Deconstructing size selectivity to evaluate the influence of fishery management

Abstract

Fisheries are managed through various means to regulate harvest and maintain the sustainability of the stock. These regulations also influence which size fishes are removed from the population: A minimum mesh or hook size directly affects the size of fish retained by the fishing gear. A minimum fish size leads to discards, and any survival of released fish decreases the selectivity for sub-legal size classes. Closed areas and seasons indirectly influence size selectivity by altering the relative availability of each size class. Collectively, these regulations control the population-level size selectivity of the fishery, which is the product of gear selection (modulated by discard mortality) and spatiotemporal availability. Stock assessment models are particularly vulnerable to incorrectly specified selectivity, yet it is difficult to independently validate the structure of this variable because it is influenced by several processes. We analyzed the Gulf of Maine fishery for Atlantic cod (Gadus morhua), which has been intensively regulated through a complex management system for several decades, and deconstruct the mechanisms through which regulations have influenced fishery selectivity. Gear selection-at-size was estimated for four fleets under two regulatory regimes by comparing the size distribution of survey and fishery catches, sampled from the same time and area. Availability-at-size was estimated by weighting predictions from a random forest model of fish density-at-size by the spatiotemporal pattern of fishing effort. Several regulations caused fishery selectivity to be distinctly dome-shaped, and regulatory changes altered the shape of this curve over time. The approach of deconstructing selectivity into its gear and availability components can be adapted to other stocks to elucidate the underlying mechanisms, inform assessment models, and evaluate alternate regulation scenarios.