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 CerbèreBanyuls 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.