436 



Fishery Bulletin 89(3). 1991 



Figure 9 



Example of sea-surface temperature (SST) gradient image. Lighter colors indicate higher gradient magnitude, as at frontal zones. 

 Crosses indicate locations of longline sets, with circles encompassing length of sets. 



approach used by Maul et al. (1984) and others, and 

 may work well if the interest is in a dominant and clear- 

 ly defined feature such as an edge of the Loop Current 

 or the Gulf Stream. We did not use this approach for 

 two reasons. First, the additional time and computa- 

 tional effort seemed unwarranted, based on the nega- 

 tive outcome of our gradient analyses. Secondly, such 

 an approach would require us to define a "front" using 

 an arbitrary criteria: we would necessarily have to 

 define a given gradient magnitude, extending over a 

 given distance, as comprising a suitable front. Then, 

 because we would expect to define multiple fronts in 

 the northwest Gulf of Mexico (cf. Fig. 1), defining 

 distances to the fronts would be problematic. For ex- 

 ample, in the case where a longline set was located 



40 km from one of the defined fronts, and 60 km from 

 another, it would be difficult to determine to which 

 front the set should be related. Additionally, elevated 

 catches near a front do not necessarily indicate fish 

 were directly associated with that front; animals prox- 

 imal to a front have not necessarily reacted to its 

 existence. We believe our approach, which asks, "Are 

 catches elevated when SST varies over some region 

 encompassing those catch locations?", to be a more 

 conservative and objective assessment of possible fish- 

 SST associations. It is also one less likely to be in- 

 advertently biased by preconceptions concerning fish- 

 front associations. 



There are also several possible biological explanations 

 why we did not find a tuna-temperature relationship: 



