Intraspecific aggression in rosyside dace, a drift-feeding stream cyprinid

Individual rosyside dace Clinostomus funduloides in a semi-natural, artific ial stream displayed substantial differences in their aggressiveness and co uld be classified as: (1) non-aggressive (NA, 18 of 30 rosyside dace), (2) moderately aggressive (MA, 9 of 30) and (3) highly aggressive (I IA, 3 of 3 0). Rosyside dace groups, however, did not exhibit linear dominance hierarc hies and fish size was only weakly correlated with the number of aggressive acts performed per individual. Small rosyside dace (< 56 mm L-p) were alwa ys non-aggressive, but larger fish were present in all three aggression cla sses. The difference in size between the contestants was significantly, alt hough not very strongly, correlated with the probability of winning an agon istic interaction (r(2)=0.39). Aggressive rosyside dace may have ultimately gained higher fitness than less aggressive ones. HA individuals occupied t he upstream-most position within foraging groups significantly more often t han other rosyside dace. This location should be the most profitable one be cause its occupant will be the first to encounter prey. IIA rosyside dace a lso occupied significantly higher focal velocities that were closer to ener getic optima than MA and NA ones. They also had greater foraging rates and were less solitary than less aggressive fish, but these differences only we re significant at the P=0.066 and P=0.081 level, respectively. Finally, IIA fish performed significantly more aggressive acts and feedings backwards t han other individuals. Despite these differences, the effects of intraspeci fic aggression in rosyside dace appeared less substantial than those that h ave been observed in stream salmonids. (C) 2001 The Fisheries Society of th e British Isles.