Forty kilometres off the entrance to New York Harbor, wedged between two of the busiest shipping lanes in the United States, one buoy has spent years listening to animals that almost nobody has ever seen.
Sei whales are the third-largest baleen whales on the planet, and probably the hardest of them to catch in the act of existing. They swim fast, they favour deep water, and they surface without ceremony. NOAA Fisheries lists the species as endangered and concedes, in the same breath, that the total population in United States waters is unknown.
Difficult to count an animal that will not sit still for a photograph.
What the buoy heard
Sei whales do make one thing easy. They talk.
A study led by Maria Papadopoulos of the Wildlife Conservation Society, published in Scientific Reports, worked through four years of audio from a buoy moored in the Empire Wind lease area. Detection software developed at the Woods Hole Oceanographic Institution did the first pass, and human analysts checked every hit twice. The sound they were hunting is called a downsweep, a low groan that slides from about 82 hertz to 34 hertz in a second and a half, delivered singly or in pairs or triplets. Deep enough that a human ear would read it as pressure rather than pitch.
Of the calls logged between 2017 and 2020, 95 per cent arrived between March and May. Detections climbed sharply in mid-March every year, then thinned out within weeks.
The nine-degree deadline
Temperature explained nearly all of it. Calling fell away once the sea surface reached about 9°C and was scarce below 5°C, which leaves the whales a window measured in weeks rather than seasons. Their preferred food, a fat-rich copepod called Calanus finmarchicus, peaks in these waters from March to May and thrives in a similarly narrow band of cold.
Chlorophyll, the standard satellite stand-in for a productive ocean, was no help at all. Presence and greenness showed no significant relationship, per the paper, which is a mild embarrassment for a variable that habitat models lean on heavily.
Then there is 2018, which misbehaved. Calls ran roughly five times higher than in any other year, peaking in mid-April rather than late March, in a spring when the water stayed under 9°C for about three weeks longer than usual. Papadopoulos and her co-authors offer a weak La Niña as a possible cause, with the caveat that this is one buoy, one type of call, and one paper.
Whales nobody sees
Eyes have almost nothing to show for those four years. Sei whales have never been logged on a vessel-based survey in the New York Bight, and monthly aerial surveys from 2017 to 2020 produced two sightings between them: one lone animal, one group of six. Earlier acoustic monitoring across the wider bight, published in PLOS ONE by a team led by Bobbi Estabrook, had already picked up a matching spring-only pattern at offshore sites.
Then, in late May 2024, a single aerial survey near Hudson Canyon counted 31 sei whales in one day, sharing the water with dozens of North Atlantic right whales. Weeks earlier, a sei whale that turned up dead in the Port of Brooklyn was found on necropsy with a gut packed with recently eaten prey, mostly copepods. Based on digestion rates recorded in other baleen whales, that meal was probably eaten within about ten hours of death.
Something out there was worth stopping for.
Ships, speed and the quiet hours
That Brooklyn whale had not swum in. It arrived draped across the bow of a cruise ship, 44 feet of endangered animal delivered to a working port on camera, as reported by CBS News in May 2024. Median vessel speeds near the study buoy during the peak weeks of whale presence generally ran above the 10 knots recommended seasonally in the area to protect right whales.
The timing data adds an unpleasant wrinkle. Whales were detected significantly more often by day than by night, and one leading explanation is that the copepods rise towards the surface after dark, so the whales stop chatting and start feeding. If that holds, the animals spend their nights closest to the surface and closest to silence at once, invisible to lookouts and inaudible to hydrophones during the hours when a hull is least avoidable.
Right whale detections from these buoys already trigger temporary slow zones in the region. Sei whale calls hit the same hydrophones, get classified in near real time, and trigger nothing whatsoever. Howard Rosenbaum, who runs the Wildlife Conservation Society’s New York Bight Program and co-authored the study, makes the point that round-the-clock listening is exactly what a species this rarely seen requires.
Which means the hard part is finished. The gear is built, moored and running; the whales have been announcing their arrival every March for as long as anyone has bothered to record it. Acting on the announcement is a policy decision, not an engineering one.
About this article
This article is for general information and reflection. It is not professional advice. For your specific situation, consult a qualified professional.