"Mothers of this population tend to go to the southeastern United States to give birth," Kraus said. "They basically run the gauntlet of all the shipping channels that come out of the East Coast of the United States."
Last year 28 North Atlantic right whale calves were born, up from an average of 23 calves for each of the last five years.
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The death rate, however, may be significantly higher, though it can be tough to track.
The detection rate for mortality is only 17 percent, meaning most dead right whales are never found. "When you see 8 animals dead on the beach, does that mean as many as 47 animals actually died last year?" Kraus said.
Entanglement deaths are particularly difficult to identify, Kraus says, because whales entangled in fishing gear tend to stop eating and lose a lot of weight, which makes their corpses less buoyant.
"When they die they tend to sink, so we don't see many [whales killed by] entanglements on the beach," he said. "The ship kills are usually fat and healthy animals, so they float and we end up with them on the beach."
Some whale-reporting systems have been implemented in an effort to reduce the number of ship strikes. For example, when large ships enter two key right whale habitats off the U.S. East Coast, they are required to report to a shore-based marine station for information about recent whale sightings.
In the Bay of Fundy the International Maritime Association moved an outbound shipping lane that overlapped with right whale distribution, reducing by 95 percent the probability of a ship encountering a right whale.
The National Marine Fisheries Service (also known as NOAA Fisheries) recently updated its formal plan to promote the recovery of the right whale through a series of management and research efforts.
In May the service asked the U.S. Coast Guard to collaborate on an effort to encourage a speed limit of 12 knots (about 14 miles an hour) in areas used by right whales. The Coast Guard rejected the offer in June, saying that, among other things, limits could hamper international trade and military rescue operations.
"The Coast Guard is concerned from an international law and policy standpoint with the imposition of new restrictions on vessels engaged in international navigation, such as speed and routing restrictions," Lt. Cmdr. Jeff Carter, a spokesperson for the Coast Guard in Washington, D.C., told National Geographic News this week.
"The imposition of such restrictions must account for the potential for other nations to impose operational restrictions for other purposes on U.S. vessels, citing U.S. restrictions as precedent," Commander Carter said.
Greg Silber, coordinator of recovery activities for large whale species at NOAA Fisheries in Silver Spring, Maryland, says the dialogue with the Coast Guard will continue over ways to reduce whale mortality rates.
"This doesn't mean that we're not going to keep on trying," Silber said. "I view [the speed limit request] as a relatively small step in a much bigger structure we're putting together. It's not an easy process."
Kraus, the New England Aquarium scientist, says immediate action is necessary.
"We need to turn around the human causes of mortality in this population if it is going to survive," he said.
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