Effect of substrate size on lake trout egg predation by mottled sculpin

Salmonines, including lake trout (Salvelinus namaycush), deposit their eggs in aggregates of rock. Eggs settle into interstitial spaces where they are safe from epibenthic predators, but vulnerable to interstitial predators such as sculpins. This study examined how rock size and mottled sculpin (Cottus bairdi) size affected predation by mottled sculpin on lake trout and rainbow trout (Oncorhynchus mykiss) eggs in field and laboratory experiments. In Lake Michigan rock piles of two types were constructed: rubble (10 to 22 cm, glacial till) and stream gravel (5 to 10 cm). Mottled sculpins collected 19 to 26 days later from these rubble piles were significantly larger than those collected fron gravel. Whether rubble or gravel piles with rainbow trout eggs attracted more mottled sculpins than those without eggs was determined next. Rock piles seeded with rainbow trout eggs for 1 to 3 days did not contain more mottled sculpins than piles without eggs. Mottled sculpins from rubble piles were larger and had more eggs in their stomachs than mottled sculpins from gravel piles. Laboratory feeding by mottled sculpins on lake trout eggs indicated that the maximum number of items consumed,was approximately proportional to fish body volume (i.e., approximately proportional to length cubed). The relationship between eggs consumed and length of mottled sculpin was similar to that documented for mottled sculpins collected from rock piles. Using mixtures of rubble and gravel it was found that substrate size can minimize predation pressure on eggs by excluding larger mottled sculpins from interstitial spaces.