Dynamics underlying fisheries stock-recruitment relationships are often obscure, especially with relatively short-lived invertebrate species such as brown shrimp. Nonetheless, disentangling these dynamics can help reveal optimal management strategies for long-term sustainability. We used a theoretical framework to link stage-specific demographic parameters to fishable stock abundance. We then investigated the long-term effects of changing the degree of density dependent settlement. By working successively through Beverton-Holt relationships and incorporating long-term fisheries catch data and fisheries-independent abundance estimates, we assessed how density dependent settler survival could have led to observed population trends over a 25 year period. A stage-based model, parameterized with survival estimates from literature, catch data, and the fitted degree of density dependence, served as the baseline for model simulations. We explored how reduced carrying capacity affected equilibrium population and stage-specific abundances. We also determined the relative sensitivity of different life stages to changes in marsh carrying capacity versus catch rates.
Results/Conclusions
Results suggest that fishing rates have already met the level of compensation potentially provided during the settlement phase, when shrimp are most vulnerable to changes in marsh habitat availability. Less than a 5% increase in catch rates, at the fitted degree of density-dependence, caused the population to crash. If any further compensation occurs during the recruitment phase, as defined by our model, nearshore fishing efforts should be carefully monitored to avoid potential overfishing. Increased carrying capacity had a greater effect on brown shrimp abundance than reduced fishing, with an increase of 0.2% in the carrying capacity causing a 10-25% increase in the population. Whereas a 0.1% decrease in carrying capacity caused nearly the same reduction in stage-specific abundances. Adults were most positively affected by decreases in catch, but all classes showed significantly higher long-term abundance with reported catches reduced by as little as 5%. This study provides a framework for evaluating settlement-related compensation in the brown shrimp population and emphasizes the need for catch quotas that incorporate habitat considerations to ensure the long-term viability of this important fishery.