(CN) --- North Atlantic right whales are shrinking, both in size and number.
While the population decline is better documented, right whales in the North Atlantic are also significantly shorter today than those born as recently as forty years ago, according to a study published Thursday in the journal Current Biology.
“On average, a whale born today is expected to reach a total length about a meter shorter than a whale born in 1980,” explained Joshua Stewart, of the National Oceanic and Atmospheric Administration in La Jolla, California, in a statement.
The reduction represents an average length decline of 7.3%. But scientists say some young whales are several meters shorter than expected.
“Major impacts on life history like this have been documented in heavily exploited commercial species, especially fishes, but to our knowledge this is the first time these kinds of impacts are being recorded in a large mammal,” Stewart added.
Using aerial photogrammetry measurements from aircraft and drones over a 20-year period, Stewart and his colleagues from NOAA, the New England Aquarium, Oregon State University and Woods Hole Oceanographic Institution documented changes in the body lengths of right whales, also known as black whales. The new research utilized drone technology to extend the time series in recent years, allowing scientists to examine right whales without interfering with their natural behaviors.
Because they’ve been monitored consistently since the 1980s, North Atlantic whales were an ideal case study, allowing researchers to accurately measure the effects of fishing gear entanglements along with other stressors such as vessel noise, ship strike and shifting prey availability.
Scientists already know that the right whale’s primary food source, small crustaceans called copepods, have fluctuated over the past four decades, affecting reproductive rates and shifting traditional foraging grounds. But the new data, collected from 129 individual whales, alarmed researchers.
While previous studies showed that the increased drag from entangling gear requires right whales to expend excessive energy just to go about their normal activities, the new data suggests even non-lethal entanglements can have lasting impacts on right whales. Stunted growth can reduce reproductive success and increase the likelihood of life-threatening gear entanglements.
“The smaller you are, the less energetic reserves you have, and the harder it might be to survive a serious entanglement or sustained food shortage,” Stewart said.
This means shorter lengths could impact the population’s chances of survival.
“In baleen whales, larger maternal size and body condition are associated with faster calf growth rates and larger calves,” the study concluded. “Decreasing body size may therefore be associated with smaller calves and lower calf survivorship, or potentially delayed first calving and lower reproductive success in females.”
The data is particularly worrisome for North Atlantic right whales, which generally exhibit poor body condition compared to other populations of right whales in the Pacific Ocean and elsewhere. The results leave researchers concerned about how large whales worldwide are being impacted by entanglements.
“Our results suggest that humans are impacting the demographic characteristics of endangered and protected marine mammals through indirect and incidental pressures on vulnerable populations,” the study concluded.
Based on the findings, researchers called for stronger management actions to reduce the impacts of human operations.
“Implementing proven solutions such as reduced vessel speeds, lower breaking strength ropes, and ropeless fishing gear more broadly throughout their range are critical and urgent steps needed to stave off the extinction of this species,” said study co-author Amy Knowlton.
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