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Grey Nurse Sharks

By Iain Anderson

Grey Nurse Sharks

Grey Nurse Sharks

With its long and pointed teeth the Australian grey nurse shark

(Charcharius taurus)

is a mean and fierce looking creature. For many years humans feared it as a potential man-eater and this encouraged intensive fishing and spearfishing during the 1950s and 60s, resulting in vastly depleted numbers of this shark on Australia’s East coast. Their fierce reputation has proven to be undeserved. Australian diving colleagues refer to them as the labradors of the sea. Swimming and diving with these graceful and gentle giants, that can grow to 3.6m in length, is a rewarding activity and one of the many high points of diving on the New South Wales Coast.

I had a first time meeting with the sharks near the coastal village of South West Rocks; a day’s drive north of Sydney. Local dive guides took us on a 20-minute trip to a small island very near the coast called Fish Rock. This was an exciting dive for it began with a trip through a cave that was about 125 metres long. We entered the cave at a depth of 24 metres. I slowly finned my way several metres up a narrow shaft and then down into a large gallery. In the distance, dozens of metres away I could see the ghostly blue-lit opening to the cave – guarded by several large sharks. As I got closer I realized that there were many more, at least a few dozen swimming nearby. I settled down at the cave mouth and reached out my hand to position myself for some photography. While doing this I touched something soft that also moved under my right hand. This was a very large wobbegong (almost three metres long). These carpet sharks are docile but also dangerous and will bite if annoyed by divers. I counted myself lucky that it swam away and left me alone. I carefully surveyed the area to see if there were any more. This was wise for the cave opening was literally carpeted in carpet sharks and beneath my left hand was another large wobbegong. Above us the circuit-swimming nurse sharks swam one behind the other up to the cave opening, where they turned a sharp 180° to return to open water beyond the limit of visibility. Behind and above the sharks there was a large school of fish, perhaps a source of food for the sharks. The sharks were largely unperturbed by our presence, some swimming to within a metre of me before turning. Out over the reef away from the cave I noticed that some of the sharks were swimming quite slow, almost hovering above the bottom in the slow current. Like other sharks, grey nurses do not possess a swim bladder and will sink unless they swim. However, these gentle giants can improve their buoyancy, perhaps by swallowing air. The editor of this magazine (Dave Moran), who has witnessed the collection of sharks for aquaria, told me how divers gently squeezed them, to burp air from the captured sharks on the way to the surface.

The giants came so close to me that I could see a hairy growth on some of their teeth that resembled hydroids. Hydroids are commonly seen growing on kelp and rocks but seem a little out of place on teeth. Clearly the sharks were in need of a good tooth brushing! Surely the act of eating would wipe the teeth clean – unless they were fasting. According to one source, females stop feeding during pregnancy and this might be responsible for the growth of hydroids on teeth. I wonder how long they stop eating during pregnancy given that gestation takes 9-12 months. The shark I photographed was probably a male! A view of its underside shows structures that resemble ‘male claspers’. Perhaps males fast too – when they are preoccupied with more important matters concerning the opposite sex!

Grey nurse sharks give birth to two pups. Females have two uteruses within which the young are hatched (at 55mm long). When they grow big enough (~100mm) the embryos develop teeth and cannibalize their brothers and sisters and unfertilized eggs until at most only two survive to be born.

Grey nurse sharks can be seen at a number of sites along the NSW coastline and inhabit a depth range from the surf zone down to 190 metres. I had further encounters with them at Broughton Island near Nelson Bay. Our guide led us single file along one of the steep walls of a canyon. Alongside us large nurse sharks swam in the channel. The geography of the dive site and the close proximity of large sharks made it an exciting dive.

This species also lives at other locations including the coasts of North and South America, South Africa and Western Australia. The tendency for a number of individuals to congregate at certain locations on a reef suggests that some sharks might not stray too far from their home. Thus there is a real danger of them going locally extinct and for this reason they have been officially declared critically endangered on Australia’s east coast and vulnerable to extinction on the west coast. There might be only 500 individuals left in East Australian waters and genetic evidence supports the view that they don’t mix with their West Aussie cousins. There has been some dispute in the media about how accurate the population estimates are. Some suggest that scientists are underestimating shark numbers. On this point I think it is important to put the size of this shark population into perspective. Even if the scientists are out by 100% in their estimates the population for this species is pretty small compared with the Aussie human population, which must number well over 10,000 humans per shark! Lets stop quibbling over numbers and save these gentle giants