Intraspecific variation in the metabolism of juvenile Atlantic salmon Salmo salar and northern pike Esox lucius
Basal metabolic rate (BMR), the sum of maintenance energy costs, represents a major component of the energy budgets of ectothermic vertebrates and varies between individuals within a species. Individual ectotherms are generally assumed to have a constant BMR at any given temperature. A strategy of flexibility in BMR might have evolved to cope with differing environmental conditions. Within-individual variation in BMR was examined in two fishes, juvenile Atlantic salmon Salmo salar and juvenile northern pike Esox lucius, whilst the effects of exercise and ration on BMR, maximum metabolic rate (MMR), enzyme levels and body composition were studied in detail for pike. In the first part of the study, measurements of BMR were made for first-summer Atlantic salmon parr at rest by respirometry. In 1996, initial measurements of BMR were made for 25 fish held in a stock tank. Fish were then allocated in small groups to channels to represent a change in environment and BMR re-measured after a period of several weeks. The procedure was repeated for 30 fish in the summer of 1997, when fish were given a reduced food ration. Variation in BMR in each experiment was analysed for individual fish, and for all fish using a linear mixed model. There were statistically significant differences in BMR values between the two times, the within-individual, between-time variation representing approximately ± 21% of BMR in 1996 and ± 28% of BMR in 1997. Reduced-rafion fish (1997) displayed a significant decrease in the mean elevation of the allometric scaling relationship between body mass and BMR between time periods. To further explore possible mechanisms for flexibility in BMR and relationships with MMR, juvenile pike were used. Initial measurements of BMR and MMR (following exhaustive exercise) were made and factorial metabolic scope calculated (MMR/BMR). Fish were then split into a high ration no-exercise group (n = 10), low ration no-exercise group (n = 10) and sustained exercise group (n = 13). Initial measurements were termed time 1, with subsequent measurements made after approximately 3 weeks (time 2) and 11 weeks (time 3). Exercised fish had a significantly larger MMR and scope following 3 weeks of sustained swimming. For all fish there were significant correlations between BMR and MMR at times 1 and 3 but not at time 2.After the oxygen measurements made at time 3 all fish were humanely killed. Maximal enzyme assays were performed on six tissues for each remaining fish (n = 30). Levels of each of two enzymes (citrate synthase, CS, and lactate dehydrogenase, LDH, measured in the direction of lactate oxidation) were found to be similar between treatment groups for respective tissues. Total CS activity levels and LDH levels were highest in the heart and red muscle. In general there was little difference in the relative organ masses of fish exposed to different treatments. It is concluded that in these two fish species with very different life styles, between- and within- individual variation in BMR (salmon & pike) and MMR (pike only) is apparent and that differences in ration and exercise influence individual physiology.