Over the past 25 years, invasive lake trout have reproduced rapidly in Yellowstone Lake, preying on the native cutthroat trout. Cutthroat swim into streams and rivers connected to the lake to spawn, and occupy more shallow depths in the lake than lake trout do. Lake trout eat the cutthroats and tend to stick to deeper waters, where osprey and other animals can’t catch them as easily as cutthroats.
The decline in Yellowstone cutthroat trout in Yellowstone Lake has meant that anglers in Yellowstone National Park must release any of the native fish that are caught. Unfortunately, for the park’s osprey, catch-and-release fishing is not an option.
Osprey are among the 40 species of animals in Yellowstone that rely on cutthroat trout as a food source. But unlike grizzly bears, for instance, or even bald eagles — two species that can adapt their diets somewhat to different food sources depending on available prey — osprey are almost entirely reliant on fish. While osprey in other parts of Yellowstone are holding their own, the plunge in cutthroat trout in Yellowstone Lake has been bad news for osprey nesting in that area.
That’s probably because bald eagles have a wider diet, with fish making up only about 30 percent of their total prey. Bald eagles around Yellowstone Lake can turn to everything form ducks to carrion to help make up for lost trout.
The good news for osprey is that in other parts of Yellowstone, where other prey fish are more readily available, the birds are doing well, with populations holding steady. In fact, some osprey are probably leaving the lake for other waters inside or outside the park.
With assistance from the Yellowstone Park Foundation, the National Park Service has hired commercial netters to remove as many lake trout from Yellowstone Lake as possible. While lake trout can’t be eradicated from the Yellowstone Lake, the goal is to reduce and hold their numbers at a level low enough that cutthroat can make a significant comeback. More than 1.1 million lake trout have been removed since 1994.
Annual monitoring efforts for osprey, bald eagles and other birds will also continue, as researchers and raptors wait for cutthroat to return.