New Developments in Length-Based Mortality Estimation for Data Limited Fisheries

Since size of fish is a principal observation in fisheries monitoring, development of reliable length-based stock assessment methodologies has been of great interest for a long time in fisheries science, particularly for data-limited and tropical fisheries.  The Beverton-Holt (BH) total instantaneous mortality estimator based on average size of fish in the exploited phase, despite its potential for strong biases, has been a popular tool due to its simplicity, computational efficiency, and modest data requirements.  Over the years two major concerns of BH method have been addressed through the works of Ault & Ehrhardt (AE) and Gedamke & Hoenig (GH).   The 1992 AE formulation addressed the concerns of strong positive bias in the BH equation by truncating the range of integration to be between the limits of the minimum and maximum size of fish observed in the exploited lifespan of a species, resulting in a zero-bias estimate at equilibrium.  GH in 2006 transformed the original BH model to a non-equilibrium formulation. Their model allows for either single or multiple changes in mortality over time.  Due to obvious differences in formulation, the AE and GH, while obvious advancements, at times provide conflicting results under independent testing.  In this study we set out to objectively evaluate the efficacy of these models by developing an analytical framework to evaluate the equilibrium and non-equilibrium model formulations through a stochastic simulation-optimization framework.  Here we report on the results of a new decision protocol to identify the situations under which certain formulations provided superior estimates.  We also discuss development of a hybrid model formulation that incorporates the best features of each approach and provides situation-dependent flexibility in optimal determination of mortality estimates.  Through application in the Florida Keys and Puerto Rican coral reef fisheries we show how these results support its use as a more universal tool in future fisheries management.