks occurred primarily with school-fish sets. There 

 was a shift toward larger shearwater flocks in the 

 porpoise-fish sets off Mexico; the identifications 



