Satellite Tagging Uncovers Surprising Birthing Ground of Porbeagle Sharks

Over the past 15 years, researchers at DFO's Bedford Institute of Oceanography (BIO) have been unraveling the life history of porbeagle sharks, a smaller relative of the great white and mako shark and one of 19 species of sharks that roam Canada's east coast waters. Until recently, however, a critical piece of knowledge remained a mystery: where exactly do porbeagles give birth? Little is known about the birthing grounds of most of the world's large sharks, yet information about this critical life stage can aid efforts to rebuild depleted stocks. Recent research led by Dr. Steven Campana of BIO has uncovered some surprising and unique findings about porbeagles, including where they give birth.

Rebuilding depleted porbeagle stocks

The porbeagle has been fished off of Canada's east coast since the 1960s, when overfishing by foreign fishermen depleted the stock by 80 percent in a mere six years. In recent years, collaborative efforts by DFO and the Canadian shark fishing industry, including strict catch quotas and management, have enabled the stock to rebuild.  Porbeagle mating grounds have also been closed to shark fishing. However, several nations fish for porbeagles outside of Canada's 200-mile exclusive economic zone (EEZ) and the species is unregulated in international waters. 

“We needed to find the birthing ground of porbeagles to determine whether or not it requires protection,” says Dr. Campana. “There was concern that if the birthing ground happened to be in a heavily fished area or an area outside of Canadian management control, the population could potentially be wiped out without being able to do anything about or even knowing about it.”

Porbeagle mating ground discovered

Acting on reports from commercial groundfish fishers that large female sharks were being caught near the Canada-U.S. border, Dr. Campana led an exploratory shark survey of Georges Bank in July 2008. The survey led to the discovery of a new porbeagle shark mating ground, only the second known to science. In collaboration with the Atlantic Shark Association, 21 porbeagles were also captured, measured and fitted with pop-up satellite archival tags (PSATs) to record their swimming depth and approximate location based on light level, as well as water temperature. The startling findings of this research were published, in the June 2010 issue of the Canadian Journal of Fisheries Aquatic Science

Tracking porbeagles with satellite tags

"We began satellite tagging porbeagles in 2001," says Dr. Campana. "Once we learned more about their movements in general, then we started to target sexually mature females and programmed the tags to pop up during the period the sharks give birth, which we determined from other studies is during April and May."

Each satellite tag was programmed to record information on the shark for six months to one year before physically releasing from the shark. The tag then floats to the surface of the ocean and transmits its accumulated data to an orbiting Argos satellite and back down to the researchers. This means the sharks don't have to be recaptured to gather the data.

Data reveals surprising birthing ground

Young porbeagles and adult males remained in cool waters off the coast of Nova Scotia for periods of up to 348 days, usually staying within 200 km of the coastline. However the satellite data collected reveals a different story for adult females.

"All of the adult females left the continental shelf by December, swimming distances of up to 2,356 kilometres to the Sargasso Sea — a vast area in the middle of the North Atlantic Ocean — before the satellite tag released. They remained there during that critical birthing period between March and April and, on average, stayed 500 metres below the surface. Even though we couldn't see they were giving birth, we knew by their location at that time that they had to be giving birth in the Sargasso Sea," says Dr. Campana. "This was a huge surprise because the porbeagle is a cold-water species associated with cool or cold temperate water and the Sargasso Sea is subtropical." 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, far too warm for porbeagles.

Findings reveals record southward migration

Porbeagles have never been observed so far south before — 1,000 kilometres south of the previous known southward distribution of any porbeagle shark and more than 2,000 kilometres from the East Coast of Canada where they spend most of their lives. This led researchers to ask how the entire population of mature females managed to give birth in the Sargasso Sea each year and remain undetected. The answer appears to be due to the great depths to which porbeagles dive and swim.

Of the sharks tagged, each one 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 dove 1.36 kilometres below the surface, which is the deepest ever recorded for a large shark. "For them to dive that deep to avoid the Gulf Stream is staggering because they are totally blind at that depth," says Dr. Campana.

While in the Sargasso Sea, every porbeagle spent the daylight hours at depths of around 600 metres, moving shallower to 250 metres during the night. Researchers suspect that the sharks were probably adjusting their depth to prey on vertically-migrating fish and squid.

Young porbeagle hitch a ride

"Porbeagle young of the year begin appearing off of the East Coast of Canada in June or July," says Dr. Campana. "We think the young are hitching a ride north to Canadian waters on the deep parts of the Gulf Stream, which is known to contain huge quantities of squid that are part of the porbeagle diet."

So what does this new knowledge about the location of the porbeagle birthing ground mean? "Given that the birthing ground is half a kilometer down in the Sargasso Sea in an area that is lightly fished and too deep for conventional long-line fishing, it doesn't look like any action is required at this time," says Dr. Campana. "However, if something happened to change the status quo, the population could potentially be endangered so it's important to keep a close eye on what goes on there in terms of fishing."

For more information about Canadian sharks and the shark satellite tagging, please see the web site of the Canadian Shark Research Laboratory: www.marinebiodiversity.ca/shark.