It’s a riddle. No one knows for sure why dolphins are being spotted more frequently and for longer periods in and around New York Harbor, the giant estuary where salty ocean tides mix with fresh water from the Hudson River.
“We’ve had a ton of sightings,” said Maxine Montello, an official at the New York Marine Rescue Center. “It’s a glory to see stronger populations but also a worry because there’s increased overlap with humans and shared resources,” she said, particularly during summer months when more tourist and pleasure craft ply the busy waters.
The dolphin revival around metropolitan New York — which has the nation’s most developed coastline — stands in sharp contrast to grim periods of disease and soaring death rates that have periodically plagued East Coast waters. In 2013, droves of dolphin carcasses washed ashore first in New Jersey and then in Virginia, the Carolinas, Georgia and Florida, the mammals’ winter home. Many of the bodies tumbled in the surf, badly deteriorated. The suspected killer was a deadly virus.
Now, like humans flocking to New York despite the bidding wars for apartment rentals, the marine mammals seem to be enjoying the city’s crowded waters again. Possible explanations include improved habitat quality, warmer water because of climate change, and the recovery of menhaden stocks, experts say. Dolphins feast on the schooling fish, eating up to 20 pounds a day.
New Yorkers are spotting dolphins in such places as the East River, which separates Manhattan from Brooklyn and Queens. A pair showed up in the waters off Greenpoint, Brooklyn, last year, eliciting gasps from onlookers and scientists.
To better understand the dolphins’ rebound and threats to their populations, six scientists at the conservation society, including Dr. Rosenbaum, recently studied the behavior and haunts of dolphins in and around New York Harbor. The team focused on bottlenose dolphins — the type famous for wide grins and energetic leaps. Highly intelligent creatures, they live in coastal waters and use sound waves to communicate and hunt food.
Scientists have found that bottlenose dolphins can emit a rapid series of clicks known as feeding buzzes that help them track prey. From 2018 to 2020, the team set up underwater microphones and recorders at six locations off Brooklyn, Queens, Staten Island and New Jersey to listen for the distinctive sounds.
One sensor was put near an artificial structure known as Rockaway Reef, a sighting hot spot some two miles south of Rockaway Beach, Queens. Closer to Manhattan, in the Upper Bay off Brooklyn, another was set up in an area of high shipping traffic. The overall aim was to document when and where the dolphins fed.
The team found the dolphins’ predatory activity to be highest in the Lower Bay off Staten Island, particularly near the entrance to outer New York Harbor and the mouth of the estuary, which stretches for five miles between Sandy Hook, New Jersey, and Breezy Point, Queens. Lower levels were found at the Upper Bay site. The team reported that the hunting peaked from late summer into the fall.
“To better manage potential human-wildlife conflict,” the authors wrote early this month in a marine ecology journal, “more focused research is needed on this understudied population.”
Sarah G. Trabue, who led the study and is a researcher with the Wildlife Conservation Society, said the underwater ears did “a great job of establishing a wealth of information about these animals, but we can see gaps that need to be filled,” especially in looking for possible links between human activity and the foraging patterns, she said.
Dr. Rosenbaum said the population rise “is becoming part of our new normal” and increased the importance of the acoustic research. “It’s a very powerful tool to learn about bottlenose dolphins in our own backyard,” he said, adding that it would help establish “the most rigorous understanding on how to minimize harm.”
Joe Reynolds, a nature writer and wildlife photographer, noted an uptick in dolphin sightings almost five years ago on his blog about New York Harbor. Among the factors driving their return, he saw a rising abundance of sea life, in particular menhaden, as most important. Humans avoid and dolphins favor the oily fish, also known as bunker. Experts credit the fish’s resurgence to improvements in the management of the East Coast fishery.
Mr. Reynolds said pods of bottlenose dolphins were often observed feeding on large schools of bunker in Sandy Hook Bay and Raritan Bay, and the Atlantic Ocean near such Jersey Shore towns as Sea Bright, Monmouth Beach and Long Branch.
“How could we tell the dolphins were after bunker?” he asked. “They were not alone. Crowds of hungry gulls, terns, cormorants and other seabirds also showed up.”
Ms. Montello of the New York Marine Rescue Center said an overlooked factor in recent sightings was the surge of humans seeking to escape the coronavirus pandemic by flocking to New York’s waterways, quays, piers, riverfront parks and fishing venues, becoming accidental observers.
“We’re seeing more animals but also more public awareness,” Ms. Montello said. She added that wildlife experts had growing concerns about accidental run-ins.
“A lot of people tested their ability to drive a boat” during the pandemic, she noted. “It can be scary out there.”
Last year, Ms. Montello said, the rescue center saw an increase in dead dolphins and live strandings, adding that the reasons for the incidents often remained unclear. “It takes more investigation to figure out what’s happening,” she said, including necropsies and other detailed studies of the stricken animals and their watery environment.
“We haven’t seen a tremendous increase this year,” Ms. Montello noted. “But we’re just getting into the season.”