Too Little but Not Too Late: Active Adaptive Management to Solve A Small Walleye Problem in Highly Exploited Fisheries

Following widespread collapses from angling overharvest, the densities of Walleyes Sander vitreus in Alberta’s lakes increased rapidly with large minimum size limits.  Anglers were unhappy, however, as catch rates increased (>1 Walleyes*hour-1) but fish remained small and did not exceed the minimum size limit.   The two alternate explanations for the small, yet old Walleyes were either compensatory growth because of high density (stunting) or size-selective mortality (overfishing).   Paradoxically, the management solutions for these problems are in opposition (more harvest versus less harvest), and a wrong diagnosis would exacerbate the problem.  We analysed backcalculated growth rates from pelvic fins for inferences on mortality. Walleyes that had fast-growth to an early maturity, and then subsequent slow-growth, had greater survival.   This ‘hockey stick’-shaped growth allows for successful reproduction while the Walleyes remain below the minimum size limit and are invulnerable to harvest.  We then modified angler effort and harvest at four different Alberta lakes to increase or decrease size-selective harvest and Walleye densities.  We determined that the size-selective mortality from angling rapidly truncated the population-size structure. With concerns of evolutionary consequences because of evidence of size selective harvest, we used an age- and size-structured, single-species model to evaluate the selectiveness of various management regulations.  The 50-cm minimum size limit used to recover Alberta’s Walleye populations did indeed select for the ‘hockey stick’ life history.  The optimal regulation to reduce life history selection and allow for population sustainability was a 40-50 cm ‘harvest-tag’ regulation.   Harvest tags are a new regulation for Alberta which allows managers to allocate harvest tags for fish based on fishing effort and population status.