Great white shark nursery
THIS IS SHARK paradise. Expert Barry Bruce has never seen as many juvenile great whites in one location as this isolated stretch of beach off the central coast of NSW. His team, on a research vessel tucked behind rolling breakers on a windswept sea, have just spotted several distinctive shark silhouettes.
As the vessel closes in on one of the animals, Barry casts a mullet on a barbless hook directly into the shark’s path. The ominous form seems interested, but cautious, distracted by the incessant drone of the outboard and the shadow of the vessel. “Here we go,” says Barry, as the shark lowers the tips of its pectoral fins towards the shallow, sandy seabed and makes one last pivoting turn before striking the bait. In a flurry of activity the team hauls in their uncooperative subject, guiding it to a partially submerged stretcher alongside.
First it is measured. From the tip of its conical snout to the end of the upper lobe of its crescent-shaped tail this one is 2.2m long. “Only a little guy, but about average for here,” Barry says. Sharks found in the nursery generally range between 1.5 and 3m. At this size, they’re no more than four years old – mere pups. Great whites can reach 6m and are thought to live for up to 50 years.
Great white shark icon
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This most famous of sharks is rare and vulnerable. Australia listed the great white as protected in 1999; internationally it is listed as vulnerable on the IUCN (World Conservation Union) Red List for Threatened Species, and it was also banned for trade by the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) in 2004.
Great white numbers have declined in Australian waters, with estimates suggesting a reduction of 50-70 per cent over the past 50 years. The interpretation of such trends is complicated by seasonal variations in shark movements, along with changes in beach-meshing regimes and fishing effort.
“We know so little about great white sharks,” Barry says. “Even basic notions, such as how large they actually grow, how long they live, how many are out there, and where they go to breed and pup [give birth], remain a mystery.” However, Barry and the team from CSIRO, the Port Stephens-Great Lakes Marine Park Authority and the NSW Department of Primary Industries (DPI) have discovered that between August and January each year, juvenile great white sharks congregate off Port Stephens, about 200km north of Sydney.
The aggregation of sharks here was first brought to the attention of scientists and the public by a pair of Port Stephens fishers – Kris ‘Mackerel’ Macklin, and Glenn ‘Mullet’ Connell – who posted a video on YouTube called Great White Shark Hunters – Stockton Beach, that was an instant hit. Although the hunting of this species, as depicted in the video, is illegal, what scientists have learnt since “has contributed greatly to our knowledge of great whites”, Barry says.
Within 10 minutes the shark Barry has caught for research purposes is released, now carrying two remarkable advances in tagging technology. The team has fixed a satellite tag to the shark’s dorsal fin, which records the date and time, water temperature, depth and light levels. When the animal’s fin breaks the surface, data stored in the tag are transmitted via satellite to the CSIRO laboratories in Hobart. Barry’s CSIRO colleague Russ Bradford has also surgically implanted an acoustic tag into the shark’s abdomen.
Similar to the electronic tags used by city toll-road commuters, it transmits a unique number when a tagged shark swims within 500m of an underwater acoustic listening station, arrays of which are scattered throughout Australian waters. “It’s like having a window into the secret lives of these animals that would be impossible to get in any other way,” Barry says.
Pup playground for sharks
The nursery area consists of a 50km stretch of broken coastline between Seal Rocks and Stockton Beach; it is bound by a surf zone of picturesque beaches and rocky headlands on one side and an offshore coastal reef on the other.
Beneath the surface, patchy temperate reefs teeming with marine life contrast with the meandering backwaters and tidal flow of the Port Stephens estuary.
The interaction of the East Australian Current with the structure of the coastline provides nutrient-rich conditions for schools of Australian salmon, mulloway, mullet and snapper, the favoured meals of the juvenile sharks. “Port Stephens is a great place to fish,” says Barry. “It’s just that the sharks are better fishers than most of us.”
Since CSIRO’s first expedition to the area in 2007, the team has tagged 32 juvenile great white sharks off Bennetts, Mungo Brush and Stockton beaches. The sharks live in the nursery area between late winter and mid-summer, before moving south to the Corner Inlet region of Victoria, where they stay until autumn. After this, Barry says, it’s difficult to generalise the sharks’ movements before they return.
This year CSIRO, in collaboration with the University of Technology, Sydney, began annual helicopter surveys of the nursery to estimate how many sharks go there. “It may just be that every juvenile great white shark in Australian waters visits this site, which may prove useful to predict adult numbers in the future, and also identify population changes,” Barry says.
Very little is known about the location of mating and pupping of white sharks. They are oviphagous (‘in utero egg eaters’, which means pups compete and eat one another in the womb), give birth to 2-14 live pups, and may only produce 4-6 litters in a lifetime. The size of newborn pups can be up to 1.5m, and they are highly mobile from birth, further complicating our understanding of where they originate.
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Sharks under threat
Dr Nick Otway, a senior research scientist with the DPI, undertakes about five great white necropsies (animal autopsies) each year in a purpose-built facility at the Port Stephens Fisheries Institute. A recent CSIRO report, co-authored by Barry, estimates that 50-300 great white sharks are captured each year in Australian waters as a bycatch of commercial and recreational fishing.
Given an average release rate of 40 per cent, 120 sharks are expected to die from fishing in a typical year, although only 5-10 are reported. Others are caught in shark-control programs installed for bather protection, such as the 51 nets deployed along beaches between Newcastle and Wollongong, from September to April each year.
“It’s tragic to lose an individual from this population, but we try to make the most out of the situation,” Nick says. By studying the dead sharks’ stomach contents, Nick has confirmed the juvenile sharks’ diet consists predominantly of fish, including Australian salmon, snapper, and the occasional stingray. Due to the high abundance of these prey species in the nursery area, this may be the important piece of evidence that proves the sharks visit the nursery area to feed. The diet of these sharks doesn’t expand to include mammals, until they reach around 3m in size, so it’s unlikely that the juveniles are implicated in attacks on people.
Risk of shark attack
With so many sharks near popular swimming beaches, interactions between sharks and people are inevitable, however. But bites from juveniles are rare, and only two recent cases have occurred around the nursery area – both within Port Stephens.
Researchers at the University of New South Wales have found one reason why bites are rare. PhD student Toni Ferrara used three-dimensional computer models to examine the feeding behaviour of juvenile great whites. The research reveals that their jaws are too weak to capture and kill large prey.
“We were surprised that although the teeth and jaws of our juvenile sharks looked the part and the muscles were there to drive them, the jaws themselves just couldn’t handle the stress associated with big bites on big prey,” says study co-author Dr Stephen Wroe. Until great whites reach about 3m there is insufficient mineralised cartilage in the jaws to resist the forces involved in killing large prey, such as seals and sea lions – or people.
“This study also explains why many shark attacks off NSW by great whites are aborted after a single bite, as those involved are usually juveniles who may sustain jaw injury if they persevere with the attack,” says another study co-author, Dr Vic Peddemors.
Conservation and study around this juvenile shark créche is vital – knowledge from the nursery could help hint at the health of future adult populations, say experts. Barry plans to get the most out of his research, by returning to the site in 2012, he says. “We’ve already made significant progress, but there are still some big pieces missing from the great white shark puzzle.”
Source: Australian Geographic Nov – Dec 2011