Fishery selection and its relevance to stock assessment and fishery management

•Fishery selection (selectivity) measures relative fishing mortality-at-age.•Fishing gear and the locations of the fish and the fishing influence selectivity.•VPA estimates indicate diverse selectivity curve shapes and temporal variability.•Selectivity influences fishery performance and stock assessments. Fishery selection (selectivity for short) is the term often used to describe the phenomenon whereby a fish stock experiences mortality due to fishing that is age- or size-specific. Selectivity operates both at a local scale, as in the direct interactions of individual fish with the fishing gear (contact selection), and at a stock-wide scale (population selection), as evidenced by the differential rates of fishing mortality-at-age that are generally observed in stock assessment results. All age-structured stock assessment models have some form of fishery selection to modulate the impact of fishing mortality on differing age-classes, but selection coefficients, from a stock assessment viewpoint, generally are nuisance parameters rather than a focus of attention. This paper provides an overview of the three main processes that contribute to and influence population selection: (1) physical sorting by the fishing gear or differential behavioral responses of the fish to the gear produce the phenomenon of contact selection; (2) differing selection properties of different types of fishing gear (e.g., trawl versus longline) in turn generate a composite selection curve that is a weighted average of the different kinds of contact selection; and (3) when the fish are not well mixed spatially, then the spatial distribution of fishing relative to the spatial distribution of the fish also affects population selectivity. Fishing mortality-at-age estimates derived from a published Virtual Population Analysis of Scotian Shelf haddock are used to illustrate the diversity of shapes that can be seen in population selection curves and their considerable temporal variability. A spatial model for fishery age-selectivity is then used to demonstrate that the maximum relative yield harvested from a stock can be a function of both contact selection and the spatial distribution of fishing.