The bottlenose dolphins arrive in New York Harbor in the spring and frequent the busy waterway through autumn, sometimes feeding with such frenzy that their foraging clicks sound like a buzz on underwater listening devices off Brooklyn, Queens, Staten Island and New Jersey.

“We’ve been seeing dolphins regularly,” said Howard Rosenbaum, director of the Wildlife Conservation Society’s Ocean Giants Program and senior conservation scientist at the New York Aquarium. “I think what’s interesting is that you can see them quite frequently, if you’re on the beach. It’s not all the time, and it might not be the right time of the day when you’re looking, but we do see them regularly.”

A study released this month by the Wildlife Conservation Society, based on passive acoustic monitoring between April and October from 2018 to 2020 in the New York-New Jersey Harbor Estuary, found that the dolphins came to feed in an ecosystem rich with more than 200 species of fish, most prevalently in late summer and early autumn. 

The estuary is located where the Hudson, Hackensack, Passai and Raritan rivers meet the ocean in the nation’s most densely populated metropolitan region.

The dolphin “belong to the Western North Atlantic northern migratory coastal stock,” said Sarah Trabue, a research assistant at the Wildlife Conservation Society and the study’s lead author.

Her study team included Rosenbaum and three other conservation society scientists as well as a researcher from the University of California, Santa Cruz.  

A grant for the study from the Hudson River Foundation, Trabue said, was originally made to gather and to study baseline data on larger whales that had been seen in the estuary, such as humpbacks. 

The Wildlife Conservation Society had been collecting data on the whales using the passive acoustic monitoring but was also picking up dolphin detections on these listening devices, Trabue said. Given that, the conservation society decided a study focused on the dolphins was warranted, she said. 

“It’s exciting because it’s establishing some baseline data for this population of dolphins that we really know very little about,” said Trabue. “In fact, a lot of New Yorkers don’t know there are dolphins here, right off the coast of New York.”

While the society had seen dolphins the estuary for years, the research team noticed an increase in dolphins, which seemed to be lingering for longer periods of time. 

What would bring dolphins to New York Harbor? Trabue said that is the million dollar question right now. She finds it interesting that they are coming here, especially since there is a lot of activity on and near the river, including shipping and coastal development. 

“I think people in general don’t associate the waters off New York with being diverse, ecological, ecologically rich, important areas for species like marine mammals,” said Rosenbaum. “Over the last decade or so, we’re seeing some pretty important changes here in terms of certain species, like dolphins and whales, increasingly using these habitats as a suitable prey base for these animals.” 

Trabue has theories on prey, a source of food for the dolphins, as well. The team did find a space and time match between an abundance of Atlantic menhaden, a species of prey, with a large number of dolphins. 

“The times the Atlantic menhaden are at their highest abundance, we’re also seeing dolphins at their highest abundance, but that’s very speculative,” said Trabue.

Estuaries are among the most productive and diverse natural systems on earth, and the abundance of species in the New York-New Jersey Harbor estuary is a good example of such biodiversity. It teems with more than 200 different species of fish and more than 300 bird species, according to the NY/NJ Harbor Baykeeper.

Trabue and her team found that the dolphins were feeding the majority of the times they were present in the harbor, which suggests that the estuary is an important feeding ground for them. 

In completing the study, the Wildlife Conservation Society determined when the dolphins were present, which was important in establishing baseline data not only on when they were in the harbor, but what kind of environmental variables were associated with their presence and when they’re feeding. 

Rosenbaum said this was important to establish  as the development of offshore wind turbines, a major new source of renewable energy, is coming to the waters off New York. 

“This is so important for our planet, for our society, for climate targets that the state and the federal government have set to deliver energy to all the millions of people and homes that are in this urbanized area,” said Rosenbaum.

But the Wildlife Conservation Society, he said, wants this development done in the most environmentally responsible and sensitive ways, especially when the well-being of these iconic, protected and critically endangered whales and dolphins could be affected. 

Although Trabue and her team did not analyze how dolphins behave in an urban environment, she was able to draw certain conclusions: the dolphin’s foraging clicks reduced dramatically, for example, at times of heavy shipping activity in the harbor. 

“We got way, way fewer dolphin detections,” said Trabue. “While we can’t say because of shipping there are less dolphins, because we didn’t analyze that. We can say, in this area where there is a lot of shipping activity, we did have less dolphin detections.” 

The Wildlife Conservation Society could conduct this study because dolphins echolocate sound and then use the echo of that sound and the timing of that echo to get a kind of image of their surroundings and their prey. 

When the dolphins are foraging for fish, the closer they get, the more foraging clicks they emit, which tells them they are getting closer to their prey.

“Essentially, when they’re approaching a prey item, you can imagine that they want to get a better, higher resolution image of their prey items,” said Trabue. “So as that dolphin is closing in, they’re emitting clicks faster and faster, with less time in between the clicks so they can hear the echoes, and kind of hone in on where that prey item is in space and time so they can grab it and consume it.”

Trabue and her team collected the time between the clicks as data. When the dolphins got super close together, their clicks started to sound like a buzz, she said. So Trabue and her team looked at foraging buzzes and the time between clicks that were picked up on the microphones.

With these baselines, Rosenbaum said, the Wildlife Conservation Society can work with various stakeholders to recommend best practices and mitigation measures that will help reduce risk or harm to bottlenose dolphins. 

“Now that we have this information, we need to work as a conservation science organization with all of the various industries and the various regulatory agencies to make sure that we use the information effectively to best protect these animals,” said Rosenbaum.