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 saltwater tolerance during the spring as part of the parr-smolt transformation. It has been observed that coho salmon also show increased saltwater tolerance in the autumn preceding smelting. To determine whether the high salt water tolerance observed during the autumn was similar to 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) concentration (B-max) and dissociation constant (k(D)) were examined. Fish examined in this study showed great variation in body size. By the November after hatch, some individuals in the population were greater than 100 g and developed silver coloration characteristic of smelts. In contrast, the smallest fish in the population were approximately 10 g in November and exhibited distinct parr marks, An intermediate group also 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 activity 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 increase in k(D). Increases in k(D) were positively correlated with Na+-K+ ATPase activity. Small- and medium-sized fish only showed the changes in gill Na+-K+ ATPase activity associated with smelting during the second spring. This study showed that physiological changes associated with smelting occurred in November and May in a population of coho salmon. The accelerated rate of smelt development allowed earlier transfer of fish to salt water in November.