Post by LSDeep on Oct 19, 2005 8:51:32 GMT -5
Sharks switching prey to humans: marine biologist
PM - Tuesday, 18 October , 2005 18:45:21
Reporter: Tanya Nolan
MARK COLVIN: A prominent South Australian marine biologist believes that sharks may be attacking humans more often because humans have so depleted their normal food source.
Dr Scoresby Shepherd, who's been looking at the prevalence of shark attacks in Australia and California, says that where there was once only an attack every 30 to 40 years, the figure's now grown to more than one a year.
And Dr Shepherd suggests that it may be the result of the significant decline in fish stocks such as tuna, which are high on the shark's menu.
Tanya Nolan reports.
TANYA NOLAN: It certainly seems like there've been a lot of shark attacks in recent times. In fact, there have been four such incidents in the last two months alone, and two of this year's attacks have been fatal.
Surfer Jake Heron, who recently managed to fight off the shark that would have him for lunch, called for the culling of Great Whites, arguing that their protected status has meant their numbers have exploded, making them more of a danger to humans.
Retired South Australian marine biologist, Dr Scoresby Shepherd, has offered his own explanation.
He suggests that it's the decline in the shark's natural food source that may've directed its attention towards people.
SCORESBY SHEPHERD: My speculation is that if the natural prey are decreasing, then they're more likely to be hungrier than they were before. And so they may be well be turning their attention to whatever else is available. And what they eat depends on availability as well as food value.
TANYA NOLAN: So you suspect that sharks are just broadening their menu to include more things, including humans?
SCORESBY SHEPHERD: Well, it's well known if one prey… if a preferred prey declines, then the species will switch to some other prey that's available, more available.
And in the case of sharks, it may be that – and I don't put any higher than that – it may be that they're seeing humans more often and so you would suspect that there might be exploratory bites at them.
TANYA NOLAN: Dr Shepherd describes this as a well-known biological phenomenon known as "prey switching".
But CSIRO research scientist, Barry Bruce, is not convinced by that theory.
BARRY BRUCE: 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?" (laughs)
TANYA NOLAN: 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.
And while last year's statistics have been slightly up, they're in line with the average of 1.2 fatalities per year.
Barry Bruce from the CSIRO, which has been tracking White Sharks since 1987, 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.
BARRY BRUCE: There's strong signals in good years and bad years for seeing White Sharks. And at the moment, we're in a good year.
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.
TANYA NOLAN: Barry 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.
(sound of Jaws soundtrack)
ACTOR: We're going to need a bigger boat.
MARK COLVIN: Going to have to get a bigger boat indeed.
Tanya Nolan reporting.
© 2005 ABC
source: www.abc.net.au/pm/content/2005/s1485234.htm
PM - Tuesday, 18 October , 2005 18:45:21
Reporter: Tanya Nolan
MARK COLVIN: A prominent South Australian marine biologist believes that sharks may be attacking humans more often because humans have so depleted their normal food source.
Dr Scoresby Shepherd, who's been looking at the prevalence of shark attacks in Australia and California, says that where there was once only an attack every 30 to 40 years, the figure's now grown to more than one a year.
And Dr Shepherd suggests that it may be the result of the significant decline in fish stocks such as tuna, which are high on the shark's menu.
Tanya Nolan reports.
TANYA NOLAN: It certainly seems like there've been a lot of shark attacks in recent times. In fact, there have been four such incidents in the last two months alone, and two of this year's attacks have been fatal.
Surfer Jake Heron, who recently managed to fight off the shark that would have him for lunch, called for the culling of Great Whites, arguing that their protected status has meant their numbers have exploded, making them more of a danger to humans.
Retired South Australian marine biologist, Dr Scoresby Shepherd, has offered his own explanation.
He suggests that it's the decline in the shark's natural food source that may've directed its attention towards people.
SCORESBY SHEPHERD: My speculation is that if the natural prey are decreasing, then they're more likely to be hungrier than they were before. And so they may be well be turning their attention to whatever else is available. And what they eat depends on availability as well as food value.
TANYA NOLAN: So you suspect that sharks are just broadening their menu to include more things, including humans?
SCORESBY SHEPHERD: Well, it's well known if one prey… if a preferred prey declines, then the species will switch to some other prey that's available, more available.
And in the case of sharks, it may be that – and I don't put any higher than that – it may be that they're seeing humans more often and so you would suspect that there might be exploratory bites at them.
TANYA NOLAN: Dr Shepherd describes this as a well-known biological phenomenon known as "prey switching".
But CSIRO research scientist, Barry Bruce, is not convinced by that theory.
BARRY BRUCE: 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?" (laughs)
TANYA NOLAN: 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.
And while last year's statistics have been slightly up, they're in line with the average of 1.2 fatalities per year.
Barry Bruce from the CSIRO, which has been tracking White Sharks since 1987, 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.
BARRY BRUCE: There's strong signals in good years and bad years for seeing White Sharks. And at the moment, we're in a good year.
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.
TANYA NOLAN: Barry 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.
(sound of Jaws soundtrack)
ACTOR: We're going to need a bigger boat.
MARK COLVIN: Going to have to get a bigger boat indeed.
Tanya Nolan reporting.
© 2005 ABC
source: www.abc.net.au/pm/content/2005/s1485234.htm