In the last four decades, North Atlantic right whale body lengths have shrunk by about seven percent—and fishing gear entanglements and other anthropogenic activities may be to blame for the steady decline, reports Oliver Milman for the Guardian.
A full-grown whale born today would be three feet shorter on average than whales born in 1980, according to a study published this month in Current Biology.
North Atlantic right whales were almost hunted to extinction in the 1890s by commercial whalers. The critically endangered species has less than 400 individuals remaining. While whaling is no longer threatening the species, other human activities are. Ship strikes and fishing gear entanglements are both leading causes of whale deaths, per the Guardian.
Researchers used high-resolution aerial photos of 129 right whales taken from airplanes and drones to track how the whales’ body size has changed over the years. The images were then compared to photos taken of right whales similar in age during previous decades, reports the Seth Borenstein for the Associated Press.
Scientists can easily distinguish the whales from one another by the unique pattern of calluses on their heads, the Guardian reports. The researchers then paired the length measurements with each whale’s birth year. Then they investigated whether the whale and its mother had a history of entanglement in fishing gear, reports Karina Shah for New Scientist.
When whales migrate along the East Coast from Florida to Canada, they have to navigate a perilous network of fishing lines and nets, reports NPR’s Eve Zuckoff. Over 85 percent of right whales have been caught in fishing gear at least once in their lives.
Whales that survive entanglement suffer injuries and infections that can lead to slower growth rates and smaller offspring. When trapped, the nets can cut into the whale’s body and cause serious injuries. Whales that shed the fishing gear still suffer from the aftermath of the stressful event. The time spent in the nets severely weakens whales, prevents them from eating, and forces them to use their energy to fight for survival, NPR reports. Surviving the stressful event leaves the whale without stored energy to gain weight, grow in length, and reproduce.
“If you are dragging around fishing gear you have less energy for growth, it’s a pretty clear mechanism,” study author Joshua Stewart, a NOAA marine researcher, tells the Guardian. “If I strapped a sandbag on to you and asked you to walk around a lot, you’d get skinny pretty quickly. For whales, this also means they may also produce smaller calves that have lower survival probabilities. We are seeing a long-term decline in their size.”
Tinier whales threaten the species’ survival because smaller whales do not have as many offspring. Nursing mothers who entangle themselves in nets also produce smaller calves, NPR reports.
Researchers suspect that other factors, like lack of food and boat strikes, may also be contributing to the whales shrinking length, reports Dharna Noor for Gizmodo. Right whales are baleen whales that feed on tiny crustaceans, including zooplankton and krill. However, ocean warming and acidification lower the number of resources available. The decline in sustenance hinders the mammals further because they have to change their migration routes and population distribution to look for food. As their range expands, they are more likely to encounter boats.
“Rapidly changing ocean conditions as a result of climate change is affecting their prey availability, which could be another contributor,” says Stewart to New Scientist. “They also get lots of vessel traffic which disrupts them on their feeding grounds where they can even get hit by boats.”
The team is now investigating whether other whale species are also shrinking.