A creepy, parasitic fish that thrives by sucking the blood out of its hosts – earning the nickname "vampire fish" – is making a comeback in the Great Lakes after the pandemic interrupted population control of the species.

The fish, which has a circular row of teeth, a serrated tongue, and an eel-like shape, is called the sea lamprey.

According to the National Oceanic and Atmospheric Administration, sea lampreys are native to the northern and western Atlantic Ocean but invaded the Great Lakes around the early 19th century through the Welland Canal, which connects Lake Ontario and Lake Erie.

Fish Shows Lamprey Bite
A lake trout from Lake Superior that was bitten by a sea lamprey. (Jerry Holt/Star Tribune via Getty Images)

"Within a decade, they had gained access to all five Great Lakes, where they quickly set to work predating on the lakes' commercially important fishes, including trout, whitefish, perch, and sturgeon," the NOAA wrote. "Within a century, the trout fishery had collapsed, largely due to the lamprey's unchecked proliferation."

By the 1960s, sea lampreys reduced the annual commercial catch of lake trout in the upper Great Lakes from about 15 million to half a million pounds, Wired reported.

The Great Lakes Fishery Commission, along with the US Fish and Wildlife Service and Fisheries and Oceans Canada, has been responsible for managing the population of this highly invasive species, and the agencies have done so with considerable success.

The fishery commission touts on its website that sea lamprey populations have been reduced by 90 percent "in most areas of the Great Lakes."

But between 2020 and 2021, the COVID–19 pandemic and ensuing travel restrictions interrupted the agencies' ability to go out and perform some of the population management operations.

Now, fishery managers say the population of the parasitic fish has ticked up across the Great Lakes, The Wall Street Journal reported.

It's unclear how much the population exactly increased, but according to a 2022 report from Undark Magazine, a nonprofit science publication, crews responsible for population control were only able to treat about 25 percent of the target streams in 2020. The following year, the teams reached 75 percent of their targets, the publication reported.

Treatment can be expensive and laborious, requiring the carefully-timed application of pesticides called lampricides to reduce the population.

Controlling the lamprey population is estimated to cost around $15 to $20 million a year, according to Wired.

This article was originally published by Business Insider.

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