Behavior of juvenile bluefish Pomatomus saltatrix in vertical thermal gradients: influence of season, temperature acclimation and food

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Abstract
Behavioral responses of juvenile bluefish P. saltatrix L. exposed to cold water in a vertical thermal gradient were measured under controlled laboratory conditions. The temperature regime was similar to conditions that these fish encounter in their inshore and estuarine habitats along the Middle Atlantic coast. Measures included activity, schooling, position with respect to depth and temperature, and feeding. Bluefish, acclimated to 20 or 14.degree. C, avoided cold water as it was introduced into the experimental aquarium by initially increasing swimming speed and shifting their distribution upward in the water column. Fish with a similar thermal history were distributed into progressively colder water from Aug. through Nov., reflecting a predisposition for lower temperatures. This change in behavioral response to temperature coincided with this species'' southerly fall migration. When food was presented at potentially debilitating temperatures which were normally avoided, motivation to feed modified their avoidance response, with the fish entering these temperatures and successfully feeding. Results are discussed in relation to natural patterns of migration and distribution.

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