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Right whales growing smaller, producing fewer calves

Samantha Chang/©Woods Hole Oceanographic Institution
The declining body size of North Atlantic right whales may have critical consequences for the future of the species because smaller females produce fewer calves, new research shows

Last year scientists found that North Atlantic right whales are declining in size— and now, they say, females are producing fewer calves. This link between declining body size and calving rates, revealed in new research, could have grave consequences for the critically endangered species.

By comparing the lengths of 41 females North Atlantic right whales with their reproductive histories, scientists found that smaller females are now having fewer calves because of their lower fat stores and energy reserves.

“So when a female right whale has calf, they nurse that calf for about a year. And that takes a tremendous toll on their energetic reserves,” said Amy Knowlton, senior scientist at the New England Aquarium and paper contributor. “If you're a smaller whale and you're nursing a calf for the same amount of time as a larger whale, you you've probably lost more of your body fat to that nursing year.  And so it's going to take you longer to recover from that calving event than a larger whale would need.”

The finding builds on a 2021 paper that found the whales are declining in size due to stress from entanglements in rope and fishing gear and collisions with boats. A whale born today is expected to be about one meter shorter than a whale born in 1980.

To be sure, Knowton added, the contributing scientists from Oregon State University, Woods Hole Oceanographic Institution, and SR3, found that prey availability, climate impacts, and maternal health could are also influencing reproduction rates.

“When you couple food changes with warming oceans with the human activities that have certainly increased over the past decade or two,” she said, “it's that combined effect that is really reducing the ability of this population to recover.”

Experts are calling for mandatory boat slow zones and fishing measures that would reduce the number of lines in the water to save the whales from extinction. Today, scientists estimate, just 336 are alive.

“Now knowing that the human impacts are having an influence on reproduction,” Knowlton said, “it just strengthens the need to really address this threat as quickly as possible.”

Eve Zuckoff covers the environment and human impacts of climate change for CAI.