J. Great Lakes Res. 25(2):275–281 Internat. Assoc. Great Lakes Res., 1999 Ontogenetic and Seasonal Patterns in the Energy Content of Piscivorous Fishes in Lake Superior Timothy B. Johnson 1,2* , Doran M. Mason 1,3 , Stephen T. Schram 4 , and James F. Kitchell 1 1 Center for Limnology 680 North Park Street University of Wisconsin Madison, Wisconsin 53706 2 Current address: Ontario Ministry of Natural Resources Lake Erie Fisheries Station R.R. #2, 320 Milo Road Wheatley, Ontario N0P 2P0 3 Current address: Department of Forestry and Natural Resources 1159 Forestry Building Purdue University West Lafayette, Indiana 47907 4 Wisconsin Department of Natural Resources 141 South Third Street, P.O. Box 589 Bayfield, Wisconsin 54814 ABSTRACT. Ontogenetic, seasonal, and sex-related patterns in energy density (J/g wet mass) was studied in burbot (Lota lota), lean lake trout (Salvelinus namaycush namaycush) and siscowet lake trout (Salvelinus namaycush siscowet) collected from Lake Superior in 1996 and 1997. Energy density was strongly negatively correlated with water content for all fishes (r 2 = 0.86 to 0.99). For fishes of compa- rable size, energy density of siscowet (10.8 kJ/g) was significantly higher than lean lake trout (7.8 kJ/g) and both were significantly higher than burbot (5.1 kJ/g). Energy density in spring was higher in lean and siscowet lake trout, and lower in burbot than in other seasons. No significant differences were appar- ent between sexes within a species. Energy density increased in a predictable fashion with body mass for lean and siscowet lake trout, but varied without trend for burbot. The regression common to the three species (kJ/g wet mass = 36.78 – 0.41(% water), p < 0.001) provides a straightforward method for esti- mating ontogenetic and seasonal energy density in these fishes. INDEX WORDS: Burbot, Lota lota, energy density, lake trout, Salvelinus namaycush, siscowet, Lake Superior. 275 INTRODUCTION Bioenergetic models (Kitchell et al. 1977b, Han- son et al. 1997) have gained wide acceptance in fisheries science and ecology as a means of describ- ing energetic relationships in aquatic ecosystems. However, suitable information describing the en- ergy density (J/g) of predators and prey is often lacking. Early studies estimated energy density from proximate composition and either gross en- ergy constants (Brody 1945, Klieber 1961) or di- gestibility coefficients (Phillips and Brockway 1959, Phillips 1969), but these approaches were limited by their associated assumptions (Craig et al. 1978). Subsequent direct estimates from bomb calorimetry provided an accurate method for mea- suring the energy content of a wide array of aquatic organisms (Cummins and Wuycheck 1971) but were extremely time-consuming. Limitations such * Corresponding author. E-mail: johnsot@gov.on.ca