How UK scientists are helping eels swim upstream
11 August 2013 Last updated at 05:58 BST
Once a common sight in Britain's rivers, the European eel is now on the critically endangered list - the population has plummeted by 90% in the past 30 years.
One of the reasons for their decline are man made obstructions in rivers, like weirs, which prevents young eels migrating upstream.
But now conservationists are hoping measures they are taking in waterways across the country are gradually helping their numbers recover.
Victoria Gill reports.