Like the icy ocean depths it inhabits, the Greenland shark is ancient, vast, and hella mysterious. Thanks to a new study from researchers at the Greenland Institute of Natural Resources and the Natural History Museum of Denmark and colleagues, though, we now know one more thing about this gentle arctic giant: the fact that it’s a big stinkin’ liar.
Okay, to be fair, it’s more “whoever named the species”, rather than the sharks themselves – and it was presumably not on purpose. But after careful and painstaking analysis of various Danish, Norwegian, and German museum specimens and international catch and observation databases, the verdict is in: Greenland sharks are probably born closer to Denmark than their national namesake.
“The study breathes new life into the story of the Greenland shark throughout the North Atlantic,” said Peter Rask Møller, an associate professor and marine biologist at the Natural History Museum of Denmark and coauthor of the new paper, in a statement last week. “And, among other things, [it] provides an understanding of how Denmark’s deepest sea area also plays a role in the species’ life history.”
Searching for giants
Greenland sharks (Somniosus microcephalus) may live for upwards of 500 years, but where they begin that epic lifespan so far been a complete mystery. Pupping grounds, where mother sharks deliver and their babies start their lives, are unknown, despite the pups being reasonably sized – not too far off the length of a human newborn, actually – and potentially numbering in the hundreds per litter.
That wouldn’t be a problem, of course, except that – like sharks in general around the world – Greenland sharks are struggling right now: they’re currently listed as vulnerable on the IUCN Red List, thanks mostly to overfishing and “bycatch”.
“The number one threat to sharks and their relatives, by far – so much that there is really not a number two threat – is unsustainable overfishing,” David Shiffman, shark conservation scientist and author of Why Sharks Matter, told IFLScience earlier this year. “Humans are killing more sharks than sharks are having babies.”
“Overfishing is a big problem for lots of species, and the knock-on ecological effects of some harmful fishing practices are especially harmful for a lot of species and ecosystems,” explained Shiffman, who was not involved in the new study. “But sharks in particular are extremely biologically vulnerable to this, and that’s because they have relatively few babies, relatively infrequently, relatively late in life. What that means is their populations just don’t bounce back that fast.”
Protecting the pups that are born, then, is a high priority – but actually doing so, when you have no idea where they’re hanging out, is pretty difficult.
Here’s another problem, though: how the heck do you go about finding them? There’s the whole of the North Atlantic to search – and some of that area can get pretty deep, too, with depths of up to 4,000 meters (13,123 feet) in some places around Greenland – and so far, no real hints on where to start. That is, until the researchers looked a little closer at the historical logs – including tens of thousands of unpublished or unofficial records – and found a handful of clues from the past century-and-a-half to light the way.
Following the clues
It’s not that nobody has been looking for these Greenland pups. “More than sixty years ago, Hansen suggested that pupping grounds in Greenland were located offshore in deep waters beyond the reach of commercial fisheries,” the paper explains.
“This is still a valid hypothesis considering the overall rarity of gravid [pregnant] females and neonates.”
When they say “rare”, they mean it. “In fact, only a single gravid female and two free- swimming neonates are known from the scientific literature,” they note; similarly, “records of small juveniles are scarce […] with one specimen each in Arctic Canada south, mainland Norway, Svalbard, and Faroe Islands, and two specimens from Iceland. A single small juvenile from southwest Greenland has also been identified in a museum collection.”
For those keeping count, that’s 10 altogether. Ten records of either a pregnant female Greenland shark, a newborn pup, or a young juvenile. In 150 years. Even when you widen the search to include older juveniles – the “teenage” sharks, if you like – and those found in unpublished surveys over the years, it still only totals a few dozen pre-adult Greenland sharks over more than a century.
It’s certainly not a lot – but it’s enough to make some deductions. “The female carrying near-term pups was caught in offshore Faroese Islands shelf waters,” the researchers write, so perhaps you’d expect this to be a potential pupping grounds for the animals. The team say no, however: it “is not likely,” they write, “because neither gravid females nor neonates have been reported from this area despite high fishing activity.”
The one place that neonates do seem to be found is around the Reykjanes Ridge, southwest of Iceland. A handful – literally fewer than 10 – have been seen there at depths of up to a kilometer (0.6 miles), and only incidentally, while surveyors were looking for other fish. Similar studies elsewhere, even in areas that are comparable to the Reykjanes Ridge, haven’t found Greenland sharks – meaning wherever these babies are living, they seem to be particular about it.
“Future targeted studies will most likely confirm that the Greenland shark gives birth to its many pups in undisturbed parts of the deep sea near the Mid-Atlantic Ridge in deep waters,” says the study’s lead author Julius Nielsen, a visiting researcher at the Natural History Museum of Denmark and former employee of the Greenland Institute of Natural Resources. “[That’s] where there is little activity from commercial fisheries.”
Teenage kicks
From their pupping grounds, these sharks then head East, to an area known to Scandinavians as the Skagerrak. We know, it sounds like an antagonist from a never-aired Thor movie – but really, it’s just the name for that little hook of sea in between Denmark, Norway, and Sweden.
“We consider the deepest areas of the Skagerrak to be a potentially important feeding ground for ‘teenage Greenland sharks’,” Rask Møller said. “In fact, the study is the first to systematically examine the occurrence of Greenland sharks in the Skagerrak.”
Indeed, about 15 juvenile sharks were reported from that area over the past 150 years – a finding which the paper notes is “remarkable, as this particular life stage was previously only associated with either high Arctic or deep-sea waters of the continental slopes.” Notably, their parents didn’t seem to be with them: “Skagerrak […] is not commonly occupied by adult Greenland sharks,” the paper points out.
This might be a problem. Like all the potential pupping grounds identified in the study, Skagerrak doesn’t seem to fulfill every criterion for a shark nursery – traditionally, they need to be comparatively full of sharks (go figure) who tend to stay for a while and come back year-on-year. Proving those to be the case anywhere, for a shark as elusive as the Greenland, would be a challenge.
But at least now, we have a starting point. “We recommend […] using, for example, satellite tracking and environmental DNA technologies,” the paper concludes, as well as standardizing data collection on Greenland sharks to “provide larger and more homogenous datasets.”
“All combined, this study provides new insights into the life history of the Greenland shark,” the team write, “which will aid the development of targeted conservation measures.”
The study is published in the journal Ecology and Evolution.
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