The Yellowstone River, among the nation’s premier habitats for trout, is a top destination for fly fishers. So alarm was widespread in certain circles when, on August 19, the state of Montana shut down 183 miles of river because a parasite known as Tetracapsuloides bryosalmonae had killed thousands of fish (whitefish, mostly, but trout, suckers, and baitfish too).
Fish mortality at the height of summer is not unheard of. Trout struggle to survive if the water where they swim hovers above 70 degrees Fahrenheit for any length of time, and many stocked trout, even if they elude May’s baited hooks, will never survive the heat of July. But nothing like that is supposed to happen on the Yellowstone River, where clean, cold water and wild, healthy trout are the order of the day, the year, the millennium.
Except maybe this millennium. Parasites such as Tetracapsuloides, according to aquatic biologists, may thrive best amid high water temperatures and low water flows—just the sort of conditions that climate change can be expected to bring to cold-water fish habitats throughout the West. Indeed, as reported by Bob Berwyn of InsideClimate News, “stream temperatures across the region are expected to warm by 1 degree Celsius by 2050 and 2 degrees Celsius by 2100.” For anglers and other lovers of the outdoors, that news is… chilling.
Meanwhile, The Atlantic reports that though Tetracapsuloides bryosalmonae may be just a microscopic parasite, it’s also an animal—one whose ancient ancestors resembled jellyfish. Very strange.