The dolphin deaths have some people worried and wondering what could be killing the animals. Might there be something unhealthy in Rhode Island waters?
Scientists don’t think so. Instead, they believe the dolphin mortalities are probably due to natural
|Common dolphin found on Block Island (Kim Gaffett)|
attrition in a large population of dolphins that is typically most active in southern New England waters in fall and winter.
Three of the four dead dolphins were common dolphins, a species that University of Rhode Island oceanographer Robert Kenney, writing in a blog in 2017, described as “the most abundant cetaceans off the Atlantic coast, with perhaps 240,000 or more between Florida and Labrador.” They also occur in tropical and temperate waters elsewhere around the world, and they sometimes aggregate into extremely large herds.
Kim Gaffett, a naturalist with The Nature Conservancy and a board member of the Rhode Island Natural History Survey, was on hand when the aquarium volunteer responded to the Block Island dolphins. She said that common dolphins are regularly observed around the island in winter – and occasionally in summer – with most sightings coming from passengers on the Block Island ferry. She said dead dolphins are observed on the island shoreline about every other year.
Neither of the dead Block Island dolphins had any visible signs of injury, according to Gaffett. Based on photos of the animals Gaffett provided, Kenney believes the animals were relatively old in age.
The aquarium did not conduct a necropsy – an animal autopsy – on any of the recently reported dolphins, so their cause of death is unknown. Animals that appear to have died more than 24 hours previously are usually left to drift back out to sea, said Sarah Callan, assistant manager of the aquarium’s Animal Rescue Program, since the decomposition process would make their tissues too deteriorated to be useful in determining cause of death. Since necropsies require several people working in close proximity, the aquarium is conducting fewer necropsies during the COVID19 pandemic to reduce the risk to its staff and volunteers.
Callan wouldn’t speculate about the cause of death of the dolphins found recently in Rhode Island waters, though she said it could be from any number of factors, including disease, respiratory infections, vessel strikes, fishing gear entanglements or various natural causes. She also noted that it isn’t uncommon for as many as 10 dolphins to strand in local waters in a typical year.
“Every year is different,” she said. “Often when animals die, whether from natural causes or something else, where they wash up depends a lot on the weather and currents. It could be a fluke of the currents that pushed those two dolphins to Block Island. Animals that died on Cape Cod can even end up here. There are so many factors involved. It doesn’t necessarily indicate something has happened off our shoreline.”
Data from an April 2020 assessment of common dolphins in the western North Atlantic by the National Oceanic and Atmospheric Administration estimated that 419 common dolphins are killed as fisheries bycatch each year. The same report indicated that 28 common dolphins were found stranded on Rhode Island beaches between 2013 and 2017 and 359 on Massachusetts beaches during the same period, including 166 in 2017 alone.
Kenney said that when individual dolphins are found dead, it is typically because the animal was sick or injured. And while there are occasionally spikes in mortality due to disease, which the federal government labels an “unusual mortality event,” no such event has been declared for common dolphins anywhere on the East Coast in recent years.
Kenney isn’t concerned about the health of the common dolphin population in southern New England, despite the number of animals found dead this winter.
“If a marine mammal population is stable, an equal number of animals should be expected to die and be born every year,” he said. “Given that the current estimate for common dolphin abundance in the regional population is 172,825, if natural mortality is only one or two percent a year, there should be 2,000 to 3,000 dead ones every year.”
In addition to common dolphins, Callan said that mid-winter is also a common time for gray seal pups to be found washed up on area beaches, both dead and alive, some of which may have originated as far away as Canada or Greenland. Anyone who finds a stranded marine mammal should call the Mystic Aquarium hotline at 860-572-5955, extension 107.
This article first appeared on EcoRI.org on February 4, 2021.