Predicting changes in the catchability coefficient through effort sorting as less skilled fishers exit the fishery during stock declines

Effort sorting is a process in fisheries where fishers of various skill levels sort according to fish density so that the mean catchability of remaining fishers increases as stock size declines. The resulting hyperstability in catch rates masks declining density, sometimes until fish populations have effectively collapsed. Effort sorting as a potential mechanism leading to hyperstability has been known for a while, but the ability to detect it using existing fisheries data has been limited. We present a way to detect effort sorting in fisheries and evaluate it using published recreational fisheries data. Specifically, we propose that catchability among anglers is log-normally distributed, but the anglers remaining fishing on any particular lake will have catchabilities high enough to exceed a minimum acceptable catch rate given available stock size. It is then possible to discern between hypotheses about causes of hyperstability, namely effort sorting or range contraction. However, the fitted model cannot reliably be used to predict fish density from catch-per-unit effort (CPUE) data, reiterating the importance of fishery-independent data, and serving as a warning against using CPUE as an index of density in management.