Sharks that circle our shores

Thu, Oct 28, 2010, 01:00

A new doctoral thesis throws fresh light on the presence and activities of some 40 species of shark off our waters, writes CLAIRE O’CONNELL

SHARKS! WHAT image does that word evoke in your mind? Blade-like fins circling in tropical waters? In Ireland we often think of sharks as being elsewhere in the world, yet we have several species on our doorstep.

Irish waters host over 40 species of “elasmobranch” (or shark, skate or ray) and of those about 28 are sharks, according to Dr Edward Farrell.

His recent doctoral thesis at University College Dublin’s School of Biology Environmental Science and the Marine Institute looked at the abundance and life-history of one particular group of sharks – the smooth-hounds – and came up with some surprising findings.

People generally think there are two species of smooth-hound in the Irish Sea, explains Farrell; the starry smooth-hound with white spots on its back, and the completely grey common smooth-hound. But appearances may be deceptive, as he found out.

“There’s a lot of confusion about the identification of the two species,” he says, pointing out that the spots on the starry smooth-hound aren’t always obvious. “If you put the two [species] side by side on a table you might have a bit of difficulty telling the difference.”

Rather than trying to identify the fish by morphological characteristics, for which you would have to kill them, Farrell and his supervisors Dr Stefano Mariani and Dr Maurice Clark developed a genetic test to identify species of live caught smooth-hounds, which can be returned to the sea.

“We take a tiny little piece of the shark’s fin,” explains Farrell, who describes how they pull out tiny pieces of mitochondrial DNA from the sample to identify which species they are dealing with.

And after testing around 850 smooth-hounds from waters along the Irish coast, the North Sea, English Channel and Bay of Biscay, the results suggest that the common smooth-hounds are not so common after all, at least in those waters.

“We found no common smooth hounds whatsoever,” says Farrell. “More than likely they don’t occur this far up north. They are a more southerly species and you get them in the Mediterranean and in South African waters but they probably don’t go further north than the southern Bay of Biscay,” says Farrell.

He adds that that also looked at the life-history of the smooth-hounds – which use their flat crushing teeth to feed on swimming crabs and prawns – by ageing caught samples that could not be returned to the sea: “There are little rings laid down on the vertebrae like the rings on a tree, so you can age them that way.”

The Ircset-funded project logged the ages of 106 males and 114 females in the region and again the results challenged previous assumptions.

“They were much slower growing, about twice as slow-growing than they were previously estimated ,” says Farrell of the findings, published this month in the Journal of Fish Biology.

“And the survey data is showing that the starry smooth-hound has increased in abundance in the North Atlantic over the last few years. We are not sure why, or if it’s a true increase or a change in the migration patterns of the sharks.”

The study provides a more concrete basis for building protection and conservation strategies for smooth-hounds, which are of commercial importance in areas of the European continent and also of interest for recreational or sports fishing, where they can be returned to the sea, according to Farrell.

“We had nothing before, we didn’t even know what species we were dealing with, so we have definitely made an advance on that,” says Farrell, adding that populations in the North East Atlantic could be vulnerable to overfishing if not strictly managed.

“We think certain areas of the Irish Sea might be a nursery, so ideally they are the areas you protect.”


For more on sharks, skates and rays in Irish waters, see irishelasmobranchgroup.org