Determining the Most Appropriate Specification of Walleye Natural Mortality in Stock Assessment Models for Mille Lacs, Minnesota

Natural mortally (M) is an influential yet difficult parameter to estimate in fisheries stock assessments.  Estimates of M are needed for most models currently used in fish population dynamics.  At Mille Lacs, Minnesota, a shared tribal and sport fishery for walleyes has existed for 15 years, and age-specific assessment models have been key tools in evaluating walleye stock status.  Recent declines in the walleye stock have led to reevaluation of assumptions of the Mille Lacs assessment models, including constant M=0.6 for age-1 and 0.24 for ages 2-17+.  Several methods for calculating M based on empirical relationships and life history parameters were evaluated with constant M estimates ranging from 0.18 to 0.35.  Total mortality (Z) was estimated from gill net survey data using multiple approaches.  Longitudinal catch curve data suggested that M for older ages may be lower than 0.24 since estimates of Z were often near or below 0.24.  Empirical methods assuming variable M with age were preferred based on literature review.  The lower 90% interval using the Lorenzen (1996) method that describes M as a function of body weight for lakes resulted in the most reasonable estimates of M ranging from 0.73 (age-1) to 0.19 for age-16 and older.