Self‐Regulation of the Okanagan Lake Kokanee Recreational Fishery: Dynamic Angler Effort Response to Varying Fish Abundance and Productivity

Abstract

AbstractThe fishery for kokanee Oncorhynchus nerka in Okanagan Lake, British Columbia, exemplifies three common issues in the management of recreational fisheries: time‐varying stock–recruit parameters, mixed stocks within a single fishing area, and open‐access fishing effort. The aggregate kokanee stock (consisting of stream‐spawning and shore‐spawning stocks) declined 50‐fold between 1971 and 1998 and then partially recovered in the most recent decade. This contrast in fish abundance provides an opportunity to gain insights into the predator–prey dynamics of anglers and fish in an open‐access sport fishery. Structural variations in the Ricker stock–recruit function were fitted to the time series of spawner escapements and fishery data to estimate the relative role of nonstationary stock–recruit parameters versus angler harvest in the observed time series. The optimal model structures selected by Akaike's information criterion suggested that kokanee dynamics were driven by external factors (e.g., lake nutrient concentration or opossum shrimp Mysis diluviana abundance) as opposed to fishery harvest. The functional response of angler catch rates to fish abundance was well predicted by a type I function (i.e., no density dependence in catchability). Conversely, the angler effort numerical response to variation in kokanee density was sigmoid, with a steep transition at an average catch rate between 1 and 2 fish/d. The interaction of angler response functions with fish recruitment dynamics led to an open‐access recreational fishery that never harvested the lower‐productivity (stream‐spawning) stock to a level below the optimal escapement abundance, despite dramatic declines in stock productivity (i.e., recruits per spawner at low abundance) over time. Standard management reactions to declining fish abundance (successive reductions in daily bag limits) were largely ineffective in altering harvest rate or effort dynamics. Our results show that the Okanagan Lake kokanee fishery was self‐regulating over a large range in fish densities and that the declines in kokanee abundance resulted from time‐varying stock productivity rather than from overharvest.Received January 15, 2013; accepted June 17, 2013