Freshwater pearl mussel breeding project in Northumberland
2 March 2011 Last updated at 09:39 GMT
The Environment Agency and scientists are celebrating encouraging signs in a lengthy project to try to get pearl mussels to breed.
They have been trying to recreate breeding conditions at Kielder Hatchery in Northumberland for seven years but with little success.
Now the work has finally paid off with 6,000 baby sea trout playing host to pearl mussel larvae.
The endangered molluscs' unusual breeding process means that fertilised eggs need to 'latch' on to fish to develop.
Jeremy Cooke reports.