Right whales in the North Atlantic have gotten significantly smaller on average since the 1980s, scientists reported Thursday, suggesting that entanglements in fishing gear are taking a toll on the critically endangered species.

The researchers used aerial photography to identify individual whales and track how large they grew. To their dismay, they discovered that right whales that survived serious entanglements reached shorter body lengths than their peers. The team published the findings in Current Biology.

"The fact that something that doesn't kill them can have such a major impact on their fitness or their life history and growth was really shocking to me," said Joshua Stewart, a postdoctoral research associate at the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center and first author of the study. "To see such a large effect is really striking and really worrying." 

Right whales were hunted to near extinction by the late 19th century, as their slow movements and tendency to float after being killed made them prime targets for whalers. Since receiving protection from an international agreement in 1935, right whale populations have recovered more quickly in some places than others. North Atlantic right whales, which can reach lengths of 52 feet, have "fared comparatively poorly," Stewart said.

"They were recovering for a while, but much more slowly," he said. "And over the past 10 years or so, that trend has kind of reversed, and there's been this alarming decline in abundance."

Deaths from increased entanglements and ship strikes are thought to be the primary drivers of the decline, Stewart and his colleagues noted in their paper.

His team first realized that something about the animals' growth was amiss when several of the researchers captured drone images of a number of right whales they assumed were calves. When the scientists later identified the whales using distinctive markings on their heads, they realized that the animals were actually five to 10 years old.

"They turned out to be these really tiny whales, which were the impetus for looking more closely at the length to see if those were anomalies," Stewart said. "Was this some rare extreme case, or was this part of a more general trend?"

Scientists have monitored North Atlantic right whales intensively over the past four decades, creating a wealth of information about nearly every member of the species, including its age, number of offspring and how many times it has been caught in fishing gear. Stewart and his colleagues drew upon this information and examined body size measurements collected by crewed aircraft in the early 2000s and by drones between 2016 and 2019. They collected measurements of 129 individual whales between the ages of less than one year to 37 years old, with birth dates ranging from 1981 to 2019. 

The researchers discovered that the young whales described in the more recent set of measurements were smaller than whales that had been measured at the same age two decades earlier. The team calculated that a whale born in 2019 will grow to a maximum length about one meter (3.3 feet) shorter than a whale born in 1981, indicating a 7.3% decline.

"Some of the photos that we have of a 10-year-old whale that's smaller than a calf are honestly mind boggling," Stewart said. "Some of them are 10 feet shorter than you would expect them to be for their age, and that's just a humongous impact."

To investigate what might be stunting the whales' growth, the researchers consulted entanglement records. Not only were whales with a history of entanglements more petite, but calves whose mothers had become entangled while nursing also wound up smaller than their peers.

The findings suggest that stressful events, such as becoming caught in fishing gear, can have lasting impacts even on whales that survive the encounter. But even more may still be uncovered, Stewart and his colleagues will next investigate whether smaller right whales have shorter lifespans or produce fewer calves.

"The physical drag of having to lug around a bunch of trailing fishing gear might be diverting enough energy ... to prevent them from reaching a normal size," Stewart said.

Still, Stewart said, these observations don't fully explain the trend toward reduced body length. It's possible some whales became snared in fishing gear and freed themselves before scientists noticed, leading them to underestimate the impact of entanglements. Other explanations are that increased vessel traffic or climate change are disrupting the young whales' foraging.

"The Gulf of Maine is a really rapidly changing part of the world; it's warming pretty quickly," Stewart said. "That might be affecting their food source."

And climate change may also be a factor in increasing entanglements. One likely reason why entanglements are on the rise is that, as waters warm, whales are moving into new areas where they haven't traditionally been spotted.  

"In some of those places, there aren't as stringent measures for reducing entanglements and things like that," Stewart said. "It's challenging to predict where they're going to show up and how to prevent those entanglements."

Entanglements of whales and other marine mammals are also rising worldwide as vessel traffic increases. So, the trend of decreasing body size that Stewart and his team reported may not be limited to North Atlantic right whales, he says.

"It's kind of a warning or a wakeup call that these kinds of impacts are probably pretty widespread," Stewart said. "We should be thinking about them and acknowledging them as these sublethal stressors that are still probably really important for population health in these other species."

The study, "Decreasing body lengths in North Atlantic right whales," published June 3 in Current Biology, was authored by Joshua D. Stewart, Morgan S. Lynn, Jacob Barbaro and Wayne L. Perryman, National Oceanic and Atmospheric Administration; John W. Durban, National Oceanic and Atmospheric Administration and Southall Environmental Associates, Inc; Amy R. Knowlton, New England Aquarium; Holly Fearnbach, SeaLife Response, Rehabilitation and Research; and Carolyn A. Miller and Michael J. Moore, Woods Hole Oceanographic Institution.