Effects of management spatial scale in fisheries outcomes

Background/Question/Methods

The success of any spatial explicit management approach will rely on the scale of management. Specifically, determining the appropriate spatial scale of management is critical to the success of ecosystem-based fisheries management, an inherently spatial approach management. For any managed region, managers must decide how much to subdivide the area under concern: from implementing a uniform approach across the region to considering a unique approach in each of one hundred patches and everything in between. For example, a central component of ecosystem-based fisheries management is a consideration of spatially explicit fisheries dynamics. However, there is no general theory for how spatial scale affects management outcomes such as optimal profit of fisheries and population abundance. To explore this question, we construct a mathematical model that follows an age-structured-harvested population in a one-dimensional patchy environment connected by larval dispersal. The spatial scale of management determines the number of managed segments in the region, within which fisheries take place so as to maximize equilibrium net profit of fisheries. We test our model with parameter values for Cabezon (Scorpaenichthys marmorata) and Red abalone (Haliotis rufescens).

Results/Conclusions

Assuming no management cost to subdividing the region (e.g., implementation cost of marine protected areas [MPAs]), we find that the net profit increases with the number of segments. If patches are randomly distributed without spatial autocorrelation, then the net profit increases almost linearly with the number of segments. However, if patches are positively autocorrelated, then the net profit increases with diminishing returns. Therefore, if management cost increases with the number of segments and the patch distribution is spatially autocorrelated, an intermediate spatial scale of management is likely to maximize net management profit. On the other hand, when the habitat landscape is not autocorrelated to neighboring patches, an optimal spatial scale can only exist when the management cost increases faster than the net profit of fisheries with increasing number of segments.