The cost of replacing fish stocks in a Swindon lake devastated by a mystery incident is expected to run into tens of thousands of pounds.

Water samples from Peatmoor Lagoon are being analysed by the Environment Agency after a sharp drop in oxygen levels led to the death of thousands of fish.

There has been no official confirmation of the cause of the phenomenon, but theories that have been suggested range from algae to warm weather.

Members of the Peatmoor Angling Club have had the heartbreaking task of collecting dead bream, roach, pike and perch from around the lake for destruction by council workers.

For the moment, all fishing is banned at the West Swindon lake.

The club, which has nearly 200 members, says it will take several years to restore fish stocks.

Members have removed 550 bream, dozens of tench, 20 pike and thousands of smaller fish from the lake.

One estimate for the cost of replacing the bream alone is £25,000.

Club vice-chairman Richard Reeves said: “I don’t know how the club is going to recover. We can’t find that sort of money at the moment.”

Carp, which have a higher tolerance for reduced oxygen levels, seem to have mostly survived.

Other fishing clubs have offered Peatmoor members access to their lakes, and air pumps borrowed from another club were today helping to reoxygenate the water.

Mr Reeves said: “Other clubs have been great at helping out.”

On the club’s Facebook page, a runner who uses the track around the lake has suggested that he had smelled dead fish for around a month.

A club member said seeing the lifeless fish had been “the saddest sight in all my near 60 years of angling”.

Another commentator said oxygen levels could have been reduced by a build-up of organic material on the lake bed, or by an increase in temperature – or both.

Club members say they think it unlikely that a pollution incident is responsible.

Thames Water is supporting investigations at the site, but has stressed that the deaths are not connected with its sewage network.

The Environment Agency said today that the results of water sample analysis would not be back until next week.

Swindon Borough Council is supporting the agency and the club.

It said: “We are working with the Environment Agency and Peatmoor Angling Club to ensure the lagoon can be a thriving hub once again.

“Our warden team has visited the site to assist in the clean-up of the fish and we are awaiting more information from the Environment Agency with regards to what has happened.”