The Round Goby (Apollonia melanostomus) is a small benthic fish, native to the Eurasian Ponto-Caspian region, that has rapidly spread through the entire Laurentian Great Lakes system since its 1990 discovery in Lake St. Clair. Tolerant of high population densities, the exotic Round Goby competes aggressively with native fish for food and habitat, and has increasingly been exploited by endemic Great Lakes predators. A management program for the Upper Niagara River, initiated in 2004, has provided an opportunity to study the interactions between these invaders and the Double-crested Cormorant (Phalacrocoiax auritus), a native top predator. The gut contents of 1,119 cormorants nesting at two sites in the Upper Niagara River from 2004 to 2007 were examined, and the species composition of ingested prey (by number and weight) was quantified for the 600 stomachs that contained identifiable prey. Results of these analyses indicate that Round Goby can constitute up to 85% of the biomass in cormorant diet during periods of the breeding season, and that gobies are consumed by cormorants through all dates sampled (May through August). Lengths of Round Gobies recovered in the cormorant diet were skewed towards larger members of the goby population, suggesting non-random selection relative to the range of size possibilities, and displayed significant declines in length between and within seasons.
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