U.S. Fish and Wildlife Services crews will study the presence of sea lampreys in the St. Louis River in August.

From August 6-15, a crew from the federal agency will study the amount of lamprey larvae in the St. Louis River to decide on possible lamprey prevention, according to a news release from Fish and Wildlife Services.

The parasitic fish, which is native to the Atlantic Ocean, became a Great Lakes invasive species in the 1920s. Since then, they’ve been a “permanent, destructive element of the fishery,” the news release read.

Using their suction cup mouths, they attach to fish and scrape through its scales and skin to feed on bodily fluids. During its parasitic feeding stage, a lamprey can kill upwards of 40 pounds of fish. Fish are often unable to survive after a lamprey attaches, according to the Great Lakes Fishery Commission.

Lampreys threaten fish in the Great Lakes when the larvae drift from streams and tributaries, like the St. Louis River, into the lakes.

Before lampreys reached Lake Superior, 4.5 million pounds of fish was harvested annually. After the lampreys invaded, the annual catch decreased to 368,000 pounds by 1961, according to the commission.

The study is among hundreds of other lamprey surveys done annually in streams that feed into the Great Lakes, according to the news release.