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.