Performance of length limits to sustain a data- and capacity-limited fishery

Abstract

Length limit regulations are widely recommended to sustain fish biomass and fishery yields, particularly for data- and capacity-limited fisheries. However, previous findings were generally derived from equilibrium conditions, used hypothetical fisheries, and/or did not explicitly quantify the performance of length limits under a combination of implementation levels, increasing fishing effort, and discard mortality. Here, we apply the management strategy evaluation (MSE) framework to test the performance of three length-limit management procedures (MPs)—maturity and high maturity length limits (i.e., minimum length limits set at and above the length at maturity, respectively) and slot length limit (i.e., excludes both immature and very large fish from retention)—in Iran's largely unregulated, data- and capacity-limited kingfish fishery. We subject these MPs to different conditions, including strong and weak implementations, discard mortality, and an expansion in future fishing efforts, and quantify their overall performance in terms of fishing mortality, biomass, and yield. Our results show that strong implementation of length limits results in the largest gains: the lowest future long-term fishing mortality and the highest biomass and yields relative to the maximum sustainable yield (MSY) level. When illegal-sized fish are selected but discarded with high discard mortality under strong implementation, or retained under weak implementation, the benefits of length limits are severely reduced. Still, length limits are projected to result in much lower fishing mortality and maintain biomass and fishery yields near MSY levels compared to the current fishery's selectivity and retention. Over the entire projection period, the high maturity length limit provided the highest probability of sustainable biomass and the highest mean relative yield under weak implementation and a growing future fishing effort. Our results underscore the benefits of implementing length limit regulation in sustaining data- and capacity-limited fisheries, particularly if current fishing conditions involve selecting and retaining small and immature fish under a high and growing fishing mortality rate.