Concentrations of melatonin, thyroxine, 17β-estradiol and 11-ketotestosterone in round goby (Neogobius melanostomus) in different phases of the reproductive cycle.

The aim of this study was to determine changes in concentrations of melatonin (Mel) and thyroxine (T) in plasma, and 17β-estradiol (E) and 11-ketotestosterone (11-KT) in plasma and gonads of female and male round gobies (Neogobius melanostomus) from the Southern Baltic Sea in four phases of the reproductive cycle classified as pre-spawning, spawning, late spawning and non-spawning periods. The concentrations of Mel, T and E were determined by radioimmunoassay (RIA) whereas 11-KT was quantified using an enzyme immunoassay (EIA). The maturity stage of gonads was determined using histological analyses. The pattern of changes in Mel concentrations of females and males was similar with the greatest concentrations in the spawning and non-spawning phases. In both sexes, there was a similar tendency of change in concentrations of T and E with the increase being in the pre-spawning and non-spawning phases. The greatest concentrations of 11-KT were observed in the plasma and gonads of males during the spawning phase. In females, there were no changes in 11-KT concentrations either in plasma or gonads during all phases where quantifications occurred. This is the first study for determination of the pattern of changes in Mel and T concentrations as well as gonadal steroids E and 11-KT, supported by histological analysis of gonads, in batch spawning fish during the reproductive cycle.