Zombies of the Deep: Greenland Sharks
‘Sluggish, blind and slow-moving’ was the scientific world’s opinion of the Greenland shark – until two Canadian researchers encountered a new group of sharks in the St Lawrence river that suggested there was more to this animal that meets the eye
It’s a staple of Hollywood, and the dramatic peak of many a film – that gut-wrenching moment when the hunter realises that he has just become the hunted. It happened in real life to Canadian diver Jeffrey Gallant when the tables turned in when the tables turned in his multi-year quest to find an elusive giant predator in Canada’s saline rivers. When a massive dark shape passed right in front of him shortly after he landed on the bottom of the St Lawrence Estuary in pea-soup conditions, Gallant understood immediately that his quarry had instead found him. He also suspected that the reason the creature had chosen to inspect him might be related to its dietary habits, which are known to include large mammals.
Gallant and his expedition partner, veterinarian Dr Chris Harvey-Clark, had both been intrigued for years by the occasional news reports of Canadian anglers landing an unexpectedly large catch while ice-fishing – a Greenland shark (Somniosus microcephalus). The Greenland shark belongs to the family of sleeper sharks, so named due to their sluggish behaviour, and is related to dogfish sharks. Preferring frigid waters from below freezing to about 12°C, the Greenland shark inhabits the depths of the Atlantic to at least 2,200m, and swims under the polar ice cap. Its slow swimming pace, the minimal resistance it puts up when captured, and inability to raise its body temperature above that of the icy waters suggests that it has a particularly low metabolic rate, and raises the question of how the species captures enough food to grow to lengths of up to 7.3m – larger than a great white shark. Part of the answer may be an extremely slow growth rate – about 0.5cm a year, according to one study – leading to estimates that a 7m specimen could be more than 200 years old, making Greenland sharks possibly the longest-lived vertebrates on the planet.
The stomach contents of captured Greenland sharks have included a variety of fish and invertebrates, shark, skate, birds, seal, dog, horse, reindeer, polar bear, porpoise, beluga, and narwhal. It’s possible that much of its food is scavenged as carrion, but there is also evidence that it can be an active predator, leaping out of the water to ambush caribou, according to one account. It is suspected of being the agent responsible for stripping the blubber from seals in a corkscrew pattern. It is even claimed that these sharks have stalked humans walking on pack ice.
The dentition of the Greenland shark is clearly well suited for disassembling large prey. The upper teeth are pointed and function as the ‘fork’ to hold the food item, while the angled lower teeth function as the ‘knife’ to saw off pieces for consumption. ‘What’s astounding about these animals,’ says Harvey-Clark, ‘is that they are active predators in water that’s only about 1°C. They’re like Dawn of the Dead sharks. How do they catch large, fast-moving prey?’
Added to the mystery of how such a slow-moving zombie-like animal captures such agile prey is the question of how it is affected by its curious relationship with the copepod Ommatokoita elongata. This crustacean parasite is found on both eyes of most Greenland sharks, and feeds on the corneal tissue, scarring the eyes. It is presumed that the shark becomes nearly blind as a result. There is some speculation that the parasites might benefit the shark by attracting small prey, but this has never been demonstrated. It’s hard to imagine how blindness could be an advantage in hunting seals. At the same time, it’s not at all comforting, when diving in near-zero visibility, to know that you are in the presence of a gigantic predator that has no need of vision to satisfy its appetite.
Although the Greenland shark has been reported as far south as Georgia, and even into the South Atlantic, it is generally associated with the high Arctic. The Inuit feed the flesh of the sharks to their sled dogs, but only after it has been boiled in several changes of water, or buried and fermented for several months. Greenland shark meat contains high levels of an ammonia-like compound, so is toxic if eaten fresh. The fermented flesh is considered a delicacy in Iceland. The chemical that renders the flesh toxic apparently functions as an antifreeze/preservative to protect the shark’s body from freezing temperatures and crushing pressures (up to 9,700kg per sq cm) at the great depths they inhabit.
Since this shark was believed to live primarily in the frigid waters of the deep sea and high Arctic, Harvey-Clark’s imagination was fired by a news story of a fisherman landing one in the Saguenay River in Quebec, almost 3,000km from its supposed home territory of Greenland. When he met Gallant, a bilingual native of Quebec, he asked him to investigate. Gallant soon found that stories of giant sharks were common among the French-speaking fishermen of Quebec, but these tales had never crossed the language barrier into the English-speaking regions of Canada, where Harvey-Clark was employed at Dalhousie University in Nova Scotia. In the world of academia, almost nothing is known of the Greenland shark’s natural history.
Following the conventional wisdom that the cold-loving sharks would most commonly rise to near-surface waters during the colder months, Harvey-Clark and Gallant mounted their first expeditions to search for sharks in the Saguenay Fjord in the dead of winter. Normally this tributary of the St Lawrence has warmer fresh water flowing on top of the colder, denser salt water below. When the surface freezes, however, the thermocline is inverted with the fresh water on top becoming colder than the deep water. This is when the cold-loving Greenland sharks should rise to shallow depths where they could be observed, they reasoned. Their research determined that sharks were caught every year at a giant ice-fishing camp that convenes each winter at the head of the fjord, so this is where they concentrated their efforts, dragging generators, air compressors, shark cages, tents, lights, and all the other equipment needed out onto the 60cm-thick ice.
