Dorset salmon study: Conservationists work day and night at River Frome
Conservationists are once again surveying salmon in a Dorset river as part of efforts to tackle a decline in the species.
The Game and Wildlife Conservation Trust tagged and weighed about 13,000 fish in the River Frome in the autumn.
The next phase aims to check the tagged juveniles as they migrate out to sea.
A special trap built across the water at the trust's research centre at East Stoke, near Wareham, runs day and night and is being checked every 30 minutes.
The trust said the majority of the one-year-old fish - called smolt - migrate at night, meaning scientists and volunteers must work around the clock to recapture, measure and weigh them before releasing them further downstream.
Fisheries scientist Bill Beaumont said: "They are critically endangered - they are a red-listed species - and we need to do what we can to conserve them and keep them in the river."
Head of fisheries Dylan Roberts said, in the 1970s and 80s, about 15-20% of the salmon that went to sea would return as adults but that figure was now just 4 to 5%.
He said: "Two significant problems are declining water quality and physical habitat - together these are producing juvenile salmon less fit to survive in the ocean."
Fisheries ecologist Sophie Elliott said the smolt would head to the Arctic before returning to the same river to spawn in one or two years' time.
She said: "There are not many species that migrate such long distances and have such a unique lifestyle. It would be a shame to lose such an iconic species."
Data from the River Frome and 11 other European rivers is used to inform governments, and to encourage farmers and water companies to improve their operations.
The survey will continue until 10 May.
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