The birthing grounds of most of the world's large sharks are unknown. And until recently, the birthing area for porbeagle sharks, found off Canada's east coast, was also unknown.
But research results from a Bedford Institute of Oceanography (BIO) satellite tagging project, conducted in the summer of 2009, has found birthing grounds of east coast porbeagle sharks in the deep, cold offshore waters of the Sargasso Sea. These waters are east and south of Bermuda.
DFO and the shark fishing industry are working together to try and rebuild declining populations of porbeagles. Understanding the life stages of the species helps with these initiatives.
The research has shown that young porbeagles and adult males remained in cool waters off the coast of Nova Scotia for much of the year, mostly within 200 km of the coastline. In contrast, all adult females left Canada's continental shelf by December, swimming distances of up to 2,356 kilometres into the sub-tropical Sargasso Sea before the satellite tag released from the shark. The adult females spent the winter and spring in the Sargasso Sea, where they gave birth to four pups each.
Porbeagles have never been observed so far south before, which begs the question: How has the entire population of mature females managed to give birth there each year without being detected? The answer appears to be because of the great depths occupied by the sharks during their journey south.
Each shark swam about half a kilometre beneath the surface during its travels, swimming underneath the warm Gulf Stream to get to the Sargasso Sea. One shark dived 1.36 km below the surface, which is the deepest depth ever recorded for a large shark. Porbeagle sharks prefer waters between 5 and 15°C, which probably explains why the sharks stayed so deep. The surface waters of the Sargasso Sea and Gulf Stream often exceed 30°C, which is too warm for a porbeagle.
In 2008, BIO researchers carried out an exploratory shark survey on Georges Bank and discovered a new porbeagle shark mating ground, the second porbeagle mating ground known to science. The Sargasso Sea is well known as being the spawning area for eels, but has not previously been suggested as a pupping ground for any species of shark.
