Modeling Exploitation in Recreational Fisheries and Implications for Effort Management on British Columbia Rainbow Trout Lakes

Abstract

We developed and evaluated a new model for predicting exploitation from recreational fishing effort. The model assumes that fish exchange between available and unavailable states and that fishery catches are taken only from the available pool. This limited vulnerability model exhibits a nonlinear, negative relationship between fishing effort and catch rate that manifests itself in the effort−exploitation relationship as an apparent decline in catchability with increasing effort. Decreasing catchability leads to an exploitation rate asymptote that may be less than unity as well as to a wide range of fishing effort over which exploitation is insensitive to changes in effort. Model predictions were consistent with the observed relationship between fishing effort and exploitation obtained from creel surveys and mark–recapture studies on rainbow trout Oncorhynchus mykiss lakes in British Columbia. We combined the exploitation modeling approach with an age-structured model for a naturally reproducing rainbow trout population to evaluate both optimum fishing effort policies and critical fishing effort levels that lead to population collapse. Optimum fishing effort (±SD) was 18.8 ± 3.7 angler-days/ha (AD/ha) for a stock that exhibits high recruitment compensation and 9.4 ± 2.4 AD/ha for a stock with low recruitment compensation. Critical effort did not exist for the high-compensation scenario; the estimate of critical effort for the low-compensation scenario was 23.2 ± 5.1 AD/ha. The estimator traditionally used to predict exploitation gave a poor fit to observed data, and resulting effort levels were twice those obtained from the limited vulnerability model.