Entanglements in fishing gear, which have been among the leading causes of death of North Atlantic right whales, are likely also stunting their growth and causing the critically endangered species to reproduce less often, a new study found.
The research raises questions about the findings of a major report on the species released last month by the federal government and could have a significant impact on how it regulates the lobster industry, which has been the primary source of entanglements. The population of right whales has plummeted over the past decade by a quarter, with scientists estimating that fewer than 370 remain.
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The long-term study, published Thursday in the journal Current Biology, estimated that a calf born in recent years was likely, when mature, to be about 3 feet shorter in length than those born in the 1980s. Entanglements have become more of a problem for whales in recent decades, as ropes have increasingly used synthetic materials and become stronger.
“Stunting of growth is a classic sign that a species is in trouble,” said Michael Moore, a senior scientist at the Woods Hole Oceanographic Institution and a coauthor of the study. “Smaller mothers are less capable of being successful breeders, because they have a smaller fuel tank and less lactation. That’s the biggest concern.”
He and other authors said the findings call into question a long-awaited report released last month by the National Marine Fisheries Service that found the lobster industry isn’t jeopardizing the survival of right whales. A federal judge last year ordered the agency to produce the report, known as a biological opinion, after ruling that it had violated the Endangered Species Act by failing to reduce entanglements.
The agency’s controversial “no jeopardy” finding means that lobstermen and other trap-pot fishermen can continue using vertical buoy lines, the ropes that extend from the surface to the seafloor, which are the main source of entanglements. But if the population and calving rates don’t improve, the industry will have to do much more to protect the whales, which could mean major changes for the fishery, such as eliminating ropes from the water column, regulators said.
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Officials in Maine have long raised concerns about the potential impact of right whale protections for the lobster fishery, which contributes an estimated $1.5 billion to the state’s economy.
In a statement after the agency released the biological opinion, Governor Janet Mills said she was relieved regulators didn’t require action that could have effectively shut down the fishery, but she called the government’s conclusions and their potential ramifications “troubling.”
Moore and other authors of the new study urged the federal government to reconsider their conclusions.
“The no-jeopardy decision in the recent biological opinion should be revisited in light of this new evidence that the species has failed to thrive and has a stunted growth rate,” Moore said. “It’s a major concern that the planned governmental management actions will be seriously insufficient.”
Amy Knowlton, a senior scientist in the New England Aquarium’s right whale research program who was also an author of the study, said its results suggest regulators in the United States and Canada must do much more to reduce the threat of entanglements. In recent years, as the Gulf of Maine has warmed at an accelerating rate, the whales have moved further north into Canadian waters in search of the rice-sized copepods they feed on.
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“I’m very concerned,” she said. “Things have continued to get worse and worse for right whales.”
The study, which measured 129 whales over 20 years, relied on high-resolution aerial photos from airplanes and drones to track their size.
While the study found that entanglements were the main cause of the declining size of the whales, which can grow to 60 feet long and weigh more than 250,000 pounds, it also attributed some of the stunted growth to other factors, such as the increasing noise from vessel traffic and how climate change is shifting the distribution of their food sources.
Scientists not connected to the study said it underscores their concerns about the “sub-lethal” threats to the species, such as dragging fishing gear over long distances. The heavy gear consumes a significant amount of energy, reduces the fat reserves they need to reproduce, and makes them more susceptible to a range of diseases. It also can inflict deep wounds and drown them.
A previous study of 70 right whale mortalities between 2003 and 2018 found that, when a cause of death could be determined, nearly 60 percent had died from entanglements, mostly as a result of lobster or crab buoy lines. That study also found that entanglement mortalities increased substantially over the last 50 years.
“Sub-lethal impacts are something that needs to be taken just as seriously as direct impacts, such as ship strikes,” said Fredrik Oscar Christiansen, a marine ecologist at the Aarhus Institute of Advanced Studies in Denmark, who called the study’s findings “tragic.” “This paper really shows how bad human disturbance can be on wildlife.”
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Others noted that the number of right whale calves born in recent years has been much lower than in previous years.
For example, between 2001 and 2005, 115 right whales were born; over the past five years, just 39 were born, and at least three of those have already died.
Peter Corkeron, chair of the Kraus Marine Mammal Conservation Program at the New England Aquarium, called the study “stark” and said it “demonstrates conclusively that entanglement is an important contributor to stunting.”
“This is yet another wake-up call on how bad things are for North Atlantic right whales,” he said.
David Abel can be reached at david.abel@globe.com. Follow him @davabel.