Introduction

Marine protected areas (MPAs) are increasingly used as tools to conserve biodiversity, manage fishing effort, and facilitate recovery of degraded ecosystems (Roberts et al., 2001; Sale, 2002; Hastings and Botsford, 2003; Cinner et al., 2006). Marine protected areas are expected to produce long-lasting local increases in the density, size, diversity, and/or productivity of marine organisms within MPA boundaries (Russ and Alcala 1996; Claudet et al., 2006; Chapter 2), as well as regional increases outside of the MPA via spillover from the MPA to sites that continue to be fished (Chapter 3). Assessment of the actual effects of MPAs relative to these goals is essential for adaptive management and decision-making. Despite a large number of assessments, however, there remains considerable uncertainty about the actual effects of MPAs (e.g., Osenberg et al., 2006).

The central question (and challenge) that underlies the assessment of any MPA is easy to express but difficult to measure: “How does the state of the system (e.g., density or size of a target organism) within the MPA compare to the state that would have existed had the MPA never been established?” The former can be directly observed; however, the latter cannot and must therefore be estimated through indirect means (Stewart-Oaten et al., 1986; Osenberg and Schmitt, 1996; Stewart-Oaten, 1996a, 1996b; Stewart-Oaten and Bence, 2001; Osenberg et al., 2006). Several different approaches have been proposed to estimate this baseline (Box 6.1; see also Osenberg et al., 2006), yet, considerable controversy still exists (Box 6.2).