Sex steroids, growth and condition of Arctic charr broodstock during an annual cycle

Abstract

Changes in plasma concentrations of sex steroids, growth rate and condition of repeat spawning (3+) male and female Arctic charr were studied throughout an annual reproductive cycle. Individually marked fish (mean weight approx. 500 g) were held under conditions of liberal food supply, constant temperature (4° C) and simulated natural photoperiod (Tromsø, 70° N). Once each month fish were weighed, measured and blood samples taken for steroid analysis. Plasma concentrations of testosterone (T), 11-ketotestosterone (11-KT) and oestradiol-17β (E2) were determined using radioimmunoassay (RIA). Both male and female fish displayed distinct seasonal changes in plasma concentrations of sex steroids, growth rate and condition. From February (minimal concentrations) to March all sex steroids increased slightly and these elevated concentrations were maintained until May. Thereafter, there was a second, and far more pronounced, increase in plasma steroid concentrations which culminated in peak steroid concentrations in September–October. There was then a rapid decline during the spawning period. In winter, growth rate and condition were generally low, then increased during the spring, reached a peak during the summer, and then declined with the onset of autumn. During spring (March–May), the frequency distributions of plasma testosterone concentrations in both male and female fish were bimodal. The fish of the upper modal group of the distribution had significantly higher growth rates and condition than those in the lower modal group. In summer and early autumn (June–September) the association between T and growth rate changed. Significant negative correlations between T and growth rates were observed in females. There was an increase in endocrine activity, indicated by elevated plasma sex steroid concentrations in March, 7–8 months prior to maturation. It is suggested that this may be one factor influencing the onset of spring growth and energy deposition among maturing charr.