Older marine reserves are more effective at protecting fish than newer reserves, according to a recent study. Reserves should be maintained for at least 15 years to allow fish numbers, particularly large, locally fished species, to recover. Marine reserves are established to protect marine biodiversity, including the conservation of fish species vulnerable to overfishing. By banning, or only allowing well-regulated, minimal fishing in reserves, fish populations have a chance to recover. In addition, the spillover of adults, eggs and larvae from reserves into neighbouring areas benefits local fisheries and tourism. Using the combined results of 33 studies from around the world, researchers in Canada assessed the effectiveness of marine reserves to protect fish species. The research focused on the densities of fish found inside and outside 32 reserves of different ages (from 1-26 years). Relative fish densities were compared with the age of the reserve. The 'relative fish density' is the average number of individual fish of each species per unit area inside a reserve relative to that in a neighbouring, fished area. The researchers also considered body size, which was used as an indicator of the life history and ecology of the fish species. Overall, the study found that fish were 66 per cent more abundant inside the reserves, compared with areas outside reserves. In addition, the older the reserve, the more effective it was. Fish density increased by about 5 per cent per year inside reserves compared with neighbouring areas outside reserves. Furthermore, significantly more fish were found inside reserves that were older than 15 years, compared with near-by unprotected areas. The researchers suggest this does not imply that younger reserves will not be effective. Reserves should be maintained for at least 15 years after establishment, even if they initially appear ineffective, to see clear increases in fish densities. Most require more than a decade to provide significant benefits. The study suggests that locally fished species will recover in reserves, given enough time. Management of reserves should therefore incorporate short-term uncertainty with long-term objectives.