SEX STEROIDS, GROWTH AND CONDITION OF ARCTIC CHARR BROODSTOCK DURING AN ANNUAL CYCLE

Changes in plasma concentrations of sex steroids, growth rate and cond ition of repeat spawning (3+) male and female Arctic charr were studie d throughout an annual reproductive cycle. Individually marked fish (m ean weight approx. 500 g) were held under conditions of liberal food s upply, constant temperature (4 degrees C) and simulated natural photop eriod (Tromso, 70 degrees N). Once each month fish were weighed, measu red and blood samples taken for steroid analysis. Plasma concentration s of testosterone (T), 11-ketotestosterone (11-KT) and oestradiol-17 b eta (E-2) were determined using radioimmunoassay (RIA). Both male and female fish displayed distinct seasonal changes in plasma concentratio ns 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 conc entrations which culminated in peak steroid concentrations in Septembe r-October, There was then a rapid decline during the spanning period. In winter, growth rate and condition were generally low, then increase d during the spring, reached a peak during the summer, and then declin ed with the onset of autumn. During spring (March-May), the frequency distributions of plasma testosterone concentrations in both male and f emale fish were bimodal. The fish of the upper modal group of the dist ribution had significantly higher growth rates and condition than thos e in the lower modal group. In summer and early autumn (June-September ) the association between T and growth rate changed. Significant negat ive correlations between T and growth rates were observed in females. There was an increase in endocrine activity, indicated by elevated pla sma sex steroid concentrations in March, 7-8 months prior to maturatio n. It is suggested that this may be one factor influencing the onset o f spring growth and energy deposition among maturing charr. (C) 1998 T he Fisheries Society or the British Isles.