23 March, 2010 3:13PM AWST

Jump the shark

Can we understand and predict shark behaviour?

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Every summer on the south coast there are several shark sightings - but just what determines the number of sharks in the area?

Apparently there are a number of things which influence numbers, including water temperature and the influx of certain other fish species.

Pilchard fisherman Vryn Westerberg told John Cecil that January and February were particularly bad for sharks, especially when they are out fishing early in the morning.

"This year there seems to be quite a few around... we had a few bad attacks on the nets."

Vryn says when the fish are alongside the boat the net is stretched quite tightly, so if a shark tries to eat the fish they bite into the nets and rip them apart.

"It's not just a loss of fish: you're out of action for a couple of days trying to mend your nets... then when you do get back out there you're just about scared of having a shot in case it happens again."

Vryn's boat has recently started using a device that sends out an electronic pulse. He says it seems to keep the sharks at bay and doesn't diminish his catch.

"You still see [the sharks] swimming around the net in the early mornings but they don't come up and bite it anymore."

Vryn has also been working with the local shark fisherman, which he says helps keep the numbers down.

"If we're having trouble we let him know and it's good for him to get out and make some money as well."

Dr Rory McCauley, Senor Shark Scientist with the Department of Fisheries, says the sharks are present all the time but different species change their behaviour for different reasons.

"What we do know is that during late summer through to early autumn or even early winter some of the more prevalent species are present off the south coast and actually giving birth off the south coast," he says.

"Species such as the Dusky Shark, which is commonly referred to as Bronze Whaler, is one of the more prevalent larger shark species and we know they're giving birth at the moment so it's to be expected that people are coming across those newborns increasingly at this time of the year.

"But then other species are doing very different things so it's hard to give one general rule when it comes to sharks, because they're such a diverse group of fish."

Dr McCauley says that it's not surprising to find the Dusky sharks around pilchard fishers, as pilchards are a favourite food especially for the juvenile Dusky Shark.

He says it's a similar case when the salmon school up in dense groups and you can expect larger sharks to be in the area.

It's not just the headline grabbing Great Whites that are the big fish in the water; Dr McCauley says the Duskies can grow up to three and a half metres or 12 feet.

"Seasonally you might have larger sub-tropical or tropical species coming down, Tiger sharks we know make it at least as far as Albany and even as far over as Esperance on some occasions.

"Their movements off the south coast tend to be restricted by the strength of the Leeuwin Current which brings that warmer water down from up north."

Dr McCauley says it's a combination of the sharks liking the warmer water as well as their food source following the current.