New study points to Skagerrak as nursery area for the enigmatic Greenland shark

The Greenland shark – the world's longest-living vertebrate – is most often associated with cold Arctic waters. However, a new international study led by researchers from the Greenland Institute of Natural Resources and the University of Copenhagen shows that Skagerrak probably serves as a nursery area for young Greenland sharks. The study also points out that Greenland sharks are not born in Greenland or anywhere else in the Arctic.

It can live for several centuries and, measuring at least 5.5 meters from snout to tail tip, is one of the world's largest carnivorous sharks. The Greenland shark is usually associated with the cold deep waters of the Arctic, where it lives a slow life shrouded in mystery, with a white worm-like parasite dangling from each eye.  

A new study led by researchers from the Greenland Institute of Natural Resources and the Natural History Museum of Denmark presents important new pieces of the puzzle surrounding the mysterious life of the Greenland shark. Among other things, the study shows that the Greenland shark is found much closer to Denmark than most people imagine.

The researchers have examined catch data from over 1,600 Greenland sharks across the North Atlantic, and here the Skagerrak—between Denmark, Norway, and Sweden—stands out as the area with the highest proportion of young sharks between 90 and 200 centimeters.

“We consider the deepest areas of the Skagerrak to be a potentially important feeding ground for ‘teenage Greenland sharks’, and in fact, the study is the first to systematically examine the occurrence of Greenland sharks in the Skagerrak,” says Associate Professor and marine biologist Peter Rask Møller from the Natural History Museum of Denmark.

The mystery of shark birth

One of the biggest mysteries surrounding the Greenland shark's way of life has been where it gives birth to its (presumably) hundreds of pups per pregnancy. Newborn Greenland sharks measure approximately 40 centimeters at birth, and the new study concludes that this is unlikely to take place in fjords or on the continental shelf in the waters around Canada, Greenland, Iceland, Norway, or Russia. This is because neither pregnant females nor newborn pups have ever been recorded in any of these areas.

However, through a thorough review of Danish, Norwegian, and German museum collections, as well as unpublished scientific databases from Iceland, Norway, and Russia, the researchers have managed to find observations of newborn Greenland sharks—all recorded near the Mid-Atlantic Ridge and the Irminger Sea south of Iceland.

“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 , where there is little activity from commercial fisheries,” 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.

Denmark's deepest sea area plays a role

The Greenland shark is listed as ‘vulnerable’ on the International Red List of Threatened Species and is currently exposed to significant bycatch, not least in trawl, gillnet, and longline fisheries for halibut and cod in deep water. 

Therefore, the new knowledge that the study brings to light about the geographical distribution and distribution of different life stages is important in relation to protecting the Greenland shark across national borders in the North Atlantic.

 “The study breathes new life into the story of the Greenland shark throughout the North Atlantic—and, among other things, provides an understanding of how Denmark's deepest sea area also plays a role in the species' life history,” says Peter Rask Møller, adding that individual Greenland sharks are very likely to travel across large parts of the North Atlantic during their very long lives.

This means that even though the birth area is the Mid-Atlantic Ridge, the teenage years may well be spent in the Skagerrak, and later in life the adult shark may reside in South Greenland or Iceland, for example. However, much more knowledge from, for example, tagging studies is needed to understand exactly how the different areas are used.

150 years of observations and Swedish anglers

Historical data from Denmark also supports the conclusion that Skagerrak is a special place for Greenland sharks. The study corresponds well with the strandings and bycatches that have occurred over the last 150 years in Denmark, which have been collected and reviewed by the Fish Atlas at the Natural History Museum of Denmark.

In the new research article, the researchers have also drawn on a newer and somewhat special data source to study Greenland sharks in the Skagerrak. The catches come primarily from Swedish recreational fishermen who fish in the deep waters of the Skagerrak and often catch the juvenile sharks. This contribution has been crucial in documenting the common presence of Greenland sharks in the Skagerrak – a presence that is otherwise best known from sporadic strandings or bycatches over time.

Unlike many of the other areas studied in the 1610 Greenland shark study across the North Atlantic, the Skagerrak is also notable for the fact that adult individuals are very rare.

“This does not mean that large females cannot be found in the Skagerrak, but the probability of catching a large female is very small there,” says Julius Nielsen.

The large adult females, which typically measure over four meters in length and are over 100 years old, are most frequently found in Subarctic areas where warm Atlantic water also flows. For example, Southwest Greenland, Iceland, and southern Arctic Canada have been identified as regions where the largest females are most frequently found.

The study has been published in the scientific journal Ecology and Evolution: https://onlinelibrary.wiley.com/doi/epdf/10.1002/ece3.71564 

 

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