NR-MAR-99-40E

SCIENTISTS UNCOVER SHARK SECRETS

December 22, 1999

Dartmouth - BIO - They're not the same species of shark that starred in Jaws or Deep Blue Sea, but they are closely related. Looking very much like a smaller version of a mako shark, the porbeagle shark makes its home off the eastern coast of Canada, where it grows to 320 kg and a length of more than three metres. Until recently, very little was known about this toothy resident of our offshore waters. But intensive scientific research by Fisheries and Oceans scientist Dr. Steven Campana and his team at the Bedford Institute of Oceanography (BIO), working closely with shark experts in the US, are uncovering many of the porbeagle shark's secrets.

Sharks are fished worldwide for food and sport, but drastic declines in their numbers in other countries have made it clear that they can't sustain the type of fishing pressure that is commonly found in fisheries such as for groundfish. Part of this is due to the very low numbers of young that sharks produce - typically a handful per year - compared to the hundreds of thousands of eggs typical of a cod or haddock. With this in mind, fisheries scientists at BIO were concerned that the burgeoning fishery for the very-tasty porbeagle shark might result in stock collapse. Nor did the shark fishing industry want to see shark stocks collapse like those of northern cod. So they teamed up to study this poorly-understood shark, with the shark fishing industry offering to fund much of the research.

What they've found has been encouraging. Over 150 tagged sharks have been recaptured in recent years. These returned tags indicate that porbeagle over-winter just south of the Canada/USA boundary, then travel hundreds of kilometres to spend the rest of the year off the coast of Nova Scotia, southern Newfoundland and the Gulf of St. Lawrence. The numbers of tags returned also gave some idea of the size of the population - on the order of 300,000. What the tagging didn't indicate is how fast the sharks replenished their numbers after fishing: that is, how fast they grew and how many pups they produced each year.

To determine the growth rate of the porbeagle, Dr. Campana turned to pieces of shark backbone - the vertebrae. When cut in half and examined under a microscope, rings similar to growth rings in trees were visible, although much more difficult to count. Examination of vertebral rings in sharks tagged as new-borns and recaptured up to 6 years later, confirmed the suspicion that the rings were indeed formed yearly.

After examining the vertebrae of 315 porbeagle, Dr. Campana concluded that the sharks grew more slowly than previously suspected, with a natural lifespan of 30-40 years. While males become sexually mature after 7 years, females are 12-14 years old before they mature.

Internal examination of mature females soon revealed a disquieting fact about pregnancy in porbeagle sharks. While the females produce enough eggs to give birth to more than 10 young per year, the embryos develop large teeth while still in the uterus. In an example of sibling rivalry at its most extreme, the largest embryos then eat their siblings while still in the uterus. Only four pups, all in different sections of the uterus, survive until birth, about 8-10 months after conception.

The teeth of an adult porbeagle shark can reach up to one inch in length and are extremely sharp. They seemed to be designed mainly for fish, not for seals and other mammals. Indeed, the scientists have usually found fish in the stomachs of porbeagle, much as they expected. What they didn't expect to find were large snails from the ocean bottom, as well as some less nutritional items: rubber bands from lobster claws, rope, rocks, balloons, and in one case even a car's oil filter.

So, how is the porbeagle population doing? According to a detailed stock assessment presented in November 1999, the current population is about 1/5 the size of the original, unfished population and recent catches are too high to conserve the stock. But somewhat lower annual catches are sustainable, particularly if the sexually mature females are protected and allowed to give birth to more young. A new porbeagle management plan is now being developed which will allow the health of the population to improve while it is being fished.

Much remains to be learned about the porbeagle shark. The next stage of research will be to tag some of the female sharks with the latest in technology to discover their birthing grounds. Females will be tagged with "archival satellite pop-up tags", which record the depth, water temperature and location of the shark every hour of every day for a full year. This information will be transmitted to an orbiting satellite and then down to the scientists. How many of these sophisticated tags will be applied? Not very many - the tags cost $9,000 each!

FOR MORE INFORMATION:

Dr. Steven Campana
Fisheries and Oceans Canada
Bedford Institute of Oceanography
Dartmouth, N.S.
(902) 426-3233