The bodies kept washing ashore—dozens of mutilated harbor porpoises stranded on the Dutch beaches every year, their bloody remains discovered by screaming vacationers. Now, after 10 years of crime scene investigation–style work—complete with autopsies and DNA testing—biologists and veterinary pathologists have finally cracked the identity of the murderers: big-eyed, chubby-faced gray seals. The finding could lead conservationists to rethink gray seal rehabilitation, and it even raises the specter of a new “great white” in the North Sea.
When harbor porpoises with missing bellies first appeared on Dutch coastlines in 2006, local biologists thought someone was deliberately hurting the animals. But the numbers soon rose to dozens a year—impossible to attribute to a single person. So the scientists looked elsewhere: Perhaps it was ducted propellers that sucked the porpoises in? Or fishermen cutting up unintentionally trapped porpoises?
Then in 2012, a group of Belgian researchers noticed that some of the wounds on dead porpoises found on Belgium beaches bore the canine teeth marks of gray seals. “We thought, ‘Of course, how silly,’ ” says biologist Mardik Leopold of the Wageningen University and Research Centre in the Netherlands. “You think seals are nice, cuddly animals—they are not. They are predators.”
With a towering height of 2.5 meters and the weight of two linebackers, gray seals are the largest predators in the southern North Sea. Though they are known as fish hunters, an adult male seal could easily overpower a 30-kilogram juvenile harbor porpoise.
So the researchers examined photographs and autopsy results of more than 1000 stranded harbor porpoises collected from 2003 to 2013. The wounds further implicated gray seals. There was the torn blubber, the fatty nourishment that seals seek; the rows of canine teeth imprints on the tailstock, the thin part connecting the body and the tail; and the telltale scratch marks—four parallel lines left by seal claws grabbing onto the porpoises. Analysis indicates that close to a fifth of the stranded porpoises had a lethal encounter with gray seals—mostly naive juveniles who probably saw seals for the first time in their lives as they migrated north in the spring, Leopold says.
But finding the smoking gun proved to be a challenge. Short of analyzing the stomach contents of living seals, the only way to ascertain the identity of the predator was to find saliva DNA in the inflicted wounds: a seemingly impossible task, as seawater should quickly wash away any traces of DNA. Even human forensics rarely employs such analysis on drowned corpses, Leopold says. “Everyone thought we were crazy in even trying.”
Indeed, nothing turned up from the obvious tear wounds. But at the bottom of deep, narrow bite marks on three porpoise bodies—where the flesh veered back after the seals pulled out their teeth and formed sealed pockets—the biologists found the iron-clad DNA evidence, they report online today in the Proceedings of the Royal Society B.
That solves the “who” question, but it still doesn’t answer the “why”: What caused the gray seals to suddenly eye harbor porpoises for dinner in 2006? Humans may be to blame, the researchers suggest. As gas prices went up in recent years, Dutch fishermen switched from trolling to using cheaper set nets anchored to the seabed, which trapped harbor porpoises as by-catch. The team speculates that the gray seal, known to steal fish from the set nets, may have stumbled on a much larger, fattier “fish” and went on to actively hunt porpoises.
The situation poses a dilemma for conservationists, as both the gray seals and the harbor porpoises are protected species. The Netherlands currently operates three rehabilitation centers for seals: The marine mammals disappeared from the region in the Middle Ages due to excessive hunting and only reemerged in the 1980s. But conservationists may need to reconsider the strategy, says biologist Jan Haelters of the Royal Belgian Institute of Natural Sciences in Ostend, who was not involved with the study. “In Africa, if you take care of all the lions and release them to the wild, it would affect the natural balance.” Still, he says, the study “gives us a very good framework for coordinated, coherent monitoring of harbor porpoises.”
The seal predation could have long-term impacts on the local porpoise population, Leopold says. Researchers studying bottlenose dolphin attacks on porpoises in Scotland have found that the porpoises adapt by becoming leaner, faster swimmers. But speed comes at a cost. Compared with larger marine mammals such as whales, porpoises have more surface area relative to their body volume, which means they need to feed every hour to compensate for heat loss through the skin. Without food, a porpoise would die of starvation after 3 days, whereas a whale can fast for a month. “They are between a rock and a hard place really,” Leopold says.
That seals would hunt down such large prey hints at a bigger problem, Leopold and Haelters warn. The Dutch shores are frequented by human bathers and surfers, raising the specter of a new “great white” terrorizing the North Sea beaches. “Statistically,” Haelters says, “an accident will happen.”