This study examined the production and consumption dynamcis of the Spirit Lake fishery, with the goal of determining the carrying capacity of major piscivorous (fish eating) sport fish. Littoral fishes (fish that live close to shore) were sampled at night using a 1000 x 12 foot beach seine (1/4 inch mesh), and a 500 x 8 foot beach seine (1/4 inch mesh). All fish sampled were identified to species and enumerated. A random subsample of at least 5 fish per 1 cm interval were measured (nearest mm) and weighed (0.1 g). The abundance, biomass, and size structure of pelagic fishes were estimated using hydroacoustic equipment. This equipment utilizes sonic pulses that measure the densities of fish schools. Estimates were checked by using a combination of Tucker trawling, otter trawling, gillnetting, and purse seining.

Population estimates of all major piscivores in Spirit Lake (walleye, northern pike, muskellunge, smallmouth bass, largemouth bass, white bass, large yellow perch) were made using standard mark-recapture techniques. Piscivore diets were assessed at monthly intervals during the ice-out season using a variety of sampling methods. including electrofishing, seining, and gillnetting. All sampling for diet analysis was conducted at night. Whenever possible, food was removed from the fish's gut by flushing with water. In this way we were able to release the fish alive. Caloric densities of piscivores and forage fishes was assessed during the 1995 season at roughly two month intervals. Three piscivores from each combination of species and size class were put on ice in the field, and were frozen as soon as possible. Batches of roughly 10 individuals per size class of abundant forage fishes were handled similarly. These samples were analyzed for energy content using a Phillipson Microbomb calorimeter. A daily thermal history of Spirit Lake was obtained by using a recording thermograph moored at a fixed location at a depth of 2 m. This theromograph was placed in the lake immediately after ice-out, and was retrieved shortly before the lake froze. The Fish Bioenergetics Model 2 (FBM2; Hewett and Johnson, 1992) will be used to estimate the forage consumption by piscivores. Findings from this study will be used to develop a comprehensive plan to manage the Spirit Lake fishery.

Knowledge of the production and consumption dynamics of the Spirit Lake fishery will allow managers to predict the consequences of their decisions before implementation, and predict the probability of success.