Variations in thyroid hormone levels and in performance in the seawater challenge test accompanying development in coho salmon raised in Hawaii

Abstract

The temporal relation between variations in blood thyroid hormone levels and changes in seawater challenge (SWC) performance was examined in juvenile coho salmon. Sampling commenced in April 1983 when the fish were approximately 4 months old and continued into May 1984. The fish were held under four sets of conditions in a factorial design with photoperiod (Hawaii: longest day = 13 h 21 min; shortest day = 10 h 55 min; and California: longest day = 15 h 1 min; shortest day = 9 h 20 min) and temperature (11° and 17°C) as variables. The timing of changes in thyroid hormones and SWC performance was remarkably similar in all treatment groups. Performance in the SWC was optimal in underyearling fish during July, but was generally poor from late summer until the end of the year. Starting in January, SWC performance improved steadily, leading to a second peak in March. Throughout the study period, SWC performance showed no relation to body size. Under all treatment conditions elevations in blood thyroid hormones preceded the periods of maximal SWC performance. This pattern is strikingly similar to the relation between thyroid hormone variations and the survival of coho salmon in seawater net pens, and suggests that periods of enhanced survival may be related to improved hypoosmotic regulatory ability.