Plasma concentrations of sex steroids and fighting in male Tilapia zillii

In laboratory-held Tilapia zillii the gonadosomatic index (I-G) is an accur ate predictor of dominance in pairwise fights between males. Immediately af ter fighting no differences were detected between winners, losers or contro ls, in plasma concentrations of testosterone (T) or 11-ketotestosterone (11 KT). 11KT was correlated negatively with I-G and positively with T. 11KT ap peared to be the major androgen in this species, but there was no evidence that simple differences in levels of these sex steroids mediated aggressive behaviour. (C) 1999 The Fisheries Society of the British Isles.