Populations of wild salmon in the North Atlantic have fallen to record lows, and if current trends continue, the salmon may be headed for extinction, the World Wildlife Fund said today. In the last 25 years, the number of large salmon returning to spawn in North American rivers has fallen from 800,000 a year to 80,000, according to WWF figures. The main causes of the decline are overfishing, particularly by England and Ireland, dams on rivers, and escaped farm salmon, which interbreed with wild salmon and compete with them for food. WWF is calling on the North Atlantic Salmon Conservation Organization, which will be meeting in Canada next week, to set tighter catch limits for the fish. The conservation group is also urging an end to U.S. production of genetically modified salmon, which some scientists say could escape and breed with wild salmon.