An illicit swim in an ecologically sensitive hot spring at a national historic site in Banff has killed at least five endangered snails.

A vandal broke into the Cave and Basin historic site last week and swam in an outdoor hot spring that is home to Banff Springs snails, an endangered species under Canada's Species at Risk Act.

Losing even five snails is a big deal because the species is an important part of the Banff ecosystem, site supervisor Steve Malins said Thursday.

"We use it basically as an indicator species. If the snail is doing well in its environment, it's a fairly good indication that the rest of the thermal springs ecosystem is doing well."

Dipping a hand in a pool can kill snails

The snails, each about the size of a lemon seed, are found only in pools at the historic site on Sulphur Mountain. In 2005, there were only 34,000 left, but the population fluctuates.

Hot springs contain minerals, little oxygen, high concentrations of dissolved hydrogen sulphide gas, and even some radioactivity. Most Banff Springs snails live in the part of the springs where the water gurgles up from the ground.

According to Parks Canada, even dipping a hand in a pool on the site can kill the snails or their eggs.

Malins said the swimmer damaged the floating microbial mats on which the snails feed and lay their eggs. Some of the algae was also smeared on the side of a nearby building.

The vandal swam undetected because a motion sensor temporarily failed, but it has since been fixed, Malins said.

The historic site commemorates the birthplace of Canada's national park system, which began at the hot springs in 1885. The pools on the site are closed to the public.