It is unclear why mercury contamination at O'Keefe's Lake is so bad. ((CBC))

An alarming increase in mercury levels at a popular fishing spot on P.E.I. means only catch and release will be allowed there.

O'Keefe's Lake, just east of Charlottetown, has been a popular summer and winter site for anglers for years, because the province stocks it with rainbow trout. Nine thousand fish were put into the lake just last year, but with mercury levels continuing to rise, the government has decided to stop stocking it.

From the summer of 2007 to the summer of 2008, average mercury levels in the fish rose from 0.53 milligrams per kilogram to 0.76 milligrams. In 2007, the most contaminated fish had mercury levels of 0.83 milligrams. In 2008, the highest level found was 1.95 milligrams, almost four times Health Canada's acceptable limit of 0.5.

For several years, the province has issued warnings that children and women of child-bearing age should not consume fish from O'Keefe's Lake, and in February, it posted warning signs for the benefit of winter anglers. It now says all fish caught must be returned to the water.

It is not clear what is causing the mercury levels to rise. The lake is a closed system, with no streams running in or out. That isolation could be causing pollution that comes from the air to concentrate in the lake.