Diet Overlap and Food Habitats of Slimy Sculpin, Deepwater Sculpin, and Round Goby During Winter and Spring in Offshore Lake Michigan

Abstract

Ecological stability in offshore benthic food webs of the Laurentian Great Lakes has been recently altered by non-native species such as round goby Neogobius melanostomus and quagga mussels Dreissena bugensis, as well as steep declines in biomass of Diporeia, a native amphipod. Correspondingly, population dynamics, life histories and the diet composition of native sculpins may be affected. I examined food habits of slimy sculpin Cottus cognatus, deepwater sculpin Myoxocephalus thompsonii, and round goby collected from January to May in 2009 and 2010 in Lake Michigan offshore of Frankfort (FF), MI; Muskegon (MSK), MI; Two Rivers (TR), WI; and Sturgeon Bay (STB), WI in depths of 69-128m. Important prey (by dry weight proportion and % occurrence) for slimy sculpin were Mysis (0.34, 45%), Diporeia (0.16, 34%), and Limnocalanus macrurus (0.22, 68%). Prey important to deepwater sculpin were Mysis (0.74, 92%) and Diporeia (0.16, 54%). Round goby consumed mainly bivalves (0.68, 95%) and Mysis (0.15, 37%). Diet composition for all three species did not vary across days sampled in January through April, and little variance was explained by year or depth sampled. Variance in diet composition for each benthivore species was best explained by the site from which fishes were sampled. For example, Diporeia constituted high diet proportions in sculpins offshore of TR and STB, but was absent in sculpins offshore of FF and MSK. Significant diet overlap (Schoener’s index ≥ 0.60) was identified between slimy and deepwater sculpin offshore of FF and STB but not offshore of TR. Significant diet overlap was not found between round goby and either sculpin species. Non-metric multi-dimensional scaling and cluster analyses revealed groups of fishes at each site with similar diets: 1) slimy sculpin only; 2) round goby only, and; 3) all deepwater sculpin, some slimy sculpin individuals, and very few round goby. Cluster analysis also distinguished groups for both sculpin species that reinforced the diet overlap results. Using genetic analyses on fish eggs taken from diet samples, bloater Coregonus hoyi and deepwater sculpin eggs were confirmed prey for slimy and deepwater sculpin in each month from February through May at all four sites sampled. Round goby consumed few deepwater sculpin eggs and no bloater eggs.