The increasing prevalence of smaller fish in highly exploited fisheries: Concerns, diagnosis and management solutions

A decline in the size of fish within a population is concerning. Large-sized fish are ecologically important and valued for social and economic reasons. 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). Size-selective mortality has evolutionary consequences. Paradoxically, the management solutions for these problems are in opposition (more harvest versus less harvest), and a wrong diagnosis could exacerbate the problem.;With concerns of evolutionary consequences because of evidence of size selective harvest, I used an age- and size-structured, single-species model, parameterized with data from Alberta's Walleye fisheries, to evaluate the selectiveness of various management regulations. I found that the 50-cm minimum size limit used to recover Alberta's Walleye populations did indeed select for the 'hockey stick' life history, although this regulation allowed for sustainable populations (>5 Walleyes*hectare -1) and angler effort up to 16 angler-hours* ha-1 *year-1. The optimal regulation to reduce life history selection and allow for population sustainability was a 40--50 cm 'harvest-tag' regulation. This regulation reversed the selection for the 'hockey stick' life history, yet produced sustainable fish densities and allowed angler effort up to 30 angler-hours*ha-1 *year-1. However, increasing angler-noncompliance reduced the sustainability of this regulation.;I used nested hypotheses, and implemented active adaptive management at several Alberta lakes, to diagnose the causal mechanism creating the small fish problem. For inferences on the source of the mortality, I analysed backcalculated growth rates from pelvic fins. 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, avoiding harvest. Using changes to sport fishing regulations, I then modified angler effort and harvest at four different Alberta lakes to increase or decrease size-selective harvest and Walleye densities. I found that size-selective mortality from angling rapidly truncated the population-size structure.