Application Of A Bioenergetics Model For Brown Trout To Growth In Southeast Minnesota Streams
Doug Dieterman, Fisheries Research, MN DNR , 1801 South Oak Street, Lake City, MN 55041; (651) 345-3365; FAX (651) 345-3975; douglas.dieterman@dnr.state.mn.us
William C. Thorn, Fisheries Research, MN DNR 1801 South Oak Street Lake City, MN 55041
Charles S. Anderson, Fisheries Research, MN DNR 1801 South Oak Street Lake City, MN 55041
We examined the influence of feeding site availability, water temperature, and food consumption on growth differences of brown trout in nine southeast Minnesota streams. Four streams supported fast growth, where brown trout exceeded 141 mm at age 1, 229 mm at age 2, and 289 mm at age 3. Five streams had slow growth, where brown trout did not exceed 127 mm at age 1, 208 mm at age 2, and 268 mm at age 3. The number and frequency of feeding sites was measured in each study stream with a transect method. We used a Wisconsin-style bioenergetics model for brown trout to help isolate the effects of water temperature and food consumption. Model parameters were developed from published literature. We first developed a baseline model for each of four cohorts using standardized parameters for diet proportions, prey energy densities, and mean daily water temperatures. We used these parameters and the mean observed weight gain over all streams to estimate a baseline proportion of maximum consumption or P-value, for summer and winter seasons. We examined the effect of either water temperature or diet by incorporating stream specific temperatures or diet specific information into this baseline model and predicted weight gain. Model simulations predicted slower growth in fast growth streams in summer because water temperatures should have been too warm. In all model simulations, hypothetical diets with the highest energy densities, such as amphipods, ephemeroptera, sculpin, and small trout, always resulted in the greatest model predicted weight gains. Even though water temperatures may have been warmer than the optimum range for brown trout growth in our fast growth streams, brown trout could have grown faster there if they were consuming a more energy rich diet or greater daily ration. Neither the number nor the frequency of feeding sites differed between fast and slow growth streams for either age 1-2 or age 3 cohorts. Our data suggested that growth differences among streams were primarily due to differences in forage. We hypothesized differences in forage composition and availability may be mediated by water temperature.