Abstract
The effectiveness of marine protected areas (MPAs) to restore populations of exploited species, both within and outside of their boundaries through net movement of individuals (“spillover”), can potentially be affected by continuity of habitats across the boundaries. Sandy seabeds may reduce movement of reef-associated species across MPA boundaries, thereby increasing the ‘reserve effect’ while decreasing spillover. Underwater visual censuses were undertaken inside the Cerbère-Banyuls Marine Reserve (CBMR) (France) and adjacent non-protected areas to assess the influence of habitat on spillover. Total fish biomass and mean fish size were significantly higher within the MPA, but rapidly declined across the reserve boundary. Nevertheless, there was no indication of a sharper decline in biomass at the northern boundary where a habitat discontinuity was present relative to the southern boundary with continuous habitat. This result may reflect a number of complicating factors that make assessment of spillover potential difficult, and which may also lead to the uncertainty about which situations and how much spillover may contribute to fished populations outside reserves. In particular, the home range area of the key exploited species relative to the scale of the habitat mosaic, and potentially different levels of fishing pressure at each boundary likely contribute to variability. While the CBMR appeared particularly well-suited to investigating this question, resolving these issues and identifying general principles for where and how much spillover occurs will likely be difficult without a series of specially designed MPAs. This highlights a conundrum facing MPA establishment in the face of pressures to be successful for both biodiversity conservation and to offer fisheries benefits—the latter are clearly not ubiquitous, but a shortage of suitable MPAs that can be used as scientific tools for better understanding how and when these benefits may occur is precluded by a general lack of MPAs designed and managed for this purpose. The results of this study do, however, clearly highlight the biodiversity conservation benefits of the CBMR.
Highlights
The effects of marine protected areas (MPAs) on fish communities have been widely studied (Babcock et al 1999; Planes et al 2008), especially on species targeted by fishers (García-Charton et al 2008)
We found no clear support for our prediction that discontinuous reef habitat at the reserve boundary would limit fish dispersal, and be reflected in a more pronounced gradient in fish biomass across the boundary
Fish biomass did not differ significantly between the northern fished sites located outside the discontinuous boundary and southern sites connected to the reserve by a continuous fringing reef
Summary
The effects of marine protected areas (MPAs) on fish communities have been widely studied (Babcock et al 1999; Planes et al 2008), especially on species targeted by fishers (García-Charton et al 2008). After the establishment of a MPA in an intensely-fished area, the biomass and body size of the target species generally increases through time within MPA boundaries (Halpern and Warner 2002; HarmelinVivien et al 2008). MPAs exclusively protect those species that remain inside the protected area (Chapman and Kramer 2000). Many temperate rocky reef species are site-attached and show some degree of fidelity to certain habitats (García-Charton and Pérez-Ruzafa 2001). Random movements, relocation of home ranges (Kramer and Chapman 1999) or
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