In late August, there were reports of dead fish on Lake Koocanusa, a scene similar to one that occurred on the lake two years ago.
It’s not entirely understood what is causing the death of thousands of kokanee salmon, but one fisheries biologist has a theory.
Mike Hensler, who works out of the Libby Field Station, said typically, this type of occurrence isn’t uncommon in large lakes, especially when it comes to kokanee salmon.
“They are relatively fragile fish – canary in a coalmine type of thing,” Hensler told The Free Press. “They are susceptible to dramatic changes more so than other fish are, and when we see these kinds of occurrences, it’s usually associated with hot, calm weather followed by a fairly dramatic storm event.”
The event in this recent occurrence was a heavy rainstorm that hit the area on Aug. 21.
Hensler said what he’s seeing is dead and dying fish on top of the surface of the lake with enlarged gas bladders.
Hensler said the kokanee are limnetic fish, which means they’re out in the middle of the lake most of the time, where other fish are not so they won’t be in the zone where the die-offs are occurring.
“When we were able to sample them as they were dying to see what was happening internally, what we’re finding was they had troubles with their GI track – with the digestive system – so they were sick,” he said. “Now, how they got sick, we don’t know because we never really found full stomachs, but they ingested something that’s a gas.”
There is possibly an algae of sorts is at the surface of the water the kokanee are getting into, but Hensler said that’s just a guess.
This time of year, especially in the evening, there are lots of fish at the surface and because it’s had the summer to heat up, surface temperatures can be lethal, Hensler said, adding the last time they checked the lake’s surface temperature, it was between 71 and 72 degrees Fahrenheit.
While that temperature could be lethal, Hensler said fish have the ability to dive back down into cooler water, but when a fish gets sick, it loses its ability to keep its place in the water column and if this happens, as they go back up towards the surface, their gas bladders expand and a sick or disoriented fish can no longer dive or swim down to decrease the pressure.
“It looks like the actual death has to do with warm water at the surface, but the reason they’re at the surface is still one of those – its’ a guess – and our guess is that there’s something there; something that they’ve ingested that’s causing them to get disoriented and sick.”
As far as he knows, Hensler said the water is safe to swim in, and should someone eat a kokanee, he suggests cooking it well but said there’s no indication suggesting that whatever is causing the die-off is harmful to humans.
While there were several thousand kokanee that died earlier this summer, Hensler said considering there are millions in Koocanusa Lake, it’s nothing to be alarmed about.
“Even if 10-20 thousands of them perish, it’s not the end of the world for kokanee in Koocanusa. It’s hard to say if this is a natural phenomenon, but it looks like it is,” he said, adding it tends to be something that happens in big lakes.