Crowds of metre-long sharks lie motionless on the seafloor within gutters etched into the rocky reef. Some are piled up under overhangs, others are lying isolated and out in the open. Beneath a waving frond of kelp, one of them raises its head and pivots upward on a pair of pectoral fins. With a few beats of its club-like tail it propels forward revealing a pencil-sized cylindrical device attached to its flank.
A diver motions forward to grab the shark. He catches it by placing one hand on the back of its head, and the other around the tail. It struggles and bucks back and fourth, before finally admitting defeat and allowing the diver to swim it up to the small canoe waiting above.
“Yes, this is the recapture we are after!” the diver exclaims, as he hits the surface. This is Nathan Bass, a PhD candidate with Macquarie University, who is studying the social networks of Port Jackson sharks in New South Wales, Australia.
He passes the shark up to the team in the canoe. They work methodically to gather as much data as they can in as a short a time as possible, to limit any undue stress on the animal.
The shark is measured from the tip of the snout to the upper lobe of its dorsal fin before being placed gently in a bag and hoisted into the air to be weighed – the average size of a male Port Jackson is 95 centimetres and six kilos, while the females are slightly larger at 120 centimetres and 14 kilos.
The team collect a tissue sample for DNA analysis, and record all the data and the unique identification number on the orange tag attached to its tail. Finally, the receiving device that drew Nathan to this particular Port Jackson is removed, and the shark is released back into the sea at the survey site.
Each interaction with these gentle sharks occurs with the utmost care. The team works within the constraints of strict animal care and ethics guidelines to ensure minimal impact on the shark during the encounter. Each step of the scientific process is calculated and performed by researchers with a passion for shark conservation.
HANDS ON IN AN UNDERWATER HEAVEN
It’s Nathan’s second winter studying the sharks, which are endemic to southern Australia. Like all sharks of the primitive order Heterodomtiformes, or horn sharks, the Port Jackson has a blunt-shaped head and spines on the leading edge of the two dorsal fins. Yet its unique colour pattern is distinguishing and intricate. Dark harness-like markings cover the eyes, run along the back and cross over along the side of the body.
They gather in numbers off New South Wales between June and September, during which time Nathan divides his time between two study locations in Jervis Bay and off Sydney.
The sharks are a popular favourite amongst avid groups of divers who are also drawn to the same temperate reefs along the popular southern New South Wales coastal fringe. Peppering the reefs throughout their distribution, some of the key locations for encountering this species include Forster and Seal Rocks, Fingal Island off Port Stephens, Bare Island off Sydney and throughout Jervis Bay.
The diving in Jervis Bay has been rated amongst the best in Australia. Shallow seagrass meadows provide a haven for a diverse suite of colourful characters such a weedy seadragons and pyjama squid. The kelp-dominated, shallow, rocky reef gives way to boulder fields encrusted in sea tulips, sponges and ascidians. Large blue groupers, schools of old wives and inquisitive giant cuttlefish are regulars, whilst red Indian and blue devilfish are some of the hidden gems to be found within the Port Jackson’s domain.
During one exploratory dive in Jervis Bay, Nathan recalls discovering a huge aggregation of Port Jackson sharks off the southern side of the bay. “There were so many sharks there it was unbelievable. Hundreds of them were all sitting there on the seafloor in small groups. It was a remarkable encounter for our team,” Nathan says.
Eager to assist with the research is an enthusiastic troop of volunteers hoping to get hands-on with a wild shark. Many of them have their own marine post-graduate research projects, while others are just keen to get involved. Once properly trained, the volunteers have the opportunity to participate in all aspects of the fieldwork.
It’s the perfect project for budding marine scientists to hone their diving and field skills, which can then applied to their own projects. The volunteers spend a lot of time in the water, making up to four dives a day at the study sites, and sometimes further afield during exploratory excursions.
Jo’s own PhD examined the social networks of dolphins. “These two projects are quite similar in that they are both asking questions about the social relationships within a population and the factors that influence those relationships,” she says.
“It’s an amazing experience,” says Jo Wiszniewski the Research and Conservation coordinator for the Taronga Conservation Society Australia and co-supervisor of this project. “All the sharks have different personalities, some come quietly to the surface, while others can be feisty and whip around,” she says.
“We knew that social relationships existed amongst the dolphins, so we were looking more at the long-term changes and the stability of those relationships,” says Jo. “Because dolphins are well studied, we then wanted to move on to sharks, as there has been limited research in this field.”
“We are trying to demonstrate how the sharks interact,” Nathan says. “The social structure of a