The mysterious lifestyle of Greenland sharks makes them incredibly difficult to follow. Peter Bushnell and John Steffensen have been studying these animals since 2011 and it was only recently that they found a way to track them. On their latest expedition they employed a new tagging method that provides ground-breaking insights into not only where, but also how deep Greenland sharks go.
How do you track a shark that lives in the depths, often beneath ice floes, in the northernmost reaches of the planet where the sun glares for weeks in summer but doesn’t emerge at all in winter? SPOT tags, which rely on GPS, work perfectly for animals like white sharks that come to the surface; PSAT tags, which estimate geographic position by measuring depth, time of day and light intensity, are effective at locations closer to the equator. Neither, however, is suitable for Greenland sharks. Since beginning his research, Bushnell has tagged 20 Greenland sharks, but the only locations he has been able to pinpoint were the exact sites where the PSATs were deployed and where they popped up after three to nine months. To discover where the sharks go and whether populations on the eastern and western sides of Greenland are mixing, he needs more detailed information about their movements.
In May 2014 he flew from Indiana to Greenland for another tagging expedition and this time he had a new plan. Instead of fitting each shark with a single tag that would result in only one usable data point, he used what he terms the ‘breadcrumb technique’. By attaching four tags to each shark and programming them to release at six-week intervals, he would be able to get a much better idea of where the shark had travelled over the six-month period. The breadcrumbs provided valuable and interesting data – and one result that came as quite a surprise. At least one, if not two, of the tags on each shark released prematurely. This was not a malfunction, but rather an issue of depth: the tags have a safety feature that causes them to release at a depth of 1,800 metres.
This sheds new light on the dark world of Greenland sharks and means that all four of the tagged animals probed at least 1,800 metres down into the bathypelagic or ‘midnight’ zone. Another deep diver, the great white, has been recorded at 1,200 metres, while the Portuguese shark, the deepest dwelling of all shark species, has been fished at 2,700 metres. The ocean’s deepest diver recorded so far, Cuvier’s beaked whale, can plummet to almost 3,000 metres.
Members of the Greenland Shark Expedition team assist in releasing a Greenland shark after it has been tagged.
A remarkable creature lives beneath the ice floes of the Arctic. Greenland sharks swim excruciatingly slowly, have been known to eat polar bears and live for an implausibly long time. Peter is bringing their mysteries to the surface.