Fishing effort redistribution in response to area closures

Spatial fishery closures will induce fishing effort to either move to open areas or to cease to fish. When designing a short- or long-term closed area management regime, the expected impact of that closure will depend upon how that effort is redistributed. We present a redistribution model based upon Ideal Free Distributions (IFDs) which is intermediate in complexity between analyses in which effort is distributed uniformly over open areas and models of full fleet dynamics. The IFD models incorporate the fundamentals of the decision process invoked by fishers facing relocation and the ensuing catch rates that result from the addition or removal of effort. Two classes of models were tested: an Availability model where catch rate declines were proportional to abundance; and an Abundance model where abundance declines at an exponential rate with the entry of displaced effort into an area. Results of these models were compared with uniform and proportional redistribution methods. The IFD-based methods included relative cost of relocation, thereby illustrating the importance of both catch rates and movement costs in designing closed area regulations. To demonstrate the methods, hypothetical area closures to United States pelagic longliners in the western Atlantic were examined and the impact of those closures on bycatch rates was evaluated. Guidance for selecting an appropriate model structure for a particular closed area problem is given.