Bioenergetics Modeling of Trophic Interactions and Predatory Demands in a Pactola Reservoir, South Dakota

Bioenergetics models were applied, along with information on diets, growth, and energy density, for Lake Trout Salvelinus namaycush and Northern Pike Esox lucius in Pactola Reservoir, South Dakota to examine trophic interactions and assess annual predatory demands.  Diet, age, and growth data for Lake Trout and Northern Pike were collected during the growing seasons of 2012 and 2013. Bioenergetics models were developed for yearly cohorts of Lake Trout (age-2 to age-19) and Northern Pike (age-1 to age-12). Diets and stable isotope analyses showed ontogenetic shifts for both species.  Rainbow Smelt Osmerus mordax accounted for 52% of the annual consumption by sub-adult Lake Trout. However, adult Lake Trout annual consumption was comprised mainly (67%) of Bluegill Lepomis macrochirus.  Younger Northern Pike fed opportunistically on an array of prey items, with 27% of annual consumption being Rainbow Smelt.  Larger Northern Pike, greater than 600 mm total length, fed primarily on stocked Rainbow Trout Oncorhynchus mykiss (65%).  Except for spring consumption of Rainbow Smelt, our findings show that diets of Lake Trout and Northern Pike exhibit little overlap.  Information about the energetic demands of Lake Trout and Northern Pike should aid in management decisions concerning trout stockings and Centrarchid management in oligotrophic lakes.