DETROIT (AP) — King salmon has helped create a multibillion-dollar sport fishery in the Great Lakes, but ripple effects of invasive species have left the fish’s future less certain.

The Detroit Free Press reports that the king, or chinook, salmon was first transplanted into the Great Lakes about 50 years ago. The species led a turnaround in the area’s fishery, helping create a $7 billion economic impact.

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But the spread of invasive zebra and quagga mussels has led to steep population declines of another species that serves as the salmon’s almost exclusive diet.

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The Michigan Department of Natural Resources is reducing its salmon stocking to regain balance between predator and prey. Stocking is when fish are raised in a hatchery and released into the lake to increase the existing population.

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