Over the last decade, bottlenose dolphins have become a more frequent sight north of New Jersey, as the lively mammals enjoy the warming shallow waters not far from land in New York Harbor.

A new study from Columbia University and Wildlife Conservation Society reveals that these migratory marine animals are now avid visitors to the Big Apple, frequently entering the human-traversed waters between New York and New Jersey for food and socializing. The annual return of these aquatic mammals to the city’s harbor could signify improvements in coastal water quality and bring a renewed commitment to protecting local wildlife even in urban areas.

“It is kind of the million-dollar question: If there is all this human activity in the New York Harbor, why are the bottlenose dolphins coming here,” said Sarah Trabue, research assistant at the Wildlife Conservation Society and the lead author of the study. “It appears that it could be an important feeding area. The prey species here are of a quality or of a quantity that makes it worthwhile to come into these urbanized areas.”

For two years, Trabue’s team tried to find possible answers to why and how often bottlenose dolphins were in the city’s harbor. They rode out in boats, dropping underwater microphones around 30 feet deep in six different locations, recording in 20-minute cycles every hour, for four months at a time, very close to the ocean floor.

The recordings were retrieved, downloaded and run through PAMGUARD, a software program that detects the sounds that dolphins make when they echolocate. Fast-paced clicks mean that a dolphin is foraging, and it’s about to get its next meal.

“The time between the clicks decreases and at a certain point, when the clicks become so close together, to our ears, it starts to sound like a buzz. That's what we call the foraging buzz,” Trabue said.

From 2018 to 2020, there was a lot of buzz in the water. The most activity was recorded in the central harbor, right at the mouth of the estuary, where the gateway right into New York Harbor spans the area from Brooklyn to New Jersey, Sandy Hook and Breezy Point.

During peak season, late summer and fall, these microphones picked up rapid clicks almost every day. (These highly social carnivores spend their winters off the coast of North Carolina.) Experts and study authors believe the reason may be an improving habitat resulting from stricter environmental regulations such as the Clean Water Act

“We're changing a lot of perceptions about what people think about the New York, New Jersey Harbor estuary as a thriving environment for a top predator, like a bottlenose dolphin,” said Howard Rosenbaum, senior study author and director of the Ocean Giants program at the Wildlife Conservation Society.

The scientists recorded the least amount of buzz foraging underneath the Verrazano Bridge, on the Brooklyn side. Dolphins were detected there only three days over the course of the two-year data collection period. Trabue said this absence could be the result of the area’s high sea vessel traffic or more polluted waters — or it may also be that boat traffic is too loud and masking the dolphin sounds.

“If food presents itself where they are, they will feed on that. That's their reality,” said Arthur Kopelman, co-founder of the Coastal Research and Education Society of Long Island, who was not involved in the study. “Feeding in high traffic zones has risks, obviously, but if food's there, they will be.”

The team also analyzed whether environmental factors such as time of day or chlorophyll levels influenced the feeding behavior. Chlorophyll is found in phytoplankton, the sun-fueled microorganisms at the bottom of the ocean food chain. They are eaten by similarly small zooplankton, which in turn are consumed by the fish, squid, and crustaceans that feed dolphins. Previous studies have indicated that higher levels of chlorophyll correlate to more dolphins.

But Trabue could not conclude whether they preferred morning feedings over evenings, nor exactly how chlorophyll concentrations are affecting them. At very low levels, dolphins were foraging, but at really high concentrations, they increased foraging as well.

“Establishing where the dolphins are and how they’re using the habitat [New York City], can be used to inform policy,” Trabue said. “If there are shifts due to climate change, then that can give us a more accurate idea of when we’re trying to assess where foraging habitat is and where it might be in the future.”

Knowing where these animals are resting, socializing and feeding is valuable for planning how humans will share waterways, according to Dr. Frants Jensen, a research assistant professor in Biology at Syracuse University. It could be used for managing shipping routes and vessel traffic — and for future offshore construction that will take place to build wind turbines.

“I was surprised by how often dolphins were using New York Harbor,” Jensen said. “There's a lot of traffic, so these dolphins have to constantly negotiate these challenges.”

This study is the first of its kind, according to Rosenbaum, who added that there is a lot still to discover about the ecosystem existing along our shores. Other than the increase in visual sightings of dolphins over the last decade, the study’s authors aren’t sure of the magnitude of dolphin population in the metropolitan waters and what the cetaceans are doing there. Now that the study authors have confirmed that New York City is a seasonal home and a desirable food source for bottlenose dolphins, they said that there is an extra duty to conserve and maintain the metro area’s waters as a habitat for both humans and wildlife.

“This area is right off New York City. You see the Verrazano. You see the skyscrapers in the background,” Rosenbaum said. “It’s iconic wildlife and iconic New York City all together.”