Farming practices are killing off wildlife in Sussex rivers, according to an Environment Agency report.

Lettuce and pea farmers are endangering fish in the river Rother because they plough the land, allowing silt to escape into the water, and snail populations in the Ouse are being attacked by pesticides.

The news emerged in an annual review of water quality and natural habitats. The Costers Brook, a tributary of the Rother, has been filling up with silt, which has reduced the variety of mayflies, a food source for fish, birds and dragonflies. Previously the brook had good water quality.

Sean Ashworth, author of the report, said: "Agriculture has been converting from dairy to lettuces and peas. Soil ends up in the river. It smothers the river bed. We are changing the gravelly, stony habitat into silt. Anything that lives in the gravel is lost. It changes the community into a community which likes silt."

Some species of fish, such as chub and trout, use gravel to nest. Mr Ashworth said: "If you have a lot of silt, there is nowhere to create these nests."

The Rother and the Ouse are home to pike and roach and the snails in the Ouse provide food for fish.

John Archer, of the National Farmers' Union, said there were a number of national and local projects to inform farmers about how they can avoid polluting watercourses.

Agricultural pollution of this kind was not a serious problem but it had been attracting attention because industrial pollution had been significantly reduced in recent years. He said: "They have cleared up everything else but pollution from agriculture is outstanding."

For a long time the river Arun has suffered from the effects of having hormones in its waters, which have caused fish to change sex from male to female.

In the summer, up to 90 per cent of the Arun can be made up of water from sewage treatment works. Fish in the Lewes Winterbourne, a tributary of the Ouse, have also been killed by sewage outfalls. The number of insects in this stream is much lower than in other Sussex watercourses.

Mike Russell, of the Sussex Wildlife Trust, said: "Pesticides may damage particular insects like mayflies and dragonflies, which has a knock-on effect on the food chain. This general decline of insects has an effect on swallows and housemartins which feed on them.