Density dependence and the economic efficacy of marine reserves

Abstract

Predictions on the efficacy of marine reserves for benefiting fisheries differ in large part due to considerations of models of either intra-or inter-cohort population density regulating fish recruitment. Here, I consider both processes acting on recruitment and show using a bioeconomic model how for many fisheries density dependent recruitment dynamics interact with harvest costs to influence fishery profit with reserves. Reserves consolidate fishing effort, favoring fisheries that can profitably harvest low-density stocks of species where adult density mediates recruitment. Conversely, proportion coastline in reserves that maximizes profit, and relative improvement in profit from reserves over conventional management, decline with increasing harvest costs and the relative importance of intra-cohort density dependence. Reserves never increase profit when harvest cost is high, regardless of density dependent recruitment dynamics. I quantitatively synthesize diverse results in the literature, show disproportionate effects on the economic performance of reserves from considering only inter-or intra-cohort density dependence, and highlight fish population and fishery dynamics predicted to be complementary to reserve management.

Marine protected areas and the value of spatially optimized fishery management

“…By contrast, when density dependence is strong the increase in larval supply resulting from spatially targeted reductions in fishing has little effect on recruitment in fished locations, so spatial management has little potential for increasing profits. This result is consistent with previous theoretical work that has shown that postsettlement density dependence makes MPAs less effective (23). A high cost of fishing also reduces the utility of spatial management.…”

Ocean zoning within a sparing versus sharing framework

“…Other studies have identified scenarios in which reserves could be essential for maintaining high yields in spite of otherwise effective management regulations. These include, for example, the potentially critical function of reserves as a buffer against environmental stochasticity (Mangel 2000;West et al 2009), and the positive impact of reserves on the density-dependent survival of young fish (White 2009) which could increase the net productivity of fished populations adjacent to reserves (but see White et al 2008;Hart and Sissenwine 2009;Russ and Alcala 2011).…”

Designing marine reserve networks for both conservation and fisheries management

“…As with conservation, the optimal reserve location for fisheries prosperity need not have high productivity; areas with high productive capacity can be more profitable as fished sites than as sources of spillover or export (40,42). Protecting a source population may compensate for harvest lost from the reserve area, although this potential fishery benefit certainly depends on the life history and demography of the species and will not always be realized (18,31,43).…”

“…To benefit fisheries, the overwhelming majority of theoretical and empirical results indicate that reserve area should be divided into a network of closures whose size and spacing maximize the net effect of spillover of fishable adults and export of larvae to fished areas between the reserves (1,26,43,49,65,66). Designs characterized by a greater number of smaller reserves increase the amount of reserve edge and decrease the average distance from fished areas to the reserve edge.…”

“…Placing a still larger proportion of the area in reserves would provide even greater conservation benefits, but the revenue lost because of the displacement of fishing effort from protected areas typically would outweigh the gains generated by the reserves (e.g., via larval export, ref. 43). Furthermore, for many species, reserves may not increase long-term profits compared with those attainable under optimal nonreserve management or may increase profits only under limited conditions (18).…”