During two winters, in 2001 and 2002, the team battled fierce storms that buried their camps in snow and blew their tents away. The bitter cold resulted in frostbitten fingers, and caused plastic items to shatter and their regulators to freeze up and free-flow. Diving through holes in the ice in near-darkness, with visibility limited by the penetration of their underwater lights, they had no idea what to expect. ‘In most cases, when you are preparing to dive with a large and potentially dangerous animal, somebody else has already been in the water with the animal you are going to dive with,’ says Harvey-Clark. ‘In this case, nobody had.’ In the end, they were unable to see any sharks.
Discouraged, and having exhausted their funding, the team had to shelve their ambitions of being the first to study and film Greenland sharks swimming freely in their natural habitat. Then, in May 2003, Gallant received an email from a Quebecois diver who claimed to have just seen a fish bigger than himself while diving in the St Lawrence at the Baie du Garde-Feu.
Gallant and Harvey-Clark both loaded their gear and headed to the site, but Gallant, being closer, arrived a day earlier. Entering the water, he was approached by a Greenland shark within minutes. This happened on every dive, and was repeated the next day with Harvey-Clark, as well.
The researchers noticed that these sharks did not, for the most part, carry the eye parasites that afflict nearly all Greenland sharks in the Arctic. Furthermore, they seemed to follow the divers with their eyes. They normally moved slowly, but could turn and accelerate suddenly when spooked and could easily outmanoeuvre and out-swim the divers. They realised that this was not the sluggish, blind, scent-oriented scavenger that had been conjectured, but rather an active, curious, visually acute and highly capable predator.
The sharks were clearly on active patrol and investigating any new entry into their environment. This was not a comforting thought when the visibility dropped from poor to non-existent, and the team was not aware of the approaching sharks until they arrived within touching distance. It also made the divers more than a little nervous when they ran low on air and had to ascend, and found sharks following them to the surface. Were they merely being watched, or were they being stalked?
As one of the largest sharks in the ocean, rivalling the great white in size, and by far the largest fish in polar waters, the Greenland shark’s indiscriminate feeding habits should make it potentially as dangerous to humans as the tiger shark – another indiscriminate feeder, responsible for a large number of attacks on humans. Could it be that the lack of records of attacks (only one unconfirmed report of a human leg in a shark stomach from 1859, and one supposed 16th-century attack) is merely a result of the shortage of humans to be found at great depths in the ocean and under the ice during the Arctic winter?
The discovery by Sirois and his companions of Greenland sharks in shallow water in the St Lawrence during the summer raised disturbing questions as to whether this balance sheet might suddenly change. Scuba divers had been exploring these sites for years without encountering sharks before, yet were now seeing them regularly for weeks at a time. Furthermore, in the following years, the peak of shark sightings switched from June to July and into August – the warmest months, when Quebecois and visitors to the province enjoy swimming, skiing, and other water sports.
As always with scientific investigation, each new discovery raised more questions. Do the St Lawrence sharks form a separate population from the ones found in the Arctic? The low incidence of eye parasites strongly indicate that they might. Where do the sharks go during most of the year when divers are not seeing them? Why are they coming into shallow water during the warmer months – exactly the opposite of what was expected?
To further their research efforts and disseminate their findings, Gallant and Harvey-Clark founded the Greenland Shark and Elasmobranch Education and Research Group, or GEERG (www.geerg.ca). The first three sharks tagged stayed within the St Lawrence system for the three months that the tags were operational, although one moved 115km up the river. Two of the sharks spent the nocturnal hours searching the water column, while staying closer to the bottom during the day. One shark regularly went up to within 10m of the surface between the hours of 4pm and 5am. Could this be a strategy for ambushing juvenile seals? If so, there are strong implications for people who might take a late afternoon dip in the river.
Of two sharks subsequently tagged with pop-up satellite tags, one only moved 5km away over a period of three months, while the the other swam 100km upriver to the Tadoussac area, where the tag popped up from a depth of over 300m. Tadoussac is at the centre of Quebec’s whale-watching industry and is a feeding ground for blue, finback, minke and other whales.
There are implications for the sharks of living and feeding in this heavily used and polluted waterway. A Greenland shark captured in the Saguenay in 2006 was found to have its flesh and organs contaminated with PCBs, flame retardants, stain repellents and heavy metals. Extensive body scarring is consistent with interactions with crab pots and other fishing gear. Harvey-Clark suspects that fishing gear interaction is one of the biggest causes of deaths in the species.
Officially, the Greenland shark is listed as near-threatened on the International Union for Conservation of Nature’s (IUCN) Red List. They are categorised by the Food and Agriculture Organization (FAO) as a species with life history characteristics that make it especially vulnerable to overfishing. As many as 50,000 a year were reportedly harvested for their oil in the 1940s, before vitamin A was synthesised, and they are still taken around Greenland and Norway. Inuit take at least few hundred a year.
The status of worldwide populations, like most of the basic biological characteristics of this elusive giant predator, remain mysterious. While he his determined to crack the shark’s secrets, Harvey-Clark relishes the difficulty of the task, quoting Steinbeck: ‘An ocean without its unnamed monsters would be like a completely dreamless sleep.’