The concept that no-take marine reserves can benefit nearby fisheries by supplying them with larvae and adult fish is central to reserves’ potential role in fisheries management. According to the theory of the reserve effect, fish that are protected inside reserves live to maturity and reproduce, and some of the young and/or adults cross the reserve boundary into unprotected waters. There they can be caught by fishers. Much of the attraction of the reserve effect is that it offers benefits both for conservation and fisheries.

Research on the effect has typically been difficult to carry out. This is due to multiple challenges, including tracking very small larvae and distinguishing potential reserve effects from other variables (including management- and gear-related ones) that can also affect fisheries yield. However, a handful of studies in 2009 offered some evidence of the reserve effect in action:

  • Robin Pelc of the University of California at Santa Barbara led a study that demonstrated higher catches of mollusks in South Africa due to the reserve effect (this was described in “What mollusks can tell us about larval export from marine reserves”, MPA News 10:11). The abstract of this study, published in the journal Marine Ecology Progress Series, is at; and
  • Richard Cudney-Bueno of the Packard Foundation (and formerly the University of Arizona) found enhanced recruitment of fished species “downstream” from a network of reserves in the Gulf of California, consistent with modeled predictions. The study, published in the free journal PLoS ONE, is at

In light of these studies, should the reserve effect no longer be viewed simply as a theory but as a documented, expected phenomenon – as “settled science”? Importantly, can it now be used to plan MPAs to meet conservation and fisheries goals? Over the years, MPA News has asked experts for their views on the reserve effect (MPA News 6:9; 5:7; 5:6). Here, we check back with three scientists and one manager for their thoughts on the latest developments:

  • Louis Botsford, fisheries biologist, University of California at Davis;
  • Trevor Ward, marine ecologist, University of Queensland (Australia);
  • Russ Babcock, marine ecologist, CSIRO Marine and Atmospheric Research (Australia); and
  • Fiona Gell, senior wildlife and conservation officer, Department of Agriculture, Fisheries, and Forestry, Isle of Man Government, British Isles.

In most cases, their comments below were excerpted by MPA News from longer responses. Those longer responses, some of which included journal references, are available on the MPA News website as indicated.

Loo Botsford: Not every site will produce a reserve effect

[Editor’s note: Botsford co-wrote his response with Will White, a post-doctoral researcher. Their full response is available as a PDF.]

“One cannot conclude from those two studies [Pelc et al. and Cudney-Bueno et al.] that every MPA will produce a so-called reserve effect for every species and every system of MPAs. In fact, one of the three MPAs examined by Pelc et al. did not produce a reserve effect, because larval production was similar inside and outside of the reserve. The lack of a reserve effect in that case is consistent with model predictions, and it illustrates why the reserve effect is not ‘settled science’.

“We would say that the set of interacting factors (and associated parameters) that determine whether there will be a reserve effect is settled science. But one must examine the specific combination of those factors in a specific MPA to determine whether there will be a reserve effect in each case.

“There are several factors relevant to the interpretation of the Cudney-Bueno and Pelc publications that MPA scientists and decision-makers should be aware of. First, the overall effect of the MPAs on population distribution and abundance will be seen only after the ecosystem has gone to equilibrium. The current level of recruitment as reported in Cudney-Bueno et al. likely depends on increased reproduction from individuals recruited before the MPAs existed. In the future it will depend on how MPAs affect recruitment at the source location, which in turn will depend on the dispersal matrix (i.e., the fraction of larvae leaving each location that settles successfully at each other location) and the future distribution of fishing effort.

“Another implication of the question of whether MPAs will have the ability to benefit adjacent fished populations is whether they increase overall yield in the fishery. Does the increase in yield over the local (still fished) area outweigh the loss of fishing due to the closed area within the MPAs? Modeling results to date indicate that MPAs are more likely to provide a direct benefit if the population was heavily fished (or overfished) prior to MPAs. This is the rationale underlying the point made by Pelc et al. that there was not a detectable increase in recruitment outside the MPA, where the fishery was well-managed. Related to this point, another important question regarding benefit is whether the implementation of MPAs is the best approach of those possible. For example, in some cases reducing fishing could provide the same benefits at less cost.

“Finally, another aspect that makes this question difficult is the multi-species nature of MPAs. When designing or evaluating MPAs one must consider the effects on a variety of species, and they will all have different movement rates and levels of fishing. Because of these differences they will have different responses to MPAs – i.e., an increase in MPAs beyond some point may increase yield for one species but decrease yield in another.”

For more information: Loo Botsford, Department of Wildlife, Fish and Conservation Biology, University of California at Davis, US. E-mail:

Trevor Ward: Three central dilemmas that need to be resolved

“To secure benefit for fisheries from the reserve effect, there are three central dilemmas that need to be resolved in applying the potential embodied in spillover and larval export.

“First, capturing the benefits of adult spillover or larval export to create substantive benefits for both conservation and fisheries (the double-payoff reserve) requires a high level of process understanding in both the ecology of the reserved systems as well as the fished species and their management. In many cases, a revision of fishery management systems may be required to be able to capture such benefits. For example, in fisheries where there is maintenance of breeding biomass of the fished species through escapement rules (such as application of a minimum size/age at capture), it may be more efficient and cost-effective to abandon (or simplify) size constraints of fished species. These constraints can be replaced (or supplemented) with a system of space/time closures that deliver the same level of breeding biomass. While space/time closures are widely used in fishery management systems, they are usually established to protect habitats that are important for purposes of recruitment, breeding, etc. Their biodiversity conservation benefits are usually coincidental at best. There is therefore a very significant challenge in simultaneously optimizing fishery production benefits and biodiversity conservation benefits within reserve design processes.

“Second, the fine-scale design parameters (size, placement, network span, inter-reserve distances, etc.) can have a very significant impact on the potential for delivery of spillover or larval dispersal to fisheries, and hence on the potential value to a fishery. Designing reserves that contribute to both fisheries and biodiversity conservation therefore requires an optimization across both sets of objectives. To be robust, this is a data-rich process that requires the clear specification of the conservation objectives for any such reserves, as well as the mechanisms for the reserves to deliver effective spillover or larval dispersion into the production system.

“Third, in many countries there are jurisdictional issues that have long prevented effective ecosystem-based management of marine living resources. Biodiversity conservation is not the principal focus of a fishery management agency, and fishery management is not the principal focus of a conservation or environment management agency. Thus there are often substantive differences in the science base and expertise (not least in the sciences of fishery management and ecology) that separate such jurisdictions, making achievement of a genuinely integrated approach to marine reserve design a difficult problem.

“So for this question, the answer can be best summarized as yes, but no. The potential is certainly demonstrated, but the costs of using double-payoff reserves in fishery management systems are still largely perceived by fishery managers to outweigh the benefits. The science of reserve design to optimally benefit both conservation and fisheries remains embryonic. Despite extensive theoretical research and advocacy, there appear to be few initiatives underway targeting double-payoff reserve designs, so progress in this area will continue to be slow.”

For more information: Trevor Ward, University of Queensland, Australia. E-mail:

Russ Babcock: Is the effect large enough to be meaningful?

[Editor’s note: Babcock’s full response is available as a PDF.]

“The reserve effect is contingent on increases in the biomass of the parent stock in a reserve. This effect is common but not a given. So the question has to be asked, are the species of interest responding? In my experience this often has to be determined on a case-by-case (reserve-by-reserve, species-by-species) basis.

“Then there is the question of how large any such larval export effect might be. Is it big enough to have a meaningful ecological effect on the exploited population? How about a meaningful economic effect on the fishery? Is it big enough to compensate for changes in fishing practice, displaced effort, etc.? Is it even big enough to measure using the methods we have at our disposal? Recruitment is highly variable, notoriously so in fact, and variations in larval supply are caused by a wide range of factors, not just stock size. Consequently all these questions need to be answered with reference to time series of data that take into account variability before and after changes in reserve management, link recruitment to spawner biomass, and provide data on the relevant fishery.

“Spillover of adult fish is usually not well demonstrated. While there must be cross-boundary movements of fish, the question of relevance to commercial fishers may be: what is the direction of net movement of biomass? (For trophy recreational fishers the question might be slightly different: what is the modal size of fish moving out?) I believe it is important to be clear how ‘spillover’ is defined: i.e., whether or not we are talking about density-dependent directional movements since density-dependent behaviors have important implications for the questions above. Many of the papers referring to spillover are equivocal due to limitations of sampling design (e.g., before-after-control-impact design, or BACI) and replication, not to mention population-level evidence of density-dependent effects.

“There are a lot of open questions here, not least because species-level variation and differences between sites (e.g., sources and sinks) are likely to mean very different responses in different locations. While larval export and spillover are possible and even likely, the devil is in the detail of the question ‘how much?’. We must do the necessary science if reserves are to be used in an informed, practical sense as an active part of a fisheries or conservation management system.”

For more information: Russ Babcock, CSIRO Marine and Atmospheric Research, Cleveland, Queensland, Australia. E-mail:

Fiona Gell: Fishermen believe in the reserve effect

[Editor’s note: Gell’s full response is available as a PDF. In contrast to the other people interviewed for this article, Gell works solely on the management side of the MPA field. MPA News asked her what role the consideration of reserve effects plays in her management work.]

“In the Isle of Man, there are two complementary Marine Protected Area programs being implemented by the Department of Agriculture, Fisheries and Forestry. These are Fisheries Closed Areas (specifically for scallop fisheries management) and Marine Nature Reserves (primarily for habitat and species conservation). The Port Erin Closed Area, for example, has been closed to scallop fishing for 20 years and the effects of closure – including build-up of biomass and spawning potential – have been well documented through the work of Andy Brand and his research team at the former Port Erin Marine Laboratory.

“After about 15 years of closure, fishermen’s support for the Port Erin Closed Area began to grow as they witnessed benefits to fisheries in adjacent areas, including increased catches. There is not yet conclusive scientific evidence that the Closed Area is supporting the increased catch per unit effort that has been recorded. However, the fishermen believe that they are benefiting from increased larval supply, and this is supported so far by particle tracking work carried out by Bangor University scientists. This has culminated in the industry-led closure of a second site (Douglas Bay) in 2008, two further sites (Niarbyl and Laxey Bay) for scallop ranching in October 2009, and a fifth site (Ramsey Bay) closed temporarily in December 2009.

“In the consultation sessions I hold about Marine Nature Reserves for conservation, Manx fishermen are not questioning the mechanism by which closed areas could benefit fisheries. But they are concerned about MPAs for conservation. We are currently working toward the Isle of Man’s first Marine Nature Reserve designed primarily for conservation purposes. It also has the potential to play a role in fisheries management if we can get the design right. Research looking specifically at designing MPAs to maximize conservation and fisheries benefits – and at how MPAs for conservation can contribute more widely to healthier ecosystems and more sustainable fisheries – is most useful for my current work.

“In the Isle of Man we can use our scallop Closed Area experiences to illustrate the potential for MPAs to show similar effects for other species of conservation and commercial interest. What we do not have is clear evidence to show how protecting other marine habitats could affect fisheries. We need to protect maerl (red algae) beds, horse mussel reefs, seagrass meadows, and many other habitats for their conservation importance. There is now great science on the value of maerl beds for juvenile scallops and some fish species. But for other habitats we do not really know whether protecting them will lead to tangible benefits for our current fisheries, now almost exclusively limited to a few species of shellfish.

“What I say to fishermen is that I think that we can work together to design conservation MPAs that can offer fisheries benefits, combining fishermen’s knowledge and the latest science. There is real potential for our Isle of Man combination of Fisheries Closed Areas and conservation MPAs to improve wider ecosystem health in our territorial sea, giving us more resilience against climate change and more options in an uncertain future.”

For more information: Fiona Gell, Department of Agriculture, Fisheries and Forestry, Isle of Man Government, British Isles. E-mail:

BOX: Research spotlight: Why are older reserves more effective than younger ones at building biomass?

A recent study in the Journal of Applied Ecology (“Effects of marine reserve age on fish populations: a global meta-analysis”) found that no-take marine reserves older than 15 years consistently harbored more fish compared with unprotected sites, whereas younger reserves were less effective at building biomass. We asked one of the study’s authors, Isabelle Côté of Simon Fraser University (Canada), why older reserves are more reliably effective.

“There are a number of potential reasons,” says Côté. “First, true protection may take a while to be implemented (via patrolling or community acceptance of the reserve), so reserves might become better protected over time. Second, if reserves have positive effects on habitat quality, these might take a while to manifest themselves. Third, it may be that periods of time on the order of a decade or more are needed for an area to experience strong recruitment pulses which occur asynchronously across species, and which are so important in determining population levels. It could also be a combination of these things – for example, recruitment success could become enhanced as habitat quality improves.”

Côté’s is the latest research to draw a link between reserve age and fish abundance. Ronald Maliao of the Florida Institute of Technology (US) led a similar study, with findings published in the journal Coral Reefs in 2009 (“Trajectories and magnitude of change in coral reef fish populations in Philippine marine reserves: a meta-analysis”). “Our own paper also pointed out that fish density is higher in older reserves,” says Maliao. The conclusions of both papers are encouraging, he says, and appear to coincide with those of earlier studies of MPAs in Kenya and the Philippines. “The finding on reserve age is probably a global trend,” he says.

For more information:

Isabelle Côté, Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada. E-mail:

Ronald Maliao, Department of Biological Sciences, Florida Institute of Technology, Melbourne, Florida, US. E-mail:

BOX: Study finds spillover of lobster from closures in Atlantic Canada

A study of lobsters inside and outside of closed areas on the Atlantic coast of Canada concludes that the closures have produced several benefits over the past decade, including the spillover of adult lobsters to adjacent fished areas. The study, published by Fisheries and Oceans Canada, also found a higher abundance of large lobsters, a broadening of population size, and increases in average size of lobsters inside the closures. A report on the study, “Assessing Marine Protected Areas as a Conservation Tool: A Decade Later, Are We Continuing to Enhance Lobster Populations at Eastport, Newfoundland?” is available at