THE INTERACTION AMONG AGE, THERMAL-ACCLIMATION AND GROWTH-RATE IN DETERMINING MUSCLE METABOLIC CAPACITIES AND TISSUE MASSES IN THE THREESPINE STICKLEBACK, GASTEROSTEUS-ACULEATUS

Thermal acclimation may directly modify muscle metabolic capacities, o r may modify them indirectly via effects upon physiological processes such as growth, reproduction or senescence. To evaluate these interact ing effects, we examined the influence of thermal acclimation and accl imatization upon muscle metabolic capacities and tissue masses in 1+ s tickleback, Gasterosteus aculeatus, in which confounding interactions between temperature and senescense should be absent. Furthermore, we e xamined the influence of thermal acclimation upon individual growth ra te, muscle enzyme levels and tissue masses in 2+ stickleback sampled a t the beginning of their final reproductive season. For 1+ stickleback , cold acclimation more than doubles mitochondrial enzyme levels in th e axial muscle. Thermal acclimation did not change the condition of 1 stickleback at feeding levels which could not maintain the condition of 2+ stickleback. Compensatory metabolic responses to temperature wer e not apparent in field acclimatized 1+ stickleback. The growth rate o f 2+ stickleback was markedly affected by temperature: warm-acclimated fish generally lost mass even at very high levels of feeding (up to 7 8 enchytraid worms per day) while cold-acclimated fish gained mass. Th is suggests that warm temperatures accelerate the senescence of 2+ sti ckleback. Generally, muscle enzyme activities increased with growth ra te. In axial muscle, the relationships between CS activity and growth rate differed with acclimation temperature. Independent of the influen ce of growth rate, CS activities were consistently higher in cold- tha n warm-acclimated 2+ stickleback, suggesting compensatory increases of CS activity with cold acclimation.