THOUSANDS of tonnes of gravel are being tipped into a river to encourage salmon to breed.

Fish have already been seen spawning on the first batch of 1,000 tonnes put into the River Balder, in Teesdale, County Durham, under an Environment Agency action plan.

Another 1,500 tonnes will be introduced later this year.

The rivers Balder and Lune are important tributaries of the Tees but do not contain sufficient amounts, or the right type of gravel for salmon to breed.

This is because a chain of reservoirs west of Barnard Castle disrupt the natural flow of gravel from the headwaters.

Northumbrian Water is supporting the move by releasing water from the Hury Reservoir to slow up the gravel's move downstream.

Fisheries staff at the Environment Agency say the habitat in the Balder is good and would support more salmon if their numbers could be increased.

Gravel encourages salmon to breed because the fish need clean gravel in which to lay their eggs.

The female digs a nest known as a Redd, then lays her eggs for the male to fertilise.

She then covers the eggs, where they incubate slowly in the gravel throughout the winter and hatch in early spring.

The agency's Tees fisheries officer David Bamford said: "In the past, the Balder Valley was one of the best salmon spawning areas in the River Tees catchment but this changed when Hury reservoir was built in the 1890s. Subsequently more reservoirs were added up the valley.

"We need a healthy stock of salmon in the river if the Tees is to recover to the same extent as the River Tyne which is now one of the best salmon fisheries in England and Wales.

Regular surveys are planned to monitor the effects on the number of salmon in the river.