Too warm water is being blamed for the deaths of hundreds of fish introduced to Muskoseepi Park last week. Alberta Environment and Parks brought 500 rainbow trout from Cold Lake to the pond on June 1st, but by the end of the weekend, upwards of 300 had died.
Senior fisheries biologist Adrian Meinke says the testing they normally do for dissolved oxygen and pH levels were in the right range, but the water temperature was at the high end of the limit for rainbow trout. Staff then placed a group of fish in the pond using a net to see how they would react.
“Typically, response to fish for high water temperatures is fairly acute and quick,” explains Meinke. “These fish seemed to not exhibit a typical sign and there has been times when rainbow trout have been able to be successfully released in higher water temperatures.”
The decision was made to stock the pond, and it wasn’t until the next morning that 20 to 50 fish were found floating on the surface of the water. Hundreds more have since been killed by the shock of the warm water.
Meinke says water this warm is unexpected this time of year, but the city has seen summer weather over the past few weeks. The Muskoseepi Park pond is also shallow, so the temperature of 23 degrees celsius was likely consistent throughout.
“In years [past], these fish would be given a number of days to weeks for those temperatures to reach in to the 20s and they’re able to acclimate, but because of the sudden temperature shock, unfortunately a large number succumbed to the high water temps.”
Having gone through this experience, Meinke says they’ll be re-assessing the criteria they use for fish in the pond, and will avoid stocking it when it’s this warm. He adds that they likely won’t add more fish this season.
“If conditions did change, we might consider putting some more fish in, but those conditions would include the weather getting fairly cool, maybe having a substantial dump of rain to help cool that water and turn it over, but at this point in time, based on our current trends and weather pattern, we aren’t considering stocking.”
A chemical called Rotenone was used in the pond last fall to get rid of hundreds of goldfish that had been there as an invasive species. However, Meinke says that has nothing to do with the rainbow trout deaths.
“Now this chemical has a fairly quick half-life, meaning that the chemical breaks down and becomes inert fairly quickly. Our estimate’s that four weeks post-treatment, the concentration of Rotenone in the water would have been below water quality drinking guidelines.”
Any fish that have survived the weekend are likely acclimated by now, and are safe to fish for and eat. However, being in the warm water, Meinke says their flesh will likely be softer than trout from a colder lake.