Endangered sei whales are regularly using the waters of the New York Bight each spring, an exciting new study has found, carrying significant implications for the management of one of the busiest and most industrially active stretches of coastline in the United States.

Led by scientists from the Wildlife Conservation Society (WCS) and Woods Hole Oceanographic Institution (WHOI), the research used acoustic monitoring technology to track the presence and vocal behaviour of sei whales in the waters off New York and New Jersey between 2017 and 2020.

What the data revealed was not the occasional, incidental passage of a wandering individual, but a consistent, structured seasonal pattern – the hallmark of a species that knows exactly where it is going, and when.

“This is the first detailed analysis of daily and seasonal acoustic patterns for sei whales in the New York Bight,” said Maria Papadopoulos, lead author of the study with WCS and graduate researcher at Columbia University. “We found that 95% of sei whale vocalisations were detected between March and May, with activity peaking in the late spring, when temperatures are between 41° and 48° Fahrenheit. This highlights a narrow environmental window when the whales are most likely to be present, potentially linked to prey availability.”

The findings are published under the title Seasonal and Diel Acoustic Activity of Sei Whales (Balaenoptera borealis) in the New York Bight.

The third largest whale species on the planet after blue and fin whales, adult sei whales average 47 feet in length and weigh around 44,000 pounds – roughly the equivalent of four African elephants. They produce loud, low-frequency calls that drop in pitch, typically arriving in pairs or triplets. Yet despite their size and their voices, they remain one of the least-studied large whales in the North Atlantic, listed as Endangered under both U.S. and international law, and rarely encountered at sea.

That elusiveness is what makes the acoustic record so valuable. Rather than waiting for a sei whale to surface within sight of a research vessel, the WHOI-developed monitoring systems listen continuously – and, as it turns out, there is a great deal to hear.

The study also identified a clear daily pattern in sei whale calling behaviour, with vocalisations occurring significantly more often during daylight hours. Researchers believe this reflects reduced calling at night, when the whales are thought to be feeding.

It is a finding with direct conservation relevance: the hours when sei whales are most vocal are also the hours when vessel traffic in the New York Bight is at its most intense – and when underwater noise is most likely to disrupt communication between animals.

The New York Bight has long been understood as a transit zone – a stretch of water that migratory species move through rather than linger in. This study challenges that assumption, at least for sei whales.

“These results suggest that sei whales may not just transit through the New York Bight but may also use these waters more seasonally in certain years,” said Papadopoulos. “Continuing to document trends in sei whale presence will help inform policies and practices to improve protections for this endangered species.”

Crucially, the same acoustic systems used in this research are already detecting sei whales in near-real time – the same infrastructure deployed by WHOI to protect critically endangered North Atlantic right whales. The overlap is significant. A single monitoring network, it turns out, can serve multiple endangered species at once.

For WCS, which has worked in the New York Bight for more than a decade across species including fin, humpback, minke, and North Atlantic right whales, the sei whale findings add an important new dimension to an already complex conservation picture.

“Acoustic monitoring provides us with an amazing opportunity 24- seven, 365 to learn about whales and dolphins in the waters off New York and New Jersey,” said Dr Howard Rosenbaum, co-author and Executive Director of the WCS New York Bight Program.

“That capability is especially important for species like sei whales that are rarely seen but clearly present. Understanding when and where whales occur is essential for effective conservation. This study gives us that knowledge about sei whales – and an opportunity to act on it.”

The New York Bight is not quiet. Instead it encompasses the busiest port complex in the United States, sits beneath active shipping lanes, and is increasingly the focus of offshore wind development. Sei whales, the data now tells us, arrive here in spring, predictably and repeatedly, during the hours of daylight when human activity – and its acoustic footprint – is greatest.

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