By Ian Marshall
It is hard to imagine a pregnant shark swimming about 2,000 kilometres to give birth. But that is exactly what female porbeagle sharks do.
They mate in Canadian waters off Georges and Grand Banks, carry their embryos for nine months and head to the deep waters of the Sargasso Sea – between Bermuda and Cuba – to give birth.
Until very recently, researchers didn't know where this smaller relative of the great white shark delivered its pups. But now, this mystery has been solved.
Dr. Steve Campana, a scientist with Fisheries and Oceans Canada, spearheaded the research that led to this discovery. Campana and his team satellite-tagged about 21 female sharks off the eastern coast of Canada and Georges Bank over the past few years. To do this, the sharks were caught on baited hooks and hoisted aboard a fishing vessel in a cradle, where they were placed on “life support” using a hose to bring salt water to their mouth and gills to provide oxygen.
Additionally, a wet cloth was placed over the shark’s eyes to keep its stress level down. The hook was then removed, the tag placed just below the dorsal fin, and the shark was released. It is all done in a matter of minutes.
The satellite tag is quite sophisticated; it has a sensor attached to it much like a minicomputer that measures the water temperature, location and depth of the shark’s movements every five minutes or so. After almost a year of sampling, the tag separates from the shark, floats to the surface and transmits this data to a satellite. The information is then transmitted to researchers.
This data led to the discovery of the world’s first known birthing place for porbeagles – in the Sargasso Sea. Fortunately, this is a relatively safe place for porbeagles to give birth. Although the Sargasso Sea is an area where very little fishing occurs, the main factor that keeps the porbeagle safe is the great depth at which it remains while there – an average of 500 metres below the surface, sometimes extending to depths of more than one kilometre.
It is suspected that young porbeagles follow the depths of the Gulf Stream to Canadian waters where they remain until the age of about thirteen when they begin to mate. As it is a coldwater shark, the surface waters of the Sargasso Sea and Gulf Stream are simply too warm for the porbeagle.
This previously unknown information about the life cycle of porbeagle can help in the management of the species; it comes at a valuable time, as porbeagle stocks are now showing signs of recovery after overfishing in the 1960s. DFO and shark harvesters have worked together to help stocks rebuild. Efforts are continuing on development of guidelines for the fishing of porbeagle in international waters when the species migrate outside Canada’s fishing jurisdiction.
(Ian Marshall is the DFO Area Director for Southwest Nova Scotia.)