Anglers pike fishing on Bassenthwaite are are being asked to share details of their catches with the Lake District National Park Authority.
"Fishermen and women are catching good numbers of pike, but say they now rarely get large ones weighing over twelve pounds," said Lake District National Park Authority ranger manager and Bassenthwaite Lake Restoration Programme member Shirley Muir.
"Some of the most significant differences seem to have been among pike stocks. As pike are top of the fish food chain, they are a good indicator of the health of the lake and, if changes are taking place, it is essential we find out more."
Shirley said the Lancaster-based Centre of Ecology and Hydrology had already done a lot of research into Bassenthwaite and was keen to investigate further.
"The centre wants to compare fish caught in 2007 with previous hauls to see if there are any significant changes in fish populations.
"We want to know what's happening to our freshwater species and why. We are asking anglers to send us catch returns throughout 2007 and we are very keen to contact people who have kept their previous years' records."
Letters have gone out to the all the lake's angling season permit holders appealing for help.
"We want to work closely with our fishing fraternity and now have two angling representatives on the Bassenthwaite Forum, which brings together conservation and recreation groups to advise on the lake's management," added Shirley.
For more information on the survey, click here.