“Part of it is the size, but the significance of this is that most of the marine protected area is a no-take area, and that involved 25 countries and complex, long-term environmental negotiations. It is one of the biggest steps for the international community,” Mr. Bloom, a director of the State Department’s Office of Oceans and Polar Affairs, said in a telephone interview from Hobart.

Protecting the Ross Sea, in the Southern Ocean, had been on the commission’s agenda for around six years, and conservationists had been arguing for a no-fishing zone there for a decade, said Andrea Kavanagh, a director of the Southern Oceans Sanctuaries Campaign at the Pew Charitable Trusts in Washington.

Delegates from the United States and New Zealand first proposed the reserve, but the plan could not go forward without the approval of Russia, which was secured on Friday.

Russia had blocked the creation of a reserve, which required unanimous consent, at last year’s meeting. But developments since then — including Moscow’s appointment of a government adviser on the environment and the expansion of a Russian national park in the Arctic to include the world’s northernmost island chain — had raised hopes that this year would be different, Ms. Kavanagh said.

“With Russia, there were signs that things were moving ahead,” she said from Hobart.

Although the commission did not reduce the total tonnage of fish that can be taken from the Ross Sea, vessels have been pushed into areas further out to sea and away from ecologically significant sites that include breeding and feeding grounds for whales, large fish, penguins and other sea birds.