Forty eels have been freed into the River Lune by a specialist contractor after they were discovered by workmen on the site of the Luneside West housing development.
Redrow Homes and Barratt Homes are jointly building 403 homes and apartments at the former Williamson’s/Forbo works site off St George’s Quay.
It is understood the reservoir was previously used by Forbo’s fishing club in the 1980s and 1990s and Lancaster City Council had specified that any European eels found there should be safely moved.
They are an endangered species and it is estimated that numbers have fallen by 95 per cent since the 1980s.
Castle ward city councillor, Jon Barry, who urged planners to check that the eels had been safely relocated, said: “It would have been awful if they had been buried alive and I am grateful that they have been released unharmed.”
A Redrow spokeswoman said: “We were assured that all of the fish thrived following the move.”
Work on the new homes will begin next month and could take six years.