RELATIONSHIP BETWEEN SIZE, GILL CORTICOSTEROID RECEPTORS, NA-K+ ATPASE ACTIVITY AND SMOLTING IN JUVENILE COHO SALMON (ONCORHYNCHUS-KISUTCH)IN AUTUMN AND SPRING()

Coho salmon (Oncorhynchus kisutch) normally develop an increase in sal twater tolerance during the spring as part of the parr-smolt transform ation. It has been observed that coho salmon also show increased saltw ater tolerance in the autumn preceding smelting. To determine whether the high salt water tolerance observed during the autumn was similar t o that occurring during smelting in the spring, a group of coho reared in lake net pens in British Columbia, Canada, were sampled from June through to the following May. Changes in gill Na+-K+ ATPase activity, plasma cortisol and gill cytosolic corticosteroid receptor (CR) concen tration (B-max) and dissociation constant (k(D)) were examined. Fish e xamined in this study showed great variation in body size. By the Nove mber after hatch, some individuals in the population were greater than 100 g and developed silver coloration characteristic of smelts. In co ntrast, the smallest fish in the population were approximately 10 g in November and exhibited distinct parr marks, An intermediate group als o existed that were smaller than the large fish in November and March, yet were beginning to silver and exhibited less distinct parr marks. Large fish exhibited significantly increased gill Na+-K+ ATPase activi ty in both the autumn and spring compared to the levels found in parr, Concomitant to the changes in Na+-K+ ATPase activity were an increase in plasma cortisol concentration, a decrease in CR B-max and an incre ase in k(D). Increases in k(D) were positively correlated with Na+-KATPase activity. Small- and medium-sized fish only showed the changes in gill Na+-K+ ATPase activity associated with smelting during the sec ond spring. This study showed that physiological changes associated wi th smelting occurred in November and May in a population of coho salmo n. The accelerated rate of smelt development allowed earlier transfer of fish to salt water in November.