Power and May: Sea-surface temperatures and Thunnus albacares catch and effort 



437 



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Mean polygon SST 



Figure 10 



Yellowfin tuna CPUE versus corresponding satellite-derived 

 sea-surface temperature (SST) in the circular region equivalent 

 to longline set length. Curve is fit using robust locally-weighted 

 regression. 



Mean polygon gradient 



Figure 12 



Yellowfin tuna CPUE versus corresponding satellite-derived 

 sea-surface temperature (SST) gradient in the circular region 

 equivalent to longline set length. Curve is fit of data using 

 robust locally-weighted regression. 



| TO- 



S 40 



a 



Polygon SST coefficient of variation 



Figure 1 1 



Yellowfin tuna CPUE versus corresponding satellite-derived 

 sea-surface temperature (SST) coefficient of variation in the 

 circular region equivalent to longline set length. Curve is fit 

 of data using robust locally-weighted regression. 



C5 40 



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j- 30 

 1 



.2 20 

 2 



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Polygon gradient coefficient of variation 



Figure 13 



Yellowfin tuna CPUE versus corresponding satellite-derived 

 sea-surface temperature (SST) gradient coefficients of varia- 

 tion in the circular region equivalent to longline set length. 

 Curve is fit of data using robust locally-weighted regression. 



(1) Tunas are renowned for their swimming ability, 

 and telemetric studies have demonstrated that 70 cm 

 skipjack tuna can readily traverse a distance of over 

 100 km/day (Dizon et al. 1978). Although yellowfin tuna 

 in this study may have remained in the vicinity of a par- 

 ticular front for an extended period, it is also possible 

 the yellowfin tuna were actively moving over a wide 

 geographic area. (2) The particular longline set may 

 have been targeting other species, and so hooks may 

 have been set at a depth or time of day inappropriate 

 for the capture of yellowfin, also masking any yellowfin 



CPUE-temperature relationships. (3) There is some 

 evidence that yellowfin tuna aggregate at fronts dur- 

 ing certain life-history stages. Beardsley (1969) com- 

 pared numbers of yellowfin tuna caught by longlines, 

 purse seines, and bait boats at a frontal zone in the 

 eastern tropical Atlantic. He concluded that the smaller 

 surface-schooling yellowfin taken in the purse seines 

 were more abundant near the front, but that there was 

 no apparent association between the front and the 

 numbers of larger fish captured on longlines. Addi- 

 tional biological information such as length, sex, gonad 



