Diet choice by the exotic round goby (Neogobius melanostomus) as influenced by prey motility and environmental complexity

This laboratory study examined the influence of substratum complexity and water clarity/visibility on non-indigenous round goby (Neogobius melanostomus) diet choice between dreissenid mussels (Dreissena polymorpha and D. bugensis, 6 to 9 mm length) and the exotic amphipod Echinogammarus ischnus. When both prey items were offered simultaneously in bare 20-L aquaria holding clear ambient water, 6.5 to 8-cm round gobies chose primarily amphipods (> 85% of diet numerically) and consumed fewer dreissenids (< 2/h) than when mussels were offered alone (5.2/h). Round gobies could ingest substantially more biomass when feeding on a mixed diet (∼17 to 24 mg/h dry weight, not including dreissenid shells) or on amphipods alone (∼26 mg/h), than feeding on dreissenids alone (∼12 mg/h). Longer handling time of mussels may thus have influenced the round gobies' preference for amphipods. Added substrata (stones or gravel) and/or diminished visibility (turbid water or darkness) shifted round goby diet markedly towards sessile dreissenids as motile amphipods found refuge. Two-way ANOVA indicated that both substratum and water clarity/visibility significantly influenced round goby diet, but did not interact. It is possible that the large contribution of dreissenids to round goby diet in the Great Lakes may not necessarily reflect a preference for them, but rather lower encounter rates with more profitable prey.