By Trevor Willis, Russell Millar, Russ Babcock & Nick Tolimieri

Many recent scientific papers on the subject of marine reserve effects contain statements within their introductions along the lines of “It is well known that exploited species exhibit increases in density and mean size within reserves”, supported by a number of citations. A closer look at the cited papers shows that many are review articles. Of the empirical studies published, most present ambiguous evidence for recovery. In fact, between 1990 and 2001, only 42% of published papers in this area contained empirical data, and many of these were difficult to interpret because of inadequate experimental design.

In the marine reserve context there are many reasons why researchers might have limits on their sampling designs. However, a critical evaluation of the experimental designs employed by many published studies brought to light the following problems with replication and lack of control sites:

  • insufficient sample replication (e.g., only one site sampled inside and outside a reserve, or no control sites sampled at all)
  • spatial confounding (e.g., all control sites located only at one end of the reserve, so that comparisons are confounded by unknown location effects)
  • lack of temporal replication (most studies consist of surveys done at only one time)
  • lack of replication at the reserve level limiting the generality of results (although in many cases this reflects the number of reserves available).
  • non-random placement of reserves – i.e., often reserves are sited to include “special” or unique features, which causes difficulties in selecting valid control sites (this is obviously no fault of the researchers).

To date, there are no studies that avoid the above problems as well as possessing a time series of “before” and “after” data.

How many studies unambiguously demonstrate significant within-reserve increases in the density of exploited species? With a sufficiently large sample size, a statistically significant difference between two sites (separated either spatially or temporally) can almost always be obtained due simply to true natural biological variability between the sites. That is, the null hypothesis of no difference between two biological entities is necessarily false. If we (conservatively) use a 100% increase in density as a minimum criterion for claiming the existence of a “reserve effect”, and ignore flaws in sampling design, then there are only a handful of instances where differences in density of individual species between reserve and fished areas can be regarded as biologically significant. In many other cases, slight trends toward higher reserve densities have been described, but these were of insufficient magnitude to confidently attribute them to reserve effects, rather than real biological variability at the spatial or temporal level. If we consider only those studies that are replicated in both time and space, to our knowledge there are only a few that establish increases in excess of 100%.

Several theoretical studies have indicated that marine reserves can provide increases or equivalence in fisheries yields under the assumed model and parameter values. However, if management decisions are based upon models built on unquestioned assumptions then we may find ourselves making costly errors. We reinforce this point by noting that there have been yield models produced which respectively predict reserves can increase fishery yield, may have no effect on fishery yield, or can be detrimental to fishery yield. Taken together, the conflicting conclusions from various plausible models lead us back to the beginning, where we must admit that, at present, we cannot predict what the effects of marine reserves might be for any given species. While the theoretical work done to date has helped to identify and formalize competing hypotheses, it should not be used to make quantitative management decisions relating to particular species. What is needed now is for models to be shaped by empirical data rather than being built purely from general assumptions and ancillary knowledge.

Since the management goals of marine reserves are many and varied, and the biology and ecology of exploited species also vary from place to place, the huge amount of effort currently being spent on “optimizing” marine reserve design is probably largely a waste of time and energy. There is probably no such thing as an optimal reserve – what is good for one species may not be particularly useful for another. A more effective role for research in the context of fisheries management might be to establish what are the minimum requirements for protection of exploited species. A return to more natural ecosystem function will probably occur as a by-product of protected areas that focus on targeted species, and more research effort should also be directed toward the effects of not fishing on unexploited species.

This comment is not intended to imply criticism of those working for the establishment of marine reserves, and it is not intended to counteract the precautionary principle. Nor should this comment be interpreted as “anti-reserve”. Rather, it is a plea for researchers to apply the same rigor to examination of the fisheries-related efficacy of marine reserves as they would apply to other environmental effects studies. Perhaps more importantly, this plea also goes out to those in a position to fund this research. They must ensure that adequate planning and resources are allocated to make it possible to implement rigorous survey designs, and that this is done far enough in advance of reserve establishment so that effects outside their boundaries can be detected. Ultimately, in a field where the division between science and politics is becoming increasingly blurry, poorly conducted studies or those with major design flaws serve to undermine the credibility of scientists, and provide ammunition to those who wish to oppose reserve proposals for reasons of their own.

(This piece was adapted by the authors from: Willis T.J., Millar R.B., Babcock R.C. and Tolimieri N. 2003. Burdens of evidence and the benefits of marine reserves: putting Descartes before des horse? Environmental Conservation 30:97-103. Copyright 2003 Foundation for Environmental Conservation)

For more information:

Trevor J. Willis, Laboratori Scienze Ambientali, Universita di Bologna, Via Sant’ Alberto 163, 48100 Ravenna, Italy. Tel: +39 0544 454 901; E-mail:

Russell B. Millar, Department of Statistics, University of Auckland, Private Bag 92019, Auckland, New Zealand. Tel: +64 9 373 7599; E-mail:

Russ C. Babcock, CSIRO Marine Research, Private Bag 5, Wembley, WA 6913, Australia. Tel: +61 8 93 336 535; E-mail:

Nick Tolimieri, 1015 NE 105th St, Seattle, WA 98125, USA