Aggressive and foraging behavioral interactions among ruffe

The ruffe, Gymnocephalus cernuus, is a nonindigenous percid in the Great La kes. Ruffe are aggressive benthivores and forage over soft substrates. Labo ratory studies in pools (100 cm diameter, 15 cm water depth) were conducted to determine whether fish density (low = 2, medium = 4, high = 6 ruffe per pool) changed foraging and aggressive behaviors with a limited food supply of chironomid larvae. All fish densities demonstrated a hierarchy based on aggressive interactions, but ruffe were most aggressive at low and high fi sh densities. Time spent in foraging was lowest at the low fish density. Th e best forager at the low fish density was the most aggressive individual, but the second most aggressive fish at the medium and high fish density was the best forager and also the one chased most frequently. A medium fish de nsity offered the best energetic benefits to ruffe by providing the lowest ratio of time spent in aggression to that spent foraging. Based on our resu lts, ruffe should grow best at an intermediate density. With high ruffe den sities, we would also expect disparity in size as the more aggressive fish are able to garner a disproportionate amount of the resources. Alternativel y, as the Great Lakes are a fairly open system, ruffe could migrate out of one area to colonize another as populations exceed optimal densities.