As female North Atlantic right whales decline in size, they’re producing fewer calves, which could have grave consequences for the critically endangered species, according to a new study.
Scientists found in an earlier study that right whales have been shrinking, a phenomenon they attributed to frequent entanglements in fishing gear. The previous study found that entanglements place greater stress on the whales by forcing them to drag heavy fishing gear over long distances, which consumes their energy, reduces the fat reserves they need to reproduce, and makes them more susceptible to a range of diseases.
Some 85 percent of right whales have been entangled at least once, and of those, a majority have been entangled multiple times, scientists say. Entanglements and vessel strikes have been the leading cause of death and serious injuries to the whales, whose population has plummeted by about 30 percent over the past decade.
In the latest study, published on Thursday by the journal Marine Ecology Progress Series, scientists determined that the declining body length and girth of the whales have likely resulted in their low birth rates in recent years.
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“Smaller females appear to have less capacity to raise calves as frequently as larger whales,” said Joshua Stewart, a research biologist with NOAA Fisheries’ Southwest Fisheries Science Center, one of the authors of the study. “Their smaller size means they may take longer to recover from the energetic cost of giving birth, especially in light of other stresses on the population.”
The scientists said their research reflects the need for further protection of the whales, whose overall population has declined to fewer than 350.
“With this study, [we] have gained further insights into how these stressors are affecting their reproduction,” said Amy Knowlton, a senior scientist at the New England Aquarium, who was also an author. “The remedies to address these threats are clear: shifting how humans operate in the ocean so that they do not inadvertently harm whales.”
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Right whales can grow to 60 feet long and weigh more than 250,000 pounds. Previous studies have 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.
The link between entanglements and calving rates underscored the need for fishermen to use weaker ropes and for federal officials to prod the lobster industry to adopt ropeless fishing gear, she said. The whales are believed to be entangled mainly in the vertical buoy ropes of the lobster and crab fisheries, which use heavy ropes that stretch from their traps on the seafloor to buoys on the surface.
Scientists based their conclusions on the aerial photos of 41 female right whales from 2000 to 2019, allowing them to compare their sizes with their reproductive histories. The relationship showed that smaller whales produced fewer offspring per reproductive year.
The study also found that larger female right whales also appeared to have more calves over the course of their reproductive years.
The scientists noted that other factors, which are more difficult to observe, could also influence the reproduction rates of the whales, including the availability of food, impacts of climate change, and the overall health of each whale.
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“Doing everything we can to relieve pressure on the population and help support their recovery and resiliency will become increasingly important in the face of a rapidly changing ocean,” Stewart said.
David Abel can be reached at david.abel@globe.com. Follow him on Twitter @davabel.