Beneath the Surface: There's more to the Grey Nurse shark than it's scary appearance

It's an impressive form, even by shark standards.

It's big, thick-bodied yet sleek and tapering from the nose and to the tail, an elegant spindle.

Our group of scuba divers kept as still as possible.

The cloud of sweepers just eased apart in the middle, like curtains framing the rock gully.

And slowly, smoothly, with the vaguest sweep of that long tail, the front of the shark pushed through.

The cone snout, the small, pale eye watching as it slid past, and the mouth, bursting with row after row of slender, curved teeth. The classic fly-by, perfect for a photographer. Gorgeous.

We call them Grey Nurse sharks although these sharks are bronze to reddish brown, certainly far less grey than many other sharks, and they are not even vaguely related to the nurse shark family.

Americans call them Sandtigers, but they're nothing like the unrelated Tiger Shark in either form or habits, and they don't have stripes as several sharks do.

They look wicked, built how a scary predatory shark is supposed to look, that nightmarish mouth bursting with sharp pointy death - yet they are widely known to be extremely docile and laid-back and easy to interact with as a scuba diver.

Most shark species are fussy eaters of a surprisingly small range of smallish fishes or squids.

Grey Nurse sharks though, eat both larger prey and a much wider variety than most sharks.

They don't seem to often if ever try to attack the sweepers that shoal here at North Rock, partly because they are too small or too agile, and partly because the sharks seem to eat less and rest more when we see them in these shallow sites during the day.

They generally forage a little more at night, heading offshore and a little deeper to do so.

The few scuba divers visiting these sharks at night describe them as very different animals than by day.

They also become perhaps a little less docile during breeding season, when males get a bit more aggressive and almost territorial around females.

But in most circumstances these sharks seem to be just about the most inoffensive of any hundred kilo shark.

We could see some sharks trailing hooks and line, not always from the mouth.

Observations from local divers and researchers suggest it may only take a small hook lodged in their throat for at least some sharks to get septicemia, stop eating and eventually starve to death.

Whether intentionally or by accident, fishing has had a big impact on its numbers in Australia.

We don't really have a good idea of what the natural populations were, but today they are listed as critically endangered.

Grey Nurse sharks seem to mature at about the size of a large man.

These are big sharks and the biggest of them can be seen at Wolf Rock off Rainbow Beach, Queensland.

Grey Nurses are infamous for uterine cannibalism.

The first youngsters hatching inside the mother will eat their siblings and slower-developing eggs.

Nine months or so after mating, two survivors are normally born.

Because they are quite mature before having any young and then have at most only two young per year, recovery rates and population growth are slow even by shark standards.

Evolutionarily, they're odd beasts.

They are lamniform sharks, which means they're closely related to only a handful of unusual large sharks, including the Basking shark, Megamouth shark and the warm-blooded predatory Makos and Great Whites.

Not really built like any of their cousins, if anything Grey Nurses are most similar in body shape to the unrelated Lemon sharks.

They're rather sedentary compared to their closer cousins, and the flat tailstock and long upper tail hint to animals that aren't built to travel either particularly far or particularly fast.

They are also more sociable than most large sharks, these sharks are usually encountered in groups, spending time at a handful of sites in each area they occur, apparently at least partly segregated by gender.

These sharks seem to change their preferred socialising areas over time.

Aggregations of large predators inevitably have an impact on the local food resources, and it makes sense for animals foraging this high in the food web to shift around periodically.

Eastern Head off Broughton Island was their preference in 2005 before they moved a short distance to Elephant Rock for five years before relocating again to nearby North Rock where they still remain.

Protected areas need to be revised from time to reflect these movements.

We'll be back, and the diving community here keeps a concerned eye on the numbers from season to season.

For now, though, after a very pleasant hour drifting about the canyon with these big, slightly dopey sharks and through the clouds of sweepers, it's time to head up and leave them to their down time.

Local dive centres such as Feet First and Let's Go Adventures run regular dive trips to North Rock.

According to the NSW Department of Primary Industries this is due to:

Malcolm Nobbs is a recreational diver and photographer from Nelson Bay. He and UK marine ecologist Jamie Watts collaborate regularly on marine life articles.