Rainbow Smelt (Osmerus Mordax) Predation on Slimy Sculpin (Cottus Cognatus) in Lake Ontario

Slimy sculpin (Cottus cognatus) and opposum shrimp (Mysis relicta) formed the primary prey of 319 adult (96–201 mm total length) rainbow smelt (Osmerus mordax) collected day and night at depths of 30–50 m in southeastern Lake Ontario during 21–25 August and 23–31 October 1984. Prey were eaten primarily at night. Between 2000–2400 hours, 77% of rainbow smelt during August and 43% of rainbow smelt during October contained slimy sculpin, compared to less than 10% during the day. Mean number of sculpin per smelt stomach was 1.2 and 1.5, respectively. Mean sizes of slimy sculpin eaten (total lengths of 12.7 mm in August and 21.1 mm in October) were significantly less than those of sculpin (30.4 mm in August and 31.6 mm in October) caught in trawls at the same depths. Occurrence of opposum shrimp in smelt stomachs also peaked at night at 80–90% and an average of up to 2.1 and 5.2 opposum shrimp were found per smelt stomach in August and October, respectively. We infer that rainbow smelt may compete with juvenile lake trout (Salvelinus namaycus) for slimy sculpin in the Great Lakes.