The establishment of the first marine national monument in the Atlantic Ocean drew mixed reactions Thursday, with environmental groups hailing the new protections that some New England fishermen denounced as a threat to their livelihood.

The designation bans commercial fishing in an expansive ecosystem off Cape Cod in a concerted effort to protect the area from the impact of climate change, President Obama said as he announced the designation at the Our Ocean Conference in Washington, D.C.

“If we’re going to leave our children with oceans like the ones that were left to us then we’re going to have to act,’’ Obama said. “And we’re going to have to act boldly.’’

The area, located about 130 miles south of Cape Cod, includes one-of-a-kind coral, rare fish, sea turtles, and marine mammals, the Obama administration said. The subject of scientific exploration since the 1970s, the swath of ocean — roughly the size of Connecticut — features three underwater canyons deeper than the Grand Canyon and four underwater mountains known as “seamounts,’’ officials said.

Waters surrounding the area are warming at some of the fastest rates in the world, forcing marine life to seek refuge there, said Peter Baker, who leads the Pew Charitable Trusts’ ocean conservation work in New England.

“We’re protecting fragile ecosystems off the coast of New England, including pristine underseas canyons and seamounts,’’ Obama said. “We’re helping make the oceans more resilient to climate change.’’

The designation bans commercial fishing in the area, called the Northeast Canyons and Seamounts Marine National Monument, within 60 days with the exception of lobster and red crab fishing, which can continue for seven years. Government vessels are already in the protected area and will enforce the fishing ban. Recreational fishing will still be permitted.

But fishermen said the area should remain open, asserting that decades of commercial fishing have not damaged the ecosystem. They accused the Obama administration of ignoring their recommendations for compromise measures.

One proposal would have allowed fishing in the area as far down as 450 meters and kept the area open to red crab fishing, said Grant Moore, president of the Atlantic Offshore Lobstermen’s Association. An average of 800,000 pounds of lobster are taken from the monument area every year, he said.

Denny Colbert, who runs Trebloc Seafood in Plymouth with his brother, said he sends two vessels to the area to catch lobster and Jonah crabs.

“It’s unbelievable,’’ he said. “I’m going to have to find another place to go.’’

Bill Palombo, president of Palombo Fishing Corp. in Newport, R.I., said lobster and red crab are plentiful in the area.

“It’s going to be devastating for us,’’ said Palombo.

The designation prevents access to the main source of red crab in New England, said Beth Casoni, executive director of the Massachusetts Lobstermen’s Association. “The red crab industry is primarily fished in these canyons,’’ she said. “I don’t see them going anywhere else. That’s where it is.’’

Despite the industry’s frustration, a legal challenge to Obama’s declaration is unlikely, she said. “You need to have a couple of million dollars just to get started,’’ she said. “We operate on a shoestring budget.’’

In a statement, a spokesman for Governor Charlie Baker said the administration was “deeply disappointed’’ by the designation and that it would “undermine the Commonwealth’s commercial and recreational fishermen.’’

Obama established the protected area using his authority under the Antiquities Act of 1906, a move some critics said circumvented federal law established in the 1970s to regulate fisheries.

But environmentalists said opponents had overstated the importance of the region for fishing. A recent report from the National Resources Defense Council said just five to eight boats fish for lobster in the area, and that the canyons are in “the red crab fishery’s least productive area.’’

“In sum, the New England Coral Canyons and Seamounts area is not an important area for New England commercial fisheries, which were valued at $1.2 billion in revenues in 2012,’’ the report said. “Closure . . . would result in negligible effort displacement, if any.’’

Scott Kraus, senior science adviser at the New England Aquarium’s Anderson Cabot Center for Ocean Life, said protecting the area will ultimately benefit fishermen, he said, by improving the health of surrounding fisheries.

“Those animals don’t care about the boundaries,’’ Kraus said. “They go outside and then they can be caught.’’

The volume of biodiversity in the monument region, he said, is a sight to behold.

“It’s like the Serengeti of the ocean,’’ Kraus said.

Laura Crimaldi can be reached at laura.crimaldi@globe.com.