Mystic — One hundred and fifty miles east of Cape Cod, a unique undersea landscape of deep canyons and high mountains supports a diverse ecosystem, abundant with colorful corals, fragile sponges, beaked whales, dragonfish and mussels adapted to living in methane hydrate seeps, that is being considered for protection as a National Monument.

Two leading advocates for the designation, which would be given by President Barack Obama under the American Antiquities Act before he leaves office in January, explained why they are lobbying for the designation Tuesday to an audience of both conservation advocates and commercial fishing representatives concerned about losing valuable fishing grounds.

U.S. Sen. Richard Blumenthal, D-Conn., and Peter Auster, retired University of Connecticut marine science professor and currently the senior research scientist at the Mystic Aquarium, made their case for declaring the New England Coral Canyons and Seamounts as a Marine National Monument during a program Tuesday evening at the aquarium.

“This would be the first marine monument in the Atlantic,” said Blumenthal, who is leading the entire Connecticut congressional delegation in advocating for the designation.

“It is a uniquely valuable area that could be valued in terms of mining and oil and gas exploration, but its value as an environmental resource far surpasses those short-term economic interests,” he said.

Blumenthal, saying he is “sensitive to the economic interests of our fishing industry,” noted that the boundaries originally proposed for the area have been changed in response to concerns raised by commercial fishermen.

Under the act, the president will decide on the proposal after the secretary of the interior and the secretary of commerce submit letters recommending either for or against the monument, said Ciaran Clayton, spokeswoman for the National Oceanic and Atmospheric Administration.

NOAA includes the National Marine Fisheries Service and is a division of the commerce department.

The letters have not yet been submitted, and it is unclear when that will occur, she said.

More than 40 environmental groups, including the aquarium, have signed letters urging the president to designate the area, located where the North Atlantic Continental Shelf meets the deep ocean.

But commercial fishing groups say the designation would cut off their access to productive areas for red crab, swordfish, tuna and offshore lobster harvests, among other species.

“Those areas have been used for hundreds of years,” said Joe Gilbert, owner of Empire Fisheries, which has operations in southeastern Connecticut and elsewhere along Long Island Sound.

He and other fishing representatives argued that if Obama uses the executive authority afforded him in the Antiquities Act to designate the area a monument, the federal and regional fisheries regulatory processes that require public input would be circumvented.

“We feel disenfranchised at this point,” Gilbert said.

Eric Reid of North Kingstown, R.I., who represents commercial fishing interests on the New England Fishery Management Council, said creating the monument would cause “localized economic damage” to the already stressed fishing industry, and advocated for a compromise being recommended by the Atlantic States Marine Fisheries Commission.

In May, the commission sent a letter to Obama stating that at least $15 million in annual fishing revenues is at stake. It recommended a smaller area be designated a monument, leaving access to key fishing areas intact, and that existing federal fisheries laws could be used to protect the remainder.

While the area in question is not a major fishing ground for most of the small Connecticut fishing fleet, it is used by the four vessels owned by New London Seafood Distributors.

Gary Yerman, owner of the company, said about one-quarter of the whiting caught by his vessels comes from two of the five canyons identified in the proposal.

Whiting and squid are the two main species for his company.

“There are a lot of other fishermen who work those areas,” he said.

In response to the concerns, Auster, who showed photographs of the area he compared to “Dr. Seuss’ garden” from explorations in 2013 and 2014, said that while he understands the fishermen’s concerns, he believes the impacts will be much more limited.

Auster said he has been studying the area for 30 years using remotely operated undersea vehicles, and that new species of corals and other animals still are being discovered there.

It is also very sensitive to human disturbance, he added.

“It’s 2 percent of the ocean” in the North Atlantic, he said. “Will there be some displacement? I’m not saying there won’t be. But the total value of the landings won’t decline.”

Blumenthal said conserving the area will help maintain sustainable fisheries.

“There is nothing inconsistent about any of those activities that produce jobs and sustain economic activity,” he said. “The idea of conservation is in many respects mutually supportive, because the fishing industry takes advantage of a renewable resource” that depends on protection of the environment.

j.benson@theday.com