Mar Ecol Prog Ser 314: 309–310, 2006 spawning stock and enable ‘spillover’of both larvae and adults into surrounding areas (Roberts et al. 2001). This is achieved by protecting particular areas of habitat from fishing so that fish can grow to become large adults. Aggregation of large numbers of adult wild fish at fish farms and the increase in their condition achieves the goals of an MPA almost perfectly. Only protection is missing. Partial protection from fishing exists in a handful of areas, but no restrictions apply in the vast majority of countries that practise coastal aquaculture.

Another good reason to keep wild fish near fish farms is that they reduce the impact of farms on the seafloor (Dempster et al. 2005). Nutrient and sediment waste flows out from fish farms in the form of food and faeces. If the amount of such waste is high, the diversity of the seafloor flora and fauna in the surrounding areas can change dramatically. Most of the wild fish beneath farms eat food lost from the cages, thereby reducing the waste that reaches the seafloor by up to 80%(Vita et al. 2004).