NEW ORLEANS — Oil and gas companies working in the Gulf of Mexico have agreed not to use seismic surveys for the next 2 ½ years in three areas considered critical to whales and along the coast during the peak calving season for bottlenose dolphins.

"The very fact of an agreement on this issue is without precedent. There has not been any settlement made with the oil and gas industry on seismic issues here — or, to my knowledge, anywhere in the world," said Michael Jasny, director of the Natural Resources Defense Council's Marine Mammal Protection Project.

He said the surveys, in which ships slowly tow arrays of air guns through the water, firing them every 10 to 12 seconds for weeks or months, can reduce whales' eating and keep baby dolphins from bonding with their mothers.

The surveys are essential for prospecting for oil and gas offshore. The industry agreed to the measures even though its scientists believe any risk of harm is minor, said Eric Milito, director of upstream and industry operations for the American Petroleum Institute, one of several trade groups that asked to join the federal government in defending against a suit brought by environmental groups seeking to stop the surveys.

"The sound produced during seismic surveys is comparable in magnitude to many naturally occurring and other man-made ocean sound sources, including wind and wave action, rain, lightning strikes, marine life, and shipping," an API statement said. "Four decades of world-wide seismic surveying activity and scientific research on marine mammals have shown no evidence that sound from seismic activities has resulted in injury to any marine mammal species."

Jasny said research by the U.S. Minerals Management Service — now the Bureau of Ocean Energy Management — found that a moderate amount of "airgun noise" reduced sperm whales' feeding and foraging by an averaging of 20 percent.

The airguns' high-intensity, low-frequency sound waves bounce off different geological layers at different speeds, letting companies know where to drill. The government uses the data to know where it's safe to drill and to decide how much to charge for leasing offshore sea bottoms.

Jasny said thousands of such surveys are done each year in the Gulf of Mexico.

"Whales, dolphins, and other ocean species depend on sound to feed, mate, navigate, maintain social bonds, and undertake other activities essential to their survival," his group said in a news release. "Airgun noise is loud enough to mask whale calls over thousands of miles, destroying their capacity to communicate and breed. It has been shown to drive whales to go silent, abandon their habitat and cease foraging, again over larger areas of ocean; closer in, it can cause hearing loss, injury, and potentially death."

The 30-month period will give the government time for environmental studies and give the industry time for research into alternatives, both required as part of the agreement, said Jasny. His group joined the Natural Resources Defense Council, the Gulf Restoration Network, the Center for Biological Diversity, and the Sierra Club in suing BOEM to stop seismic surveys after the oil spill.

API, the International Association of Geophysical Contractors, the Independent Petroleum Association of America, and the U.S. Oil & Gas Association joined the federal government in the suit.

The agreement forbids any seismic surveys in the Mississippi and DeSoto canyons and an area west of the Florida Keys and Tortugas, which Jasny described as important for sperm whales and Bryde's (BROO-duhz) whales.

Between March 1 and April 30, they cannot be used in the near-coastal waters frequented by small groups of bottlenose dolphins.

The agreement also adds manatees to the animals whose presence near a survey boat requires silencing or shutting off the airguns.