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Thames estuary now providing a useful breeding ground. The study raises concerns that some already overexploited fish stocks are under additional threat from climate change. Allison Perry, one of the researchers in the study, said: " We know these ecosystems are already under heavy pressure and this is an added stress. " Some of the species studied also moved to deeper water as they shifted north, which may also have been driven by a search for cooler seas. " The North Sea presents a gradient from around 30 to 200 metres deep northwards, " says Perry. But populations of six species, including plaice and cuckoo rays moved to deeper waters without changing latitude. The findings demonstrate how sensitive fish stocks are to climate change. In 2003, scientists at the University of Texas reported that 99 species of birds, butterflies and alpine herbs had shifted 600 metres north each year because of global warming. In this latest study, the mean rate of movement of populations of the six species whose boundaries changed in relation to both climate and time was 2.2 kilometres per year. The shifting populations of fish have implications beyond fisheries management. The study found that smaller fish with shorter life cycles were more likely to move north in response to climate change, with larger, slower-growing fish lagging behind. Scientists now fear that differing abilities to respond to warming waters will fragment ecosystems by separating predators from their usual prey. " Previous work off the eastern United States has shown that fishes with the most temperature-sensitive distributions included key prey species of non-shifting predators. Such changes could have unpredictable effects in an ecosystem already under heavy anthropogenic pressure, " they write. Further temperature rises are likely to have profound knock-on effects for fisheries. If the North Sea continues to warm as expected, at least two species of commercial fish, blue whiting and redfish, will have disappeared by 2050. As climate change drives more familiar fish north, species more commonly found in southern waters may begin to populate the North Sea. According to the study, by 2080, the bib, a small cod relative, will extend its range over the entire North Sea. And other presently unusual species, such as sea bass, red mullet and john dory, may become increasingly common. The growth in the technology and experience of rearing farming fish of several species has prompted thoughts that hatchery-reared fish …