The danger to whales and other large marine mammals from oceangoing vessels’ propellers and bows has long been recognized. And efforts are in place to track and curb such ship strikes. But a new study published in Frontiers in Marine Science finds that ships are also hitting large numbers of smaller marine animals—which are suffering severe injuries or dying at higher rates than previously thought.
The researchers sifted through necropsy results, eyewitness reports, and other anecdotal data from around the world and found that ships and smaller craft hit at least 75 species—including dolphins, sharks, sea otters, seals, penguins and sea turtles. Among them are vulnerable species such as the critically endangered Kemp’s ridley sea turtle and the endangered Hector’s dolphin. Younger animals are particularly at risk, because they are more playful and less experienced and might be left alone while a parent forages for food. Species that spend a lot of time sleeping at the surface, such as otters, also face a higher level of hazard. “When we started looking into this, I was quite surprised that all these other species are also being affected,” says Stephanie Plön, senior author of the study and now a cetacean biologist at the Bayworld Center for Research and Education, a South African nonprofit.
Strikes involving smaller species may be missed because crews are less likely to notice them than they would a collision with a massive whale, says Plön, who was at Nelson Mandela University in South Africa when she conducted the research. The bodies of such creatures might also sink or be eaten more quickly than those of larger marine mammals, which sometimes wash onshore, where they can undergo a necropsy. And previous research has found that even strikes with larger animals still remain undercounted.
Frazer McGregor, a marine ecology doctoral student at Murdoch University in Australia and lead scientist at a research collaboration called Project Manta, was not involved with the new study but says it matches his own findings. A paper he published in PLOS ONE last year reported that many injuries to manta rays in Western Australia’s Ningaloo Coast World Heritage Area that had originally been attributed to predators were actually caused by strikes, many likely made by small recreational boats. This reassessment was prompted when one of the area’s large resident female mantas suffered obvious propeller injuries: evenly spaced cuts that were deep and slightly curved.
The researchers had initially thought the scars would remain for life. But the next year they noticed the animal had healed. This result prompted another look at images in their database and a reanalysis of manta scarring and healing. “We worked out that it’s way higher than we had believed, so that’s a worry,” McGregor says. “It means a lot more animals are likely getting hit than we are recording, because they heal quickly, and the next time we see them, they’ve healed up.”
Even though mantas appear to heal well in a short amount of time, he says, such injuries can have long-term negative effects. If one of these animals survives a hit but loses its tail or parts of its wings or reproductive claspers, its competitiveness and continuing survival will be at risk. Plön’s study also notes that a struck animal needs to use energy for “body maintenance”—and that energy would have otherwise been used for foraging, growing and reproduction. McGregor says sightings of the area’s manta ray population have dropped a bit, possibly, at least in part, because of animals dying after being hit by boats.
Other research has illuminated the particular ways ship strikes affect a range of species, including some of those mentioned in Plön’s latest work. A 2019 study in the Journal of Wildlife Management found that from the mid-1980s to the mid-2010s, the number of loggerhead turtles hit by craft off the coast of Florida rose with the number of boats registered in the state. And in the Arctic, Caspian seals were more likely to be hit at night, when ships broke through ice in their breeding grounds, a 2017 study in Biological Conservation found. The seals did not move away from the ships until they were very close, possibly because the vessels’ bright lights may have stunned the animals.
One easy way to reduce strikes is simply to slow down. There is a direct relationship between injuries and craft speed, says Simone Panigada, a ship strike co-coordinator for the International Whaling Commission and president of the marine-conservation nonprofit Tethys Research Institute. For example, ship operators have voluntarily reduced speed in New Zealand’s Hauraki Gulf, which is home to Bryde’s whales. “The ships’ strike rate decreased almost by 100 percent,” Panigada says. He adds that whale-detection apps are also a useful tool to alert captains to slow down if the animals are around or to avoid areas where they congregate.
For now, though—in the absence of clear official policies such as speed limits—the increasing development of ports, shipping, and offshore oil and gas development will likely mean an increase in large boat traffic and thereby boat-related sea-animal injuries and deaths, Plön says. And that effect, she notes, will only add to the myriad pressures on marine animals—including warming ocean waters, pollution and ocean noise—leading to “more and more of these cumulative impacts.”