J. Great Lakes Res. 21 (Supplement 1):445-457 Internal. Assoc. Great Lakes Res., 1995 Predator-Prey Relations and Competition for Food Between Age-O Lake Trout and Slimy Sculpins in the Apostle Island Region of Lake Superior Patrick L. Hudson, l Jacqueline F. Savino, l and Charles R. Bronte 2 1 National Biological Service Great Lakes Science Center 1451 Green Road Ann Arbor, Michigan 48105 2 National Biological Service Great Lakes Science Center Lake Superior Biological Station 2800 Lake Shore Drive East Ashland, Wisconsin 54806 ABSTRACT. Slimy sculpins (Cottus cognatus) are an important component of the fish community on reefs and adjacent nursery areas of the Great Lakes and overlap spatially with age-O lake trout (Salveli- nus namaycush). Important interactions between these fishes are possible during the lake trout's first year of life, which could include predation on each other's eggs and larvae, and competition for food resources. We investigated the diets of age-O lake trout and slimy sculpins on a lake trout spawning reef (Gull Island Shoal) and adjacent nursery area (near Michigan Island) in the Apostle Island region of western Lake Superior during June through September from 1988 through 1991. Organisms in stomachs of 511 lake trout and 562 sculpins were identified and counted. Of the 11 major food types found in age-O lake trout stomachs from both areas, Mysis was the dominant food item (mean volume in stomachs = 68%) and occurred in about 3/4 of the fish analyzed. Copepods, cladocerans, chironomid pupae, fish, and Bythotrephes were also common in the diet (frequency of occurrence> 4%). Diets of lake trout were more diverse on the reef than on the nursery area where Mysis dominated the diet. Slimy sculpins were only found in lake trout greater than 50 mm. Mysis was an important food item of slimy sculpins over the reef but not over the nursery area, where Diporeia was by far the most important taxon. A variety of ben- thic invertebrates (Asellus, chironomids, benthic copepods, and snails) comprised the bulk of the sculpin diet over the reef Sculpins also ate lake trout eggs in November. Based on cluster analysis, diets were most similar over the reef where both consumed Mysis, calanoid copepods and chironomid pupae. Diets diverged over the nursery areas where sculpins were strictly benthic feeders and lake trout maintained their planktonic diet. In Lake Superior, where lake trout recruitment through natural reproduction has become well established, the coexistence of the two species appears amicable. However, in other Great Lakes with higher sculpin to lake trout ratios on a reef, the coexistence of the two species may be a bot- tleneck for age-O lake trout survival beginning with egg deposition and ending when age-O lake trout move off the reef and the two species no longer compete for a common food resource. INDEX WORDS: Predation, fish diets, Lake Superior, lake trout, slimy sculpins. INTRODUCTION The goal of restoring self-sustaining stocks of lake trout (Salveiinus namaycush) in the Great Lakes has only been achieved in Lake Superior (Hartman 1988, Hansen et ai. 1995). In the other lakes where egg deposition is observed over suitable substrate, the number of spawn- ers, contaminants, and predators have been dismissed as reasons for low natural recruitment beyond early fry stages (Stauffer and Wagner 1979, Wagner 1981), al- though Mac (1988) has found a link between early sur- 445 vival and contaminants. The suggestion remains how- ever, that recruitment is being inhibited prior to the year- ling life stage (Sly 1984, Perkins and Krueger 1995). Early stages of lake trout development, from egg deposi- tion until movement of larvae to nursery areas (Peck 1982, Bronte et ai. 1995), typically occur over rocky spawning reefs. Knowledge of biotic interactions within the reef community at this stage in the lake trout's life history is sparse and generally consists of laboratory studies (Savino and Henry 1991, Savino et al. 1993) or