UBC Theses and Dissertations
Ecosystem based management for Mille Lacs Lake, Minnesota under changing environmental conditions Kumar, Rajeev
Single-species and multi-species modelling was employed to seek options for sustainable management of Mille Lacs Lake, the second largest lake within Minnesota, known primarily for its walleye (Sander vitreus) fisheries. Managers of the lake face challenges in adapting to changes that include changing temperature of the water body, changing community structure, growing number of piscivorous birds, and an invasion of zebra mussels leading to greater water clarity. The lake experienced a historical decline in the population of cisco (Coregonus artedi)—a cold-water stenothermic forage species. Surplus production models of cisco population suggested that temperature explained 36-40% of change in cisco abundance and that the decline in the species was due to a combination of temperature increase and high fishing pressure. To rebuild the depleted stock, I concluded that strong restriction was required on cisco fisheries especially in warmer years. For multi-species analysis, I built an ecosystem model with the Ecopath with Ecosim (EwE) modelling suite using extensive field data on organism parameters and diet. The EwE model was fitted to abundance and catch data for the period 1985 to 2006. Influence of temperature on the model predictions were simulated by adding temperature data as forcing function in the EwE model. The model was driven forward 25 years to evaluate the ecosystem-wide predictions for prevailing harvesting strategies and other possible fisheries scenarios of interest to the lake managers. The ecosystem model was used to estimate single-species and ecosystem-level reference points for thirteen fished species in the lake with and without including the effects of temperature change. The analysis helped comparison of ecosystem effects and temperature effects on the estimated reference points. The EwE model sucessfully forecast most of the changes seen in the lake after the period of fitted-data from 2007 to 2012. The model was extended for evaluation of ecosystem-wide impact of zebra mussels invasion. The model predicted that the mussels population would stabilise in the lake after attaining maximum density. But during this period several species, including the important walleye, could be negatively affected by the filtration of phytoplankton by zebra mussels.
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