De-industrialisation and more effective environmental management and regulation has seen a significant improvement to water quality on waterways across the UK, sufficient to support the return of aquatic life. However despite these improvements, weirs continue to have an adverse impact on biodiversity and the natural heritage of the waterway and the wider catchment as follows:


  1. Impeding the migration of salmon and sea-trout (fish from both these Biodiversity Action Plan (BAP) species from other rivers now annually migrate up the river to the weir) towards their spawning gravels and therefore stopping recolonisation of the river.

  2. Impeding access for sea and river lamprey (BAP species) that migrate up the River Ouse to spawning gravels in the River Calder.

  3. Impeding eel migration (BAP species) preventing them reaching important nursery habitat.

  4. Preventing coarse fish (e.g. barbel, dace, chub) from reaching spawning gravels. 


The fish and eel pass is the primary measure required to help secure sustainable fish populations in a river. They also assist in securing the future of fish eating predators such as otters (UK BAP), heron and kingfishers (RSPB Amber List) which depend on sustainable fish stocks in the river. 



A diagram of a Larinier fish pass


A Larinier fish pass under construction at Thrybergh Hydro. 

(A rest pool can be seen half way down the slope)