Seasonal ecology and biochemistry of juvenile Atlantic salmon

The ability to hold station against a water current, the efficiency of enzyme catalysis, and various aspects of body morphology were studied in juvenile Atlantic salmon during their period of freshwater residence. This was done in order to assess the effect of varying seasonal temperatures on juvenile salmon and to compare the relative physiological performance of fish following differing life-history strategies, which are manifested by the upper (UMG) and lower (LMG) modes of a bimodal size distribution within a sibling population. The relative ability to oppose water current (critical holding velocity: CHV) varied with water temperature and was similar for both 0+ modal groups until the UMG fish became smolts, at which time they could withstand lower velocities than the LMG fish. 1+ LMG fish during their final year in fresh water demonstrated a seasonal pattern of CHV similar to that of UMG fish in their first year. Within the samples of 0+UMG and 1+LMG fish at smolting CHV was significantly correlated with the degree of silvering seen in smolting fish. The kinetic characteristics of lactate dehydrogenase (LDH) from juvenile salmon white muscle changed throughout the sampling period, so that the efficiency of this enzyme (as measured by the Michaelis constant) tended to be greatest at the temperature the fish were experiencing when sampled. There was no apparent difference in the seasonal efficiency of LDH between the 0+ modal groups. The efficiency of LDH at the different environmental temperatures also varied significantly with seasonal temperature, being most efficient at the lowest environmental temperatures. Glucose-6-phosphate dehydrogenase (G6PDH) from the liver also demonstrated a tendency to be most efficient at the environmental temperatures. However, LMG fish appeared to show greater adaptation of this enzyme to seasonal water temperatures than UMG fish. There was no seasonal variation in the efficiency of G6PDH at the environmental temperatures, except for the sample taken during May (the time when UMG fish were smolting) when the apparent efficiency of both modal groups decreased. This enzyme also demonstrated a seasonal variation in the free energy of activation' which tended to increase throughout the first year of fresh water residence. Truss measurements were taken over the whole body of juvenile salmon from both modal groups throughout the period of freshwater residence and standardized for the size of fish to allow comparisons of body shape. Principal components analysis was used to identify the measurements that differed between modal groups, and the standardized measurements were then analysed directly. With the appearance of the bimodal size distribution relative differences in body morphology became evident, with the UMG fish having relatively smaller jaws and longer heads; longer, deeper trunks; similar post-anal regions; shorter anal fins and longer dorsal and tail fins than the LMG throughout most of the first year in freshwater. However, by the time that 0+ UMG fish were smolting these differences in shape had mostly disappeared, although smolts tended to have slightly deeper heads and trunks. These results contrasted with the general view that at smolting salmon show a relative lengthening of their body. The post-anal region, which is generally considered to be the area of the body that lengthens at smolting, showed no difference between the modal groups at any time. 1+ LMG fish (i.e. in their second year of fresh water residence) demonstrated a similar pattern of overall head, posterior trunk, and dorsal fin measurements to that of 0+ LMG fish, whereas the anterior and mid-trunk measurements of 1+ LMG fish tended to be intermediate between those of the 0+ modal groups. However, by May there was no difference between the standardized measurements of both ages of smolting fish. The mesenteric lipid levels peaked in November in both modal groups of 0+ salmon, but UMG fish maintained a relatively higher level than the LMG fish. Both 0+ UMG and 1+ LMG fish had generally similar levels of mesenteric lipid, but 0+ LMG fish showed a different seasonal pattern. The results are discussed in relation to each other and in relation to the differing behaviour and ecology demonstrated by fish following different life-history strategies. It is suggested that the LMG fish are not physiologically less capable than those of the UMG, and that any differences in performance, metabolism and body shape are related to the differing ecologies of these fish.