What causes dolphins and whales to strand along the coast?

dolphinIt was not a pretty sight for David Mickey and Sue Dayton the fall day they came upon a large dark object on the beach. Walking closer, they identified what they had spotted as a dead bottlenose dolphin.

Disturbed, they called the Marine Mammal Stranding coordinator for North Carolina’s central coast, Vicky Thayer. She came to the island and, with Mickey’s assistance, performed a necropsy on the animal.

Mickey and Dayton had just moved to Ocracoke, and this was their introduction to the series of dolphin strandings in mid-Atlantic waters last year.

Over the next couple of months, two more dolphins would be found stranded on the island.

According to Thayer, the number of strandings varies from year to year, but there was an increase of bottlenose dolphin deaths from July 2013 to June 2014 in North Carolina, USA, due in part to a virus known as dolphin moribillivirus. Of the three dolphins that were stranded on Ocracoke Island last winter, one tested negative for morbillivirus and results from the others are not completed.

The deaths were part of the largest number of strandings of bottlenose dolphins and small whales ever recorded on the East Coast, said William McLellan, state stranding coordinator at the University of North Carolina Wilmington. A similar morbillivirus outbreak took place in 1987-‘88, killing up to half the bottlenose dolphin population in the mid-Atlantic during an eight-month period.

Concerned, Mickey and Dayton helped arrange for Keith Rittmaster, the natural science curator at the North Carolina Maritime Museum in Beaufort, to visit Ocracoke and give a presentation on his work with whales and dolphins. Rittmaster has been involved with many strandings, also known as beachings, that have occurred here over the years.

Entanglement in fishing line, nets and anchors; ingestion of trash, including life jackets and milk jugs; injuries from propeller blades on boats and ships; shark attacks and sting ray barbs; and viruses, bacteria and parasites are among the causes of death in marine mammal strandings described by Rittmaster. He accompanied the descriptions with slide depictions of the many beached whales and dolphins he has studied.

Whales, dolphins and porpoises, known collectively by scientists as “cetaceans,” have intrigued humans for centuries, as evidenced by the nearly full house Rittmaster’s presentation attracted. North Carolina’s coastal waters are home to at least 33 species of marine mammals; and, of the 10 “great” whales that exist, eight have been documented offshore here, according to N.C. Aquariums.

There are toothed whales, known by the scientific name odontoceti, of which the sperm whale is the largest. There are also baleen whales, or mysticeti, including humpback, blue, North Atlantic right, fin, sei and minke whales. Baleen whales are so called because instead of having teeth, they have racks of baleen — hanging sheets of fringed keratin that filter and trap small fish, shrimp, and krill. 

Two species of pilot whales, several species of dolphin and one type of porpoise swim off North Carolina’s shores, of which bottlenose dolphins are the most frequently seen. Harbor porpoises and Atlantic spotted dolphins are also abundant.

Two factors play a role in the great diversity of marine mammals off our coast, explained Rittmaster. Warm water of the Gulf Stream, coming from the south, mixes with the colder water of the Labrador Current, streaming from the north, near the N.C. coast. It is also where the inner and outer continental shelves meet, with the same result.

North Carolina’s coastal native people probably took advantage of the beached whales that washed ashore, using their meat and blubber. It was not until the Europeans arrived that the animals were hunted for their blubber that was boiled to extract its oil. This brought several species to the brink of extinction. Oil from the sperm whale was used for lubrication, that from the humpback for lighting and oil from the lower jaw of dolphins for lubrication of fine machinery and watches.

Today cetaceans are protected by the federal Marine Mammal Protection Act and the International Whaling Commission. The most endangered species, the North Atlantic right whale (called so because in former times it was the “right” whale to hunt) appears to be making a slight comeback. This winter 14 right whale calves were born off the Southeast, a number that relieved conservationists who feared there would be fewer.

The National Oceanic and Atmospheric Administration recently proposed expanding the designated critical habitat for the endangered right whales, including a portion of the southern N.C. coast where they have calves and nurse. Overall, the rule would expand their critical habitat to almost 30,000 square nautical miles.

Scientists are worried about the Navy’s sonar testing off the N.C. coast, which they think can adversely affect cetaceans. Thirty-four whales of three different species were stranded and died along the Outer Banks in January 2005 following offshore Navy testing using sonar. While there was no definitive proof that the tests caused the beachings, they were, according to McLellan, “investigated as an acoustic event, and it could not be ruled in or out.” Sonar tests are also, he added, “levels of harassment,” that may contribute to aberrant cetacean behavior.

There are many reason why marine mammals beach themselves, Thayer explained. “People want a simple answer, but it’s not that easy,” she said. “It’s very difficult to determine cause of death and there have not been many cases worldwide that have been definitively attributed to acoustic trauma. Numerous factors can combine to cause marine mammal strandings, although the burden of proof should be on the group causing the potential harm.”

Joel Reynolds, senior attorney for the Natural Resources Defense Council, thinks differently. “There is no question that sonar injures and kills whales and dolphins,” he said in 2008. According to the defense council, sonar can cause bleeding around the brain, ears and other tissues and large bubbles in organs, similar to the bends, which sometimes kills divers who surface too quickly.

The magazine Scientific America writes that sonar sound waves travel hundreds of miles under water and retain intensity of 140 decibels 300 miles from their source. “Evidence shows that whales will swim hundreds of miles, rapidly change their depth, sometimes leading to bleeding from ears and eyes, and beach themselves to get away from the sounds of sonar,” notes an excerpt.

Seismic air guns, which cause similar effects as sonar, are proposed to be used off the N.C. coast, starting this year, to search for potential reservoirs of oil and natural gas. McLellan says that seismic exploration along North Carolina’s coast was conducted last by the National Science Foundation and the Lamont-Doherty Earth Observatory at Columbia University to look at the ocean floor and channels and study continental drift.

Thayer says that efforts are made to insure that cetaceans are beyond a tolerable radius of seismic air gun use, a radius in which air guns are assumed to have the potential of altering cetaceans’ health or natural behavior. But, she explains, marine acoustics and its relation to cetaceans is complex. Beyond having direct physical effects on cetaceans or their behavior, seismic exploration and related activities — boat use, leaks, spills — may have indirect effects by affecting cetaceans’ ability to find and capture food, hear approaching predators and find mates.

 

Full story: The Outer Banks Voice

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