LETTER A theory for optimal monitoring of marine reserves Leah R. Gerber, 1 * Maria Beger, 2 Michael A. McCarthy 3 and Hugh P. Possingham 2 1 Ecology, Evolution and Environmental Sciences, Arizona State University, College and University Dr, Tempe, AZ 85287- 1501, USA 2 The Ecology Centre, School of Integrative Biology, The University of Queensland, Brisbane 4072, Australia 3 Australian Research Centre for Urban Ecology, Royal Botanic Gardens Melbourne, c/o The School of Botany, The University of Melbourne, Parkville VIC 3010, Australia *Correspondence: E-mail: leah.gerber@asu.edu Abstract Monitoring of marine reserves has traditionally focused on the task of rejecting the null hypothesis that marine reserves have no impact on the population and community structure of harvested populations. We consider the role of monitoring of marine reserves to gain information needed for management decisions. In particular we use a decision theoretic framework to answer the question: how long should we monitor the recovery of an over-fished stock to determine the fraction of that stock to reserve? This exposes a natural tension between the cost (in terms of time and money) of additional monitoring, and the benefit of more accurately parameterizing a population model for the stock, that in turn leads to a better decision about the optimal size for the reserve with respect to harvesting. We found that the optimal monitoring time frame is rarely more than 5 years. A higher economic discount rate decreased the optimal monitoring time frame, making the expected benefit of more certainty about parameters in the system negligible compared with the expected gain from earlier exploitation. Keywords Conservation, cost, marine reserve, monitoring, optimization, profit. Ecology Letters (2005) 8: 829–837 INTRODUCTION Marine-protected areas (MPAs) have become a popular approach to managing small-scale marine fisheries (Nowlis & Roberts 1999; Dayton et al. 2000; McClanahan & Mangi 2000), and the number of no-take reserves has increased in recent years (Allison et al. 1998). However, in the global effort to establish marine reserves (Lubchenco et al. 2003), some fundamental questions remain largely unresolved. One issue that has received little attention is the application of decision theory to help design monitoring strategies that aim to assess effectiveness of a marine reserve. Gerber et al. (2002) argue that, in practice, it is virtually impossible to achieve an optimal reserve design because of parameter and model uncertainty. Monitoring is therefore critical to assess effectiveness of a reserve and allow prompt modification of management and monitoring strategies. Previous approaches to designing monitoring pro- grammes have sought to maximize statistical power, but not to optimize a management objective (Thompson & Mapstone 2002). Monitoring of marine reserves has thus far addressed biological parameters in most cases (Russ & Alcala 1996) although a number of recent studies have incorporated monitoring of socio-economic effects (e.g. Dixon et al. 1993). To date, monitoring studies have provided evidence of increased density of fishes inside and outside of reserves (Alcala 1988; Bohnsack 2000), and in some cases increases in fishing yields (Gerber et al. 2003; Willis & Millar 2005), but have not addressed effectiveness in terms of the enhancement of sustainability and population persistence. There is some evidence that spawning biomass, abundance and average size of fishes may increase in marine reserves (Rowley 1994; Halpern 2003). In a meta-analysis of 19 studies describing the effects of marine reserves on overall fish abundance and species inside and outside reserves, Co ˆte ´ et al. (2001) reported an insignificant trend towards increase of overall species abundance in reserves when compared with adjacent unprotected areas. Perhaps not surprisingly, a study that considered individual fish species found that only commer- cially important species increased in abundance significantly inside the reserve (Mosquera et al. 2000). An uncharitable view of the literature on monitoring marine reserves would argue that existing research on marine reserves has allowed us to successfully reject the null hypothesis that fishing does not kill fish. Monitoring is the systematic acquisition of information over time. In the context of marine reserves, monitoring is the process of collecting information about state variables (e.g. abundance, size or vital rates) at different points in time Ecology Letters, (2005) 8: 829–837 doi: 10.1111/j.1461-0248.2005.00784.x Ó2005 Blackwell Publishing Ltd/CNRS