The Effect of Attracted Fishing Effort and Stocking Strategy on Interactions Between Stocked and Wild Fish: a Simulation Approach

In recreational fisheries, stock enhancement is a valuable and commonly used management action that sometimes elicits unintended consequences for wild fish populations.  While the effects of various biotic interactions between stocked and wild fish (e.g., predation, competition) have often been studied, few studies have examined if stocking may negatively affect wild populations by attracting additional angling effort and increased fishing mortality.  To investigate the circumstances under which such a situation might occur, I simulated the response of a wild fish population to the stocking of a related species.  Specifically, I examined three stocking strategies:  (1) no stocking, (2) constant number of fish stocked annually, and (3) a variable number of fish stocked annually to maintain constant total fish abundance (compensatory stocking).  Each stocking strategy was examined with and without stocked fish survival and reproduction, and with and without stocked fish negatively impacting (through predation or competition) wild fish.  For each scenario, I assessed the spawning stock abundance of wild fish and the total fish catch using a stochastic age-structured population model with dynamic angling related to total fish (combined stocked and wild) abundance.  In the absence of any biotic interactions, both constant and compensatory stocking resulted in decreased spawning abundance of wild fish and increased total catch, with compensatory stocking resulting in lowest wild spawning abundance but highest total catch.  This reduction in wild spawning abundance was exacerbated when stocked fish were simulated to reproduce or to negatively interact with wild fish.  The model was sensitive to wild fish catchability at low stock sizes, to differences in catchability between wild and stocked fish and to assumptions of the relationship between angler effort and fish abundance.  Nonetheless, these results suggest (1) stocking can potentially negatively affect wild populations due to attracted angler effort, (2) such an effect may be exacerbated by compensatory stocking, particularly if stocked fish establish a reproducing population and/or biotic interactions that negatively impact wild fish occur, and (3) attracted angler effort may lead to trade-offs between wild spawning abundance and total catch.  These findings emphasize the importance of understanding angler effort response to stocking and to evaluating stocking strategies in systems where stocked and wild fish exist.