21 October 2005

Are sharks switching prey to humans?

A prominent South Australian marine biologist says a rise in the number of shark attacks on humans over the past 80 years could indicate sharks are starting to see humans as a food source.

Scoresby Shepherd is studying shark attacks in Australia and California.

Dr Shepherd says where attacks used to happen once every 30 to 40 years, they are now happening at least once a year.

He says a reason for this may be a decline in the shark's natural prey, such as tuna.

"It's a well-known biological phenomenon which is called prey switching," Dr Shepherd said.

"An animal which is a generalist predator, as we know the shark is, it eats a wide range of prey.

"It eats what is available."

But CSIRO research scientist Barry Bruce is not convinced by that theory.

"For a start, white sharks are pretty cosmopolitan, so it is not as if they feed on one particular sort of prey and if something happens to that prey they kind of swim around the ocean going, 'well, what'll we eat now?'" he said.

Australia ranks second behind the United States for the number of shark attacks.

According to the Australian Shark Attack Register compiled by the Taronga and Western Plains Zoo, 60 people have been killed by sharks in the past 50 years.

While last year's statistics were slightly up, they are in line with the average of 1.2 fatalities per year.

Mr Bruce says one explanation for the spike in attacks is that more sharks have been spotted close to shore in recent times, a phenomena he describes as not unusual, but not understood all the same.

"There's strong signals in good years and bad years for seeing white sharks and at the moment, we're in a good year," he said.

"The issue is why. It's not to do with population and size. It's to do with distribution, it's to do with where they are.

"What we don't know is what drives those differences in distribution. It's not something that's just suddenly happening in the environment, because you can go back … for example, if you go back to the late 1980s, there was an incredibly poor year for seeing white sharks in these island areas of dangerous reach to the point where people were so concerned that they thought the species would go extinct.

"All it was was a shift in the distribution because two years later we saw just as many white sharks as people had seen like a decade before."

Mr Bruce says the last decade since white sharks became a protected species has not been long enough for their numbers to rise significantly. Females only begin to breed once they grow to five metres long, and only produce a small number of offspring in their lifetimes.

For now, scientists do agree on one thing – too little is known about sharks to know with any certainty why they attack humans, but it can always be guaranteed that they will continue to do so.

Source: www.abc.net.au


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