At last - somebody has done the work and provided some proof that the MPA spillover effect is real.
Hugo Harrison and others have just put out a paper in Current Biology which indicates that at their study site in Australia "reserves, which account for just 28% of the local reef area, produced approximately half of all juvenile recruitment to both reserve and fished reefs within 30 km". They conclude - and I concur - that this provides "compelling evidence that adequately protected reserve networks can make a significant contribution to the replenishment of populations on both reserve and fished reefs at a scale that benefits local stakeholders."
[see Harrison et al (in press) Larval Export from Marine Reserves and the Recruitment Benefit for Fish and Fisheries. Current Biology (2012), doi:10.1016/j.cub.2012.04.008]
I have always maintained that the precautionary approach should be applied to MPAs with fisheries goals - that the risk of displaced fishermen collapsing the stocks in surrounding areas may outweigh any benefits that MPAs may produce in terms of providing recruitment to surrounding fisheries. MPAs with fisheries management objectives need to be accompanied by firm fishery management measures in surrounding areas, or alternative fishing opportunities or livelihoods. But usually MPAs are promoted as a substitute for fishery management measures, especially in developing countries where it is difficult to institute these. I was concerned that the downside could well outweigh the upside of fisheries MPAs, both in terms of maintaining overall stock biomass, and in terms of maintaining community livelihoods.
Everyone accepts that MPAs lead to recovery of non-highly migratory fish populations within their boundaries, and can thus satisfy conservation goals. But for the first time we are seeing good evidence that the "MPA spillover effect" may well compensate for the increased fishing pressure on surrounding areas, and may thus contribute positively towards fishery sustainability goals. If 50% of the recruitment in the total area can be provided from an MPA covering 27% of this area in Australia - a country with the most stringent fishery management regime in the world - then in countries where the stocks are in worse shape the relative contribution of mature MPAs to recruitment in surrounding areas is likely to be even greater.
In short, the requirements of the precautionary approach - where an action that is suspected to be deleterious needs to be subject to a certain level of proof before that action is taken - are being satisfied when it comes to MPAs with fisheries objectives.
Of course, to be good sources of larval recruitment for surrounding areas, the fish in these reserves need to be protected from other deleterious impacts and not just from fishing. They need to be protected from pollution, agricultural runoff, reclamation, oil exploration and substrate mining amongst other things.
But by now, of course, everyone recognises this to be a no-brainer. Don't they?