05 July 2005

Tagging great white sharks - research on behavioural ecology

The ghostly grey shape makes its first cautious approach towards the tuna-head bait bobbing in the water off Seal Island with an effortlessness and grace that defies description.

Tagging great white sharks - research on behavioural ecologyWe all know that great white sharks are big, but the huge size of this animal gliding through the clear False Bay waters comes as a major surprise to someone seeing the species in its natural habitat for the first time.

It's long, very long, although University of Cape Town researcher Alison Kock estimates that it's "only" about four metres in length - still just medium-size for an animal that can reach an astounding 6m or more.

But even more surprising is its massive girth; it would take at least two, maybe three, people with joined hands to get their arms around it.

"It's like a bus," agrees Morné Hardenberg, dive-master of a commercial shark-cage diving boat in Gansbaai, who is helping Kock on this particular outing to Seal Island.

The shark has been attracted to Kock's boat by a rank mixture of anchovy oil and water ladled into the sea - a practice called chumming - and by the tuna head tied firmly to a rope.

There's also a dummy seal, Flatty, made of styrofoam, which bobs behind the boat on another rope, providing the silhouette of a Cape fur seal which is the great white shark's main prey in this area.

Surprisingly, the shark approaches the bait extremely carefully, making several passes at a distance before coming in for a taste.

But it's a male, not what Kock is looking for, so Hardenberg quickly hauls in the bait before the shark can get hold of it.

It's all restrained and surprisingly unfrenzied - there's no snapping of jaws, no thrashing or gnashing of the triangular, razor-sharp teeth which are partly responsible for this species' awesome reputation. It is definitely not like the movies.

After several unsuccessful attempts to get the bait, the shark leaves, but another is quickly on the scene - this time a female, about 3,5m.
To the trained eye, it's simple to differentiate between the sexes as males have a pair of easily visible appendages known as "claspers", one on either side of the pelvic fin - if you know where to look.

Hardenberg uses the tuna head to lure the shark close to the boat, and at the last possible moment Kock "spears" it with a long tagging pole, planting a small barbed electronic transmitter into its tough skin, just alongside the dorsal fin.

The shark doesn't react, and returns several times before it too leaves the scene.

Occasionally there are two sharks cruising around the boat, and at one time during the three-hour operation even three, but mostly there is only one at a time in view.

This day most are males, but eventually another female - slightly larger at about 3,6m and dubbed "Dorsal Dot" - is lured close enough for Kock to plant a second transmitter, and the morning's work is done.

Altogether, 18 different individuals are identified during this trip - some way short of Kock's record of 36 in a single day.

Kock is a PhD marine biology student who is researching the behavioural ecology of what is correctly called the white shark (Carcharodon carcharias) in False Bay, and she's planning to tag 35 of these creatures this year, and another 35 next year, getting a good cross-representation of size and sex. The tags are read by a scanner in one of 28 receivers attached to weighted tyres sunk around False Bay, at points from just past Kogel Bay and at Gordon's Bay on the eastern side of False Bay, to Macassar, Strandfontein, Muizenberg, Fish Hoek, Simon's Town, Partridge Point and Cape Point - and, of course, at Seal Island.

The most popular bathing areas of Muizenberg and Fish Hoek are particularly well covered with four receivers each.

Each time a tagged shark swims past a receiver, its identity and other information is recorded, including when it arrived, how long it stayed, at what depth, and when it left.

"On a day like this (a beautiful, calm day with clear water and a very small swell) the receivers will pick up signals from at least 600m or 700m, but I work on an average of 400m," Kock says.

"We're working on the assumption that if the sharks are inshore, they're swimming parallel to the coast, and we try and put in an inshore and an offshore receiver to try and cover that."

"And I'll definitely pick up any tagged animal coming into a bay like Fish Hoek."

Her research has been designed to answer specific questions, she explains.

"The main aim of this project is, firstly, to gain a better understanding of what white sharks are doing in False Bay, what times of year they occur here, which areas they frequent, and how long they're there for."

Secondly, she wants to see whether and how the sharks react to various factors.

"I'm trying to correlate these shark movements to activities like trek-net fishing, water-user activities such as bathing and surfing, the state of the river mouth at Sandvlei in Muizenberg corner, and activities at Kalk Bay harbour such as the cleaning of the boats and fish offal from there."

She is particularly interested in the relationship with trek-net fishing because it involves many potential prey species.

"I'd be very surprised if white sharks weren't attracted, and so I'd like to investigate that further," she says.

It is still too early in her research to come up with any conclusive findings, other than some of the shark movements around Seal Island.

"The white sharks seem to start arriving at the island from May and concentrate here, and we see the seal predations which last through to about September."

"And in summer the sharks aren't at the island - we might get one or two occasionally coming by."

Great Whites are known to range incredibly widely, with some having been recorded as crossing to the coasts of other continents.

There is no certainty yet about where the False Bay sharks go during the summer, but Kock thinks at least some remain in the bay.

"This is one of the main things we're trying to find out, but from aerial surveys, from interactions with fishermen and kayakers and divers, I really think we have a year-round presence of white sharks in False Bay, and we need to get a better handle on that."

She's also researching behavioural differences between different size sharks and between the sexes.

Her boat, Xiphodon (named, appropriately, for an extinct giant mako shark), the tags and the receivers are all sponsored by the Save Our Seas Foundation which operates out of Jeddah in Saudi Arabia.

Kock wants more people to have the experience of seeing great white sharks in their natural environment, and regrets the high price tag of the commercial trips - around R900 a person.

"I really wish that more South Africans could see what happens out here."

"When there's a shark attack, there's this impression that there's this 'rogue' shark that has come into False Bay and is now looking for someone to eat."

"That's just such a wrong impression, it's so far from the truth!"

"It's tragic when there's a shark attack, but I really think that for having so many of these top predators in our waters, we are actually co-existing with them."

Source: www.iol.co.za


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