Bears That Have No Fish to Eat Eat Baby Elk Instead

The illegal introduction of lake trout in Yellowstone’s lakes is having wide-reaching consequences

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Yellowstone National Park is a vast expanse of largely-untouched natural beauty, a tract of the west home to bears and wolves and geysers and mountains. But where humankind’s direct influence is deliberately kept to a minimum, that strategy of do-no-harm doesn’t always seem to work. For the past few decades, lake trout have been taking over the rivers and lakes in Yellowstone, pushing out the local Yellowstone Cutthroat trout. The Greater Yellowstone Coalition:

Yellowstone Lake and its tributaries once supported an estimated 3.5 million Yellowstone cutthroat trout. Since the illegal introduction of lake trout in the 1980s, the cutthroat population in Yellowstone Lake has plummeted. Catch rates for Yellowstone cutthroats have significantly dropped as more and more lake trout are caught every year. The precipitous drop in cutthroat numbers is a result of lake trout predating on cutthroat trout.

But more than just affecting cutthroat trout, the invasion of the lake trout is being felt throughout the ecosystem. According to new research lead by Yale’s Arthur Middleton, the replacement of cutthroat trout with lake trout is leaving Yellowstone’s local population of grizzly bears without enough fish to eat. Middleton and colleagues:

Historically, Yellowstone Lake harboured an abundant population of cutthroat trout, but lake trout prey heavily on cutthroat trout and have driven a decline of more than 90 per cent in their numbers. Although cutthroat trout migrate up shallow tributary streams to spawn, and are exploited by many terrestrial predators, lake trout spawn on the lake bottom and are inaccessible to those predators.

Without fish, the grizzlies need something, and in their place the bears have turned to eating baby elk.

In the late 1980s, grizzly and black bears killed an estimated 12 per cent of the elk calves in northern Yellowstone annually. By the mid-2000s, bears were estimated to kill 41 per cent of calves.

The researchers say that by turning to elk calves in place of the now-gone trout, the elk population growth rate has shrunk by 2 to as much as 11 percent. The research reminds that the food web is in fact a web, and that the illegal introduction of a few trout can mean a whole lot of dead elk.

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