Modeling the Influence of Parr Predation by Walleyes and Brown Trout on the Long‐Term Population Dynamics of Chinook Salmon in Lake Michigan: A Stage Matrix Approach

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AbstractPredation events during ontogeny may have long‐term consequences for fish population abundance and variability. We used a stage‐based matrix model to evaluate Walleye Sander vitreus and Brown Trout Salmo trutta predation on Chinook Salmon Oncorhynchus tshawytscha parr of the Muskegon River stock and the relative influence of parr predation on the long‐term population dynamics and recruitment of Chinook Salmon in Lake Michigan. The model predicted the number of Chinook Salmon individuals in each stage (fry, smolts, and lake age 0 [recruits] through lake age 4) and forecasted population trajectories based on demographic data (e.g., survival, growth, and fecundity). The relative influence of parr predation was compared with influences of environmental stochasticity in the egg stage and Alewife Alosa pseudoharengus abundance (prey for lake‐stage salmon) on Chinook Salmon fecundity, recruitment, and population growth. To simulate environmental stochasticity and the influence of Alewife abundance, we varied Chinook Salmon stage‐specific survival rates, growth rates, maturity schedule, and carrying capacity. Relative to a baseline recruitment scenario, removal of stocked Brown Trout resulted in a significant increase in parr survival and long‐term Chinook Salmon abundance. Walleye predation on parr had little apparent influence on Chinook Salmon population dynamics. Predation on parr during out‐migration was positively correlated with variation in Chinook Salmon population stability and was negatively correlated with population growth, suggesting that Brown Trout have a significant negative effect on Chinook Salmon recruitment and long‐term population stability. The negative effects of variation in egg survival rates and Alewife abundance on Chinook Salmon recruitment and population growth rates were similar to the negative effects from parr predation scenarios. Our study suggests that management decisions to promote Great Lakes Chinook Salmon populations may require evaluation of trout stocking practices in nursery habitats.