Competitive ability is related to metabolic asymmetry in juvenile rainbow trout

Of 55 size-matched pairs of juvenile rainbow trout Oncorhynchus mykiss, eac h consisting of one high and one low relative metabolic rate fish (relative size difference <5%), the high relative metabolic rate fish was dominant i n 36 pairs (65.5%), significantly more often than expected by chance. The p robability of a fish winning was related to its relative metabolic rate (me asured prior to introduction into the test arena); the higher the relative metabolic rate of a fish compared to its opponent, the greater its probabil ity of being dominant. In the 36 pairs or fish where the low relative metab olic rate fish was the subordinate, its competitive ability was significant ly correlated with how closely the two fish were matched in terms of their relative metabolic rates. The smaller the difference between the relative m etabolic rates of the dominant high relative metabolic rate fish and subord inate low metabolic rate fish, the greater the competitive ability and feed ing success of the low relative metabolic rate subordinate. However, no suc h relation was found in the 19 pairs of fish where the high relative metabo lic rate fish was the subordinate. In these pairs, the competitive ability of the subordinate high relative metabolic rate fish was correlated with it s size relative to its dyad partner. The larger the dominant low relative m etabolic rate fish, the lower the competitive ability of the subordinate hi gh relative metabolic rate fish. (C) 2001 The Fisheries Society of the Brit ish Isles.