Fish might not be able to ride bicycles but they could soon be climbing ladders in a scheme to attract migrating sea fish back to the River Aire.

The Environment Agency is introducing 'fish passes' on weirs along the river to allow salmon and trout to negotiate their way back to spawning ground further up the Dales.

It is currently working with the Bradford Amateur Rowing Club, which is repairing Hirst Weir in Saltaire.

As part of the repair work, the club has agreed to make room for a fish pass to be introduced, which involves lowering part of the weir.

The fish have not been seen in the rivers for the past 150 years because of pollution caused by the Industrial Revolution, and man-made structures such as the weirs, which have been too high or difficult for the fish to negotiate.

Fish passes work like ladders, allowing the migrating sea trout and salmon to leap from one stage to the next over the weirs, which can be as high as five metres.

A spokesman for the Environment Agency said: "Small numbers of salmon travel up the Ouse into the River Aire but get stuck at Chapel Haddersley, near Knottingley.

"They come in up the Ouse system and try to get into the Aire system where they get to a series of weirs. The fish passes are structures that allow fish to get up over the weirs, almost like ladders with water.

"Instead of having a vertical barrier in front of them they have a graduated structure and can leap up the different stages."

The long-term project across the north east region aims for salmon and trout to return to their original spawning grounds in about 20 years.

Kevin Sunderland, chairman of the Aire and Calder Rivers Group, said: "The River Aire has been cleaned up tremendously in recent years." Salmon had been seen in the River Yore and the River Wharfe as close as Leeds.

"A lot of trout have been trying to get up the weir to spawn and feed. This could be one of the first steps towards seeing salmon coming up the river in 20 years' time," said Mr Sunderland.

It is hoped other fish such as grayling will return to the river.

The Environment Agency stressed it was a long-term strategy and it was reviewing all the weirs in the river system to see if fish passes could be put in place.