David Sampson
Oregon State University
November 27, 2013

Most population models and stock assessment approaches assume that fish and fishing are uniform in their spatial distribution. In this Think Tank talk we will examine some of the effects that spatial structure can have on selectivity, which refers to the phenomena by which age- or size-classes of fish are not equally vulnerable to fishing. Fishery selection (selectivity) controls how fishing affects the agestructure of a fish stock, which in turn influences a stock’s productivity and ability to sustain harvests. It is possible to analyze selectivity at multiple spatial scales and selectivity can vary at these different scales. For example, the shape of the selection curve at the level of the population will be dome-shaped under a wide range of conditions, even though the gear-selection curve operating in common across all the regions is asymptotic. During this talk we will explore the factors that influence the shape of areas-asfleets selection curves, which are the form of selectivity that results from applying separate regional selection coefficients to the entire stock rather than to the regional sub-populations. Areas-as-fleets selectivity provides a mechanism for modeling spatial catch-at-age in an assessment model that is not spatially explicit. We use a simplified set of survival equations from Sampson and Scott (Can. J. Fish. Aquat. Sci. 68, 1077-1086. 2011), in which there are three distinct spatial regions and the same asymptotic selection curve operating in each region, and show that the overall population-selectivity often will be dome-shaped, but the corresponding areas-as-fleets selection curves will have shapes that differ from each other and from the population-selection and gear-selection curves. Factors that influence how much the selectivity curves differ from each other include the magnitude of the differences in the regional fishing mortality rates, the rates of fish movement and whether the movements are diffusive or directional, and the regional distribution of the recruits. Based on ad hoc explorations with the model it appears that areas-as-fleets selectivity will generally differ from the underlying gear-selectivity, except under the extraordinary conditions of equal recruitment and equal fishing mortality across all spatial regions. The talk will also explore the temporal variations that changes in the regional recruitment or the rates of fishing mortality will induce in population-selection and areas-as-fleets selection curves.