The Greenland shark’s slow swim through deep time — and into possible extinction
We tend to think of vertebrates as living about as long as we do, give or take 50 to 100 years. That notion has been completely trashed by a fish — the Greenland shark.
Everything about the Greenland shark is slow, deep and long. There are some – perhaps many, but they’re hard to count – that were alive before the Dutch colonised the Cape of Good Hope and will go on to outlive all humans presently on earth.
The oldest recorded one, caught and killed, it embarrasses me as a human to say, was more than 400 years old. They’re the longest-living vertebrate animals known to science.
At the birth of that now-dead shark, the English Civil War between the Roundheads and Royalists was raging. While the shark was still a pup, Europe entered the Renaissance and the Mughal Empire was consolidating its conquest of India.
A hundred years later, in 1700, when it was still young, Europe was in the middle of the Enlightenment and, as it was nearing midlife, the Industrial Revolution began. At that point, it had become old enough to breed.
When it was 200 years old and in breeding prime, the Anglo-Boer was taking place. For this Arctic Ocean fish, World War 2 was a short while ago and Mars landings, its yesterday.
About the future of the Greenland shark, there is good news and bad.
The good news is that in September 2022, after years of haggling and calls for more information, the intergovernmental Northwest Atlantic Fisheries Organisation prohibited the retention of Greenland sharks in international waters and called for any caught to be handled with care and returned to the ocean.
The ban didn’t make the front page – it wasn’t a fish anybody cared much about.
The bad news is that fishing is not an exact science. Nets and hooks haul out whatever gets caught by them and Greenland sharks are not exempted from what’s known as bycatch – unwanted and unintentional capture.
There’s another problem.
According to Sonja Fordham, president of Shark Advocates International, these sharks are “good at playing dead. So they’re often assumed dead and may not be treated particularly carefully and it can also be hard to get them out of the net”.
The International Union for the Conservation of Nature (IUCN) has estimated that each year about 3,500 Greenlands are caught as bycatch by bottom trawling, longlines and gill nets in the northwest Atlantic, Arctic Ocean and Barents Sea.
If you take an average lifespan at around half their possible age, that equals 700,000 years of life lost to the species.
This, along with historically targeted fishing pressure, has contributed to a decline of about 60% in the past 420 years. In 2020, the Greenland shark’s conservation status on the IUCN Red List worsened from near threatened to vulnerable.
We know almost nothing about Greenland sharks, but what we do know are the reasons for their vulnerability. They swim slowly, grow slowly and mature slowly. They inhabit both the abyssal deeps, continental shelves and shallows, leaving them vulnerable to trawling, gillnetting and longlining.
It’s estimated that before the ban, around 50,000 had been caught to support the liver oil industry (10 million years of cumulative life). In 2012 it became compulsory to report Greenland shark catches. The following year it was recorded at 22.2 tons; in 2016 it was at 210.2 tons.
As the Arctic ice recedes and the planet warms, fishing boats are pushing northwards into the core habitat of Greenland sharks. Vessel noise is known to alter both short and long-term habitat decisions, affecting overall species distributions.
As to its daily life, science offers a few clues – at a cost. Though its heart pumps at between 12 and 20 beats a minute, it’s no slouch as a predator.
Researchers who cut open 39 Greenland sharks found 25 different fish species – and wiped out a possible 8,000 years of life to discover that.
“There’s still so much we don’t know about [Greenland sharks],” Arctic fisheries adviser Brynn Devine told the environmental news site Mongabay.
“How many there are, their abundance, their population structure… We have no idea where they go to mate, or where they go to have their pups.
“We don’t know how many pups they have or how often they reproduce. And that makes conservation planning particularly challenging because those are the things that you need to know to understand how at-risk a species is to things like bycatch.”
The population of the Greenland shark is thought to have been stable for more than five million years until humans began deep-sea fishing. In the past 100 years, in addition to fishing, it has had to deal with vessel traffic, seismic survey blasts, pollution and climate change.
In a way, it’s a poster fish for our destruction of marine life – a synecdoche for the impact of humans on the planet.
Out of sight is generally out of mind and three-quarters of our planet lives there. But if this long, slow shark was to go extinct, it would be akin to losing every elephant, lion or polar bear. DM
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