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Ciscoes Discovered in Yellowstone Lake

A three-year-old female cisco discovered in August of 2019 was determined to have been born in Yellowstone Lake.

Ciscoes Discovered in Yellowstone Lake

Ciscoes are a well-known favorite food source for lake trout (shown) in the Great Lakes and Canada. Neal Herbert/National Park Service photo

How exactly nonnative ciscoes got into Yellowstone Lake is the latest mystery to plague Yellowstone Lake and Yellowstone National Park fisheries personnel. Park officials revealed the discovery of ciscoes at Yellowstone’s 2021 Annual Fish Meeting. It’s likely more bad news for the beleaguered haven of native Yellowstone cutthroat trout.

According to Todd Koel, Yellowstone’s Native Fish Conservation Program lead, one three-year-old female cisco (likely Coregonus artedi) was discovered during routine lake trout gillnetting in August of 2019. Subsequent otolith testing revealed that the fish was born in Yellowstone Lake, which indicates that there is a viable, reproducing population of ciscoes in the 85,000-acre lake. To date, only one has been found and positively identified.

Ciscoes are baitfish that are native to the Great Lakes and other coldwater lakes in the Upper Midwest and Canada. They are a well-known favorite forage fish of lake trout. The nearest nonnative population to Yellowstone Lake is in Montana’s Fort Peck Reservoir, about 250 miles away. It is this distance that led Koel to believe the ciscoes were likely planted in Yellowstone Lake intentionally.

Ciscoes Discovered in Yellowstone Lake  
Ciscoes have apparently been intentionally and illegally planted in Yellowstone Lake and are now reproducing. Neal Herbert/National Park Service photo

“It took a lot of time and care to move those fish in abundance great enough to establish a reproducing population in the lake,” Koel said at Yellowstone’s 2021 Annual Fish Meeting. “Ciscoes did not swim there on their own. They did not get dropped in there by an eagle or anything like that. They were intentionally brought to the lake, intentionally by humans. Do we think that the ciscoes were brought here intentionally as a food resource for lake trout? I don’t know why ciscoes could be brought there for any other reason.”


Invasive lake trout were first discovered in Yellowstone Lake in 1994—how they got there remains a source of great debate. But because Yellowstone Lake is one of the last significant refuges for native Yellowstone cutthroat trout, which are essential to the Park’s ecosystem and are prized by anglers, the National Park Service has worked for many years to reduce or eliminate the lake trout population.


Significant progress has been made in recent years, and cutthroat populations have shown signs of recovery prior to the discovery of the invasive forage fish.

Most anglers support efforts to remove lake trout, but there are pockets of anglers throughout the Rocky Mountain West who oppose the removal of the wild-born and gameworthy mackinaws. Because of this opposition, Koel believes it’s feasible that there could be a rogue group of monkeywrenchers actively (and illegally) finding ways to support the lakers by, for example, adding a population of baitfish for them to feed upon.

Regardless, complete eradication of the ciscoes is now virtually impossible, and they are a new reality in Yellowstone Lake. How they will affect the lake and its treasured cutts remains to be seen, but the addition of nonnative species into Yellowstone Lake’s fragile ecosystem is likely a cause for concern.

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