438 



Fishery Bulletin 89(3). 1991 



weight, maturity stage, and age is necessary to deter- 

 mine whether relationships exist between frontal 

 occurrence and life-history stages of the species. Also, 

 yellowfin tuna may aggregate at color fronts, and not 

 thermal boundaries. This phenomenon was observed 

 by Laws et al. (1984) in albacore tuna. 



Finally, the SST structure present in the northwest 

 Gulf of Mexico is dynamic, and can change rapidly 

 depending on local atmospheric conditions. Huh et al. 

 (1978) provide a sequence of Gulf of Mexico images 

 demonstrating how SST can change with time, and 

 speculated that air-sea heat fluxes can rapidly alter the 

 pattern of SST temperatures observable from satel- 

 lites. The AVHRR detect only the immediate surface 

 temperature (Schluessel et al. 1987), which may not be 

 indicative of deeper water the yellowfin tuna may 

 prefer. Consequently, the surface thermal patterns in 

 the northwestern Gulf of Mexico may not persist long 

 enough to either aggregate yellowfin tuna directly or 

 set up other conditions, such as enhanced food avail- 

 ability, that would result in a detectable fish-tempera- 

 ture relationship. 



In summary, the fisherman's belief that tuna aggre- 

 gate in response to thermal patterns is a persuasive 

 argument that such behavior occurs, and we initiated 

 this research with that preconception. We are now 

 uncertain whether such a phenomenon has global 

 applicability, and consider from our results that this 

 behavior does not reliably occur in the northwestern 

 Gulf of Mexico. This may be due to the dynamic and 

 non-persistent nature of the thermal patterns, or that 

 those patterns do not generally occur in conjunction 

 with other processes such as upwelling. 



Acknowledgments 



This research was supported by the National Marine 

 Fisheries Service Marine Fisheries Initiative (MAR- 

 FIN) program, contract number NA89WC-H-MF015. 

 We wish to offer our sincerest thanks to Tom Leming 

 and Rex Herron of the National Marine Fisheries 

 Service (NMFS), who provided much of the imagery 

 used in this study, to Joan Browder, also of NMFS, 

 who provided the tuna catch data, and to Larry Rouse 

 of the LSU Coastal Studies Institute, who assisted 

 with satellite imagery. We thank Wendy Morrison, 

 Floyd Stayner, and David Wilensky for assistance with 

 computer details. We also wish to thank Robert 

 Carney, LSU Coastal Ecology Institute, whose efforts 

 provided the VAX computers essential to this study. 

 Finally, we thank the editorial staff of the Fishery 

 Bulletin for their considerable assistance with a diffi- 

 cult manuscript. 



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