An unusual mortality event is a specific designation under the Marine Mammal Protection Act, and is defined as “a stranding that is unexpected, involves a significant die-off of any marine mammal population, and demands immediate response.”
Whale or other marine mammal die-offs are often poorly understood by scientists, and this case is no exception. Officials from NOAA Fisheries could not explain why the animals were coming into greater contact with vessels, or if there were any human-caused or climate-related disturbances that had changed their behavior.
Gregory Silber, marine resources manager in the agency’s Office of Protected Resources, said that there had not been any increase in ship traffic in the region, and that the whales might be following their prey — they mostly eat krill and small fish — to areas where there could be more shipping.
Ten whales other than those killed by ships have been examined, but officials have not yet determined the cause of death. There is no indication that they were killed by disease.
Humpback whales — which can be as long as 60 feet, weigh as much as 40 tons and can live for 50 years — are found in all of the world’s oceans. There are 14 distinct population segments — groups that follow certain migration and breeding patterns — of humpback whales, some of which are classified as endangered or threatened. The population along the Atlantic coast, which winters in the Caribbean and summers in the North Atlantic or Arctic regions, is not now considered threatened under the Endangered Species Act.
Around the world, there are an estimated 30,000 to 40,000 humpback whales, about a third of its original population. The Atlantic population is around 10,000.
Scientists have suggested that some whale deaths could be a result of marine noise, often a result of military activity, offshore drilling or exploration, which can disorient the animals and send them in the wrong direction, possibly toward beaches where they get stuck instead of into the deeper ocean. Mr. Silber, the NOAA manager, said he was not aware of a connection between ocean noise and these strandings.
A recent study has shown that dolphins, when escaping predators or the source of marine noise, might shoot up from a dive more quickly than they otherwise would, switching from slow, deliberate strokes to faster, longer ones, which can cause them to use double the energy they normally do, and exhaust them.
The last major mass casualty event for marine mammals in this part of the world took place from 2013 to 2015, when a resurgence of the morbillivirus killed thousands of bottlenose dolphins on the Eastern Seaboard.
Among humpback whales, there was an unusual mortality event in 2006, following others in 2005, which involved other large whales, and 2003, which was primarily humpback whales. In each investigation, the cause was undetermined, officials said.
NOAA officials said members of the public looking to help could report stranded or dead floating whales to numbers listed on their website.