How Uncertainty in Natural Mortality and Steepness May Affect Perception of Stock Status and Fishery Sustainability in Atlantic Surfclam: A Simulation Analysis

Abstract

The Atlantic surfclam (Spisula solidissima) is an important commercial fishery resource on the U.S. MidAtlantic continental shelf. Although the 2016 stock assessment found that surfclams are neither overfished nor is overfishing occurring, uncertainty in the scale of spawning stock biomass persists. As a consequence of this uncertainty, the MidAtlantic Fishery Management Council (MAFMC) lowered the acceptable biological catch in 2016. A simulation analysis was developed for Atlantic surfclam to estimate the overfishing risk associated with the catch recommendation and its adherence to the MAFMC risk policy. Operating models conditioned on the 2016 stock assessment model structure generated simulations of the surfclam population, with alternative models to represent uncertainty in steepness (h) of the stock-recruitment curve and natural mortality (M). Simulations were forecasted under a variety of management procedures and evaluated with estimation models that spanned uncertainty in h and M. Results showed that current management decisions are more conservative than the stated risk-tolerance policies, though overestimating steepness in assessment models could lead to the misrepresentation of an overfished stock as within management thresholds. Further analysis evaluated future economic viability of the fishery by estimating proportion of fishable clam patches given forecasted biomass and historical observations of clam density. The proportion of fishable patches able to support fishery economic sustainability was generally stable despite biological uncertainties, though declined with increasing fishing pressure. This work contributes to the efforts to evaluate environmentally and economically sustainable fishery management strategies.