PORTLAND – After 4,500 years of cod dominating the Gulf of Maine, fishing pressures have triggered two major reorganizations of its ecosystem in the last four decades, according to University of Maine research.

For millennia, cod were the top predator in the gulf and their feeding habits kept other species in check.

But fishing techniques in the 1960s pushed cod into steep decline and allowed for sea urchins, kelp, and crab to dominate coastal waters, according to the report.

The same conditions that led to a proliferation of crabs in the Gulf of Maine also allowed for the record lobster harvests of recent years, according to Robert Steneck, a marine sciences professor at Maine and one of the report’s authors.

Steneck says the trends documented in the paper he published with former graduate students John Vavrinec and Amanda Leland raise concerns for fishermen and fishery managers.

Titled “Accelerating Trophic-level Dysfunction in Kelp Forest Ecosystems of the Western North Atlantic,” the paper was published in the journal Ecosystems.

Steneck compares the state of Maine’s fisheries to an investor who put his portfolio into a single stock, one that might rise and fall like Enron.

“Over 70 percent of what we harvest is one species – lobster,” Steneck said.

He stressed that there would be little to fish if shell disease devastated lobster populations in northern New England as it did in Rhode Island.

Maine Department of Marine Resources lobster biologist Carl Wilson says many fishermen are also concerned with how lobsters had become “the only game in town.”