Combining High-Resolution Bathymetry and Recreational Catch Rates to Model the Spatial Distribution of Biomass for Reef-Associated Nearshore Species

Principal Investigator: E.J. Dick1, Melissa Monk1, and Rebecca Miller1,2

1NMFS, SWFSC, Fisheries Ecology Division
2The Cooperative Institute for Marine Ecosystems and Climate, University of California, Santa Cruz)

Stock assessments help us learn about the dynamics of fish populations and their response to fishing. One of the first steps when doing a fisheries stock assessment is to define the spatial boundaries and structure of the stock, or population of interest. A successful assessment gives managers information about the total catch (the Overfishing Limit, or OFL) that can be sustainably removed from the defined area in a given year. Often, however, the area defined by the stock assessment spans several management areas and/or political boundaries. If managers wish to allocate the OFL among management areas in a way that is consistent with stock assessment assumptions (i.e. assessments assume a spatially uniform fishing mortality rate), then it is necessary to know what fraction of the stock biomass is in each area. This can be a difficult problem to solve, particularly when the species of interest is not well-sampled by existing scientific surveys.

Our research seeks to address this problem by developing a novel and general methodology, and then applying it to a number of nearshore, reef-associated groundfish species caught off the U.S. West Coast. We use Geographic Information Systems (GIS) to analyze high-resolution maps of the seafloor to identify rocky reef habitat. The GIS analysis gives us the approximate area of each reef, as well as characteristics of the seafloor (e.g. depth and ruggedness) that might help us predict fish distribution. We then model catch rate data from recreational fishing drifts to estimate changes in relative fish density among reefs. The catch rate data is collected by onboard observers, is spatially-explicit, and measures total catch (retained and discarded fish) and effort to the minute. By combining this index of relative fish density with the areas of each reef, we can learn about the relative spatial distribution of fish biomass.

In the absence of comprehensive, fishery-independent survey data, our method provides a way forward for those interested in allocating OFL among management areas within the boundaries of stock assessment. Previous methods of allocating OFL based on catch or habitat alone make strong assumptions such as constant fishing effort and/or constant fish density among areas. Our approach uses habitat maps and a proxy for fish density to describe the relative spatial distribution of managed stocks, and can help managers allocate catch in a way that ensures consistency with stock assessment efforts.

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Southwest Fisheries Science Center (SWFSC)


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