Marine fish populations or “stocks” are subject to fishing pressure along the world’s coastlines by local fishers and further offshore by industrial fleets, whose huge capacity is capable of decimating the fish that they target. Fishing operations are therefore typically constrained by catch limits derived from stock assessments—i.e., reports on the condition of a fish stock and its estimated future abundance. Theoretically, catch limits should ensure sustainability by not exceeding the productive ability of fish populations and by allowing depleted stocks to rebuild. In reality, the fraction of overfished stocks continues to increase, and reported global catches stagnate despite increasing fishing effort, implying that fish abundance is declining (1). On page 860 of this issue, Edgar et al. (2) report a likely explanation for the apparent inability of standard stock assessment models to provide sound catch limits. Their findings underscore the importance of accurate stock estimates for sustainable fisheries management.