TRAVERSE CITY, Mich. — When Queen Elizabeth II became the longest-reigning British monarch earlier this month, cooks in Gloucester presented her with an unusual gift: a lamprey pie. They had to import lampreys from the Great Lakes, where they are considered an invasive species.

It was a necessity that made many employees at the Great Lakes Fishery Commission in Michigan chuckle.

“I was amused by the whole thing because it’s such a great old tradition,” said spokesman Marc Gaden, who sent three frozen Great Lakes lampreys for the pie. “The irony, of course, is hilarious because we’re trying to get rid of them and they’re scraping trying to find lamprey.”

Great Lakes sea lampreys were slightly more elusive this year. The species’ population dropped in 2015, according to the Great Lakes Fishery Commission’s annual lamprey report released Wednesday morning.

Lake Michigan’s lamprey population hit a 20-year low — an estimated 27,000 adults. The Lake Huron lamprey population dropped to a 30-year low — an estimated 69,000.

Sea lampreys devastated Great Lakes fisheries when they spread throughout the five freshwater basins in the early 1900s. Adult lampreys latch on to fish with rows of razor-sharp teeth and hurt or kill fish by sucking out their blood and body fluids. Each adult lamprey can kill up to 40 pounds of fish during its 12- to 18-month feeding period.

Brian Springstead, captain of True Blue Charters in Acme, Michigan, said an older generation of anglers remembers the lamprey’s effect in the 1950s and 1960s.

“There was literally nothing left in the Great Lakes but ciscoes,” Springstead told the Traverse City (Michigan) Record-Eagle. “There was not lake trout, there was no whitefish. It was just a desert.”

Springstead said he didn’t see any fresh scars or lampreys attached to lake trout this year, but the salmon he caught in August were loaded with them. He caught three salmon with lampreys attached, and estimated 20 to 25 percent of the salmon he caught had fresh wounds they likely picked up in southern Lake Michigan.

David McCool, a fly fishing guide based in Lake Ann, Michigan, said zebra mussels, another invasive species, gobble so much algae that fish low on the food chain have little left to eat. That means less prey for big sport fish. Fewer sport fish means less ideal prey for lampreys.

“In general in the environment and in life everything’s kind of connected,” he said. “I think all of it’s played a role in the reduction of lamprey.”

Great Lakes sea lamprey control isn’t left to the environment. The fishery commission spends about $20 million of its funding from the U.S. and Canadian governments on sea lamprey research and control.

The commission treats streams with chemicals to kill burrowing lamprey larvae and builds barriers, or dams, that prevent lampreys from migrating upstream to spawn.

could divert them from lakes and streams.

Springstead said he wouldn’t have a business without the commission’s lamprey work. The Great Lakes fisheries could return to that desert state without it.

“I don’t care how much money they spend, it’s worth every penny,” he said.

Thompson writes for The Traverse City (Michigan) Record-Eagle.