Why are hundreds of these Port Jackson sharks hanging together?

Neville Barrett has spent over 20 years studying Australia's reef systems, and yet the seasoned researcher is still frequently puzzled by what he finds hidden beneath the surface. The Commonwealth waters surrounding the continent now contain 40 new marine reserves, and many of these remain largely unexplored. During a recent expedition to one such swathe of protected ocean, Barrett and his colleagues encountered a veritable horde of Port Jackson sharks. The species has been documented in the region before, but this particular excursion turned up hundreds of individuals, gathered together atop a single rocky reef.

We tend to think of sharks as lone giants that scour the seas on an endless mission to gobble up formidable prey. In reality, however, the apex predator of "Air Jaws" fame is the odd "man" out in the shark world. There are over 500 known shark species, and most of them are small, unassuming and often left out of the limelight – but just as deserving of our attention. Recent studies on Port Jackson sharks, for example, suggest that these nocturnal bottom-dwellers display complex social dynamics.

"The most surprising thing about this sighting was the actual abundance of the sharks in this aggregation," says Barrett, who is a research fellow with the University of Tasmania Institute of Marine and Antarctic Studies (IMAS). "We often see small aggregations when diving, up to eight or so, but this one was clearly in the hundreds at a minimum, and potentially much greater if we could have spent time to survey it properly."

Barrett and his team weren't actually looking for sharks: their unexpected encounter occurred during a ten-day cruise to survey the reefs at Beagle Marine Park (formerly the Beagle Commonwealth Marine Reserve). This 3,000-square-kilometre area has been protected for a decade, but its remote location in Bass Strait, which separates Tasmania from the Australian mainland, has made studying the reserve a challenge over the years.

"There are more than 40 reserves in the new Australian Marine Park network," explains Barrett. "They are all in Commonwealth waters, which is a minimum of three [nautical miles] offshore."

Beagle lies about a day's travel from the nearest port – amazingly, it's one of the easier reserves to reach – but expeditions of this nature still require multiple days on the water. What's more, the seabed in the reserve reaches 70 metres deep.

Using an autonomous underwater vehicle (think the aquatic equivalent of the Mars Rover), the team was able to capture three-dimensional photographic maps of the reserve – and the Port Jackson shark party was only one of many interesting observations.

"A big part of what we are doing is going around the new marine parks one by one and documenting what is actually in them, as at present we don't have a clue!" says Barrett. "We have only recently gotten the technology to do this effectively. It's a big job, one step at a time."

Shark aggregations of this size are typically associated with mating, but Port Jackson sharks are known to lay their eggs nearly 600 miles north of Bass Strait, off the coast of New South Wales (NSW). Between breeding seasons, the animals do migrate to Tasmania, though they return to the same NSW reefs each year with incredible accuracy.

The Bass Strait footage was recorded in July, while breeding typically occurs between August and October. This has left Barrett wondering whether the mass gathering could be a sign that the sharks kick off their mating routine in southern waters.

"At such a size, it does suggest that this may be a mating aggregation, but I'm no expert of Port Jackson shark biology," he says. "If it isn't a mating aggregation, it could well be that the sharks are in good winter foraging grounds, and are using this reef as a resting refuge where they have some protection, shelter from currents, and safety in numbers against possible predators, including seals that are common in the area."

Macquarie University professor Dr Culum Brown, who runs a long-term monitoring programme of Port Jackson sharks in New South Wales, suspects the latter may be true, and offers another possibility.

"My feeling is that it is unlikely to be a breeding aggregation," he says. "It could be a mustering place prior to migration." A pre-coitus caucus seems to line up with what experts have learned about the preferred travel routes of this species. After breeding, sharks tagged by Brown and his colleagues in Sydney Harbour and Jervis Bay migrated to the Wilsons Promontory peninsula at the southern tip of the mainland, and even farther, to Bass Strait's Barron Island.

"We know very little about the southern populations," notes Brown, "[but] there is every chance that [the sharks in the footage] were about to migrate north. The timing is about right."

Both male and female Pork Jackson sharks make annual migrations to their egg-laying sites, a rarity among sharks. What's more, their individual positions within these gatherings – and which sharks they join up with – seem to remain relatively consistent from year to year. This doesn't mean Port Jackson sharks seek out former flings or finned best friends, but they do appear to hang with "familiar" faces in some capacity.

The animals often congregate in small groups to rest during daylight hours, repeatedly sharing refuges with the same individuals.

The sharks in Barrett's video, however, don't appear particularly "jiggy"– as we've witnessed before, shark sex can be a bit rough – which also indicates that the aggregation he witnessed served some other purpose.

Unlike sprawling barrier reefs, the sponge-littered rocky reef at Beagle is both relatively isolated and bordered by steep drop-offs. It makes for a good stopover, one that would allow the sharks to rest and feed on tasty invertebrates while being sheltered from strong currents.

When it comes to shark jaws, Port Jacksons rock one of the weirdest. Their signature curled snout (there's a reason these animals are also known as "pig-nosed" sharks) conceals dozens of tiny, pointed teeth and a crushing plate used to break into hard prey like urchins, molluscs and crustaceans.

Image: Richard Ling/Flickr

The recent sighting has also piqued the interest of Macquarie University behavioural ecologist Dr Johann Mourier, who has been studying Port Jackson sharks in the region along with Dr Brown.

"From our previous knowledge, it is surprising, as such aggregations have been seen on northern locations. But sharks always surprise us and there're still much to learn about them," he says. "Whether they form here for breeding or to initiate a synchronous migration, a social signal or social-learning process is required for sharks to gather at a same location."

Through their work at the New South Wales breeding grounds, the Macquarie team has become fascinated by Port Jackson social networks.

While we may never know precisely why the Port Jackson sharks got sociable at Beagle, Barrett and his team plan to return to the area next year for a larger-scale biodiversity survey with Parks Australia. The possibility of spotting a similar aggregation again – if the fish return – depends largely on timing.

Exploring these reserves, meanwhile, will help local wildlife authorities to identify key points for long-term monitoring – which means more interesting discoveries are undoubtedly in store.