Seasonal consumption of Hemimysis anomala by fish in Southeastern Lake Ontario, 2009–2010

We investigated the seasonal occurrence of Hemimysis anomala in the diets of fish that prey on macroinvertebrates at two sites with established Hemimysis populations east of Oswego, NY, during 2009–2010. In 2009, we examined 320 stomachs from 10 species and found Hemimysis only in alewife (Alosa pseudoharengus), rockbass (Ambloplites rupestris), and yellow perch (Perca flavescens). Of those species, alewife consumed Hemimysis most frequently and it represented a greater proportion of their diets. During 2009, the dry weight composition of Hemimysis in alewife diets varied seasonally between < 1% in June, 5% in July, 98.5% in August, and 18.8% in September. In contrast, we examined 667 stomachs from 15 species in 2010 and observed Hemimysis in only one alewife and two rockbass stomachs. For alewife from September 2009, we found no relationship between predator size and the number of Hemimysis consumed, or between the presence of Hemimysis in fish diets and the presence of other diet taxa or diet diversity. Fish diets collected as bycatch from other assessments revealed large numbers of Hemimysis in fishes that had not previously been observed consuming Hemimysis in Lake Ontario, including cisco (Coregonus artedi) and white perch (Morone americana). Our results indicate Hemimysis consumption by nearshore fish can be high, but that it is variable across seasons and years, and may be most prevalent in fish that feed up in the water column, at or near dark, and have the ability to consume swift moving prey like Mysis diluviana or small fish.

► In 2009–2010 we observed Hemimysis in diets of alewife, yellow perch and rockbass.
► Predation by alewife on Hemimysis varied seasonally and was greatest in August.
► We recorded new observations for cisco and white perch that consumed Hemimysis.
► Fish that consumed Hemimysis feed in lowlight and can handle large invertebrates.
► Fish that did not consume Hemimysis were strictly benthic fishes or small fishes.