Anatomic and enzymatic responses of the three‐spined stickleback, Gasterosteus aculeatus to thermal acclimation and acclimatization

The three‐spined stickleback (Gasterosteus aculeatus) from the St. Lawrence Estuary prefers temperatures between 9 and 12°C, but encounters temperatures ranging from 0 to 30°C. To determine whether despite this preference for cool temperatures this species retains a capacity for metabolic compensation during thermal acclimation, adult sticklebacks were acclimated to 4°C and 20°C for 6 weeks at various ration levels. When food rations were maximal, PFK, CPT, and CS activities were twofold higher in cold‐acclimated sticklebacks, particularly in the pectoral muscle. This pattern of thermal compensation is similar to that shown by eurythermal fish with considerably higher thermal optima. However, at sub‐maximal rations, warm‐acclimated sticklebacks can undergo a decline in condition which confounds the compensatory response. In the field, even though food is abundant, the condition of male sticklebacks also declined as temperatures rose. When the condition factor declined by 30%, the pectoral and axial muscle indices decreased by 40% and 55%, respectively, and the activities of glycolytic and oxidative enzymes in the pectoral and axial muscles dropped by up to 60%. Clearly, a temperature‐induced increase in metabolic rate can confound a compensatory response to temperature.