GLASGOW, Mont. – State fisheries biologists say that hot weather has caused the death of hundreds of cisco, a prevalent forage species, in the Fourchette Bay and Narrows area of Fort Peck Reservoir over the past few weeks.
Cisco, also known as lake herring, are typically found in the deeper, colder areas of the reservoir. But a sustained period of hot summer weather has increased surface temperatures in the upper portion of Fort Peck, some of which is relatively shallow.
Warm water temperatures — recently documented as high as 84 degrees Fahrenheit in the Fourchette area – and subsequent low levels of dissolved oxygen take their toll on some types of fish, especially cisco.
“We’re getting reports of hundreds of cisco floating and washing up near shore,” said Heath Headley, the reservoir’s lead biologist with the Montana Department of Fish, Wildlife & Parks. “While the numbers of dead fish may seem like a lot to anglers, they should remember that the area where the die-offs are occurring is quite small when compared to the rest of the reservoir.”
Headley said cisco die-offs have occurred in the same part of Fort Peck Reservoir in past years. Shallow depths make the water more susceptible to large fluctuations in temperature.
In addition, Headley said the relative abundance of cisco in the reservoir has increased over the past several years. That’s largely due to favorable spawning and rearing conditions created by higher water levels.
“This has led to an increased number of adult cisco in the current population, which in turn has led to healthier and larger game fish species in Fort Peck,” Headley said. “Anglers have likely noticed more robust chinook salmon, lake trout, walleye, and northern pike on the end of their lines the last couple of years. That’s why.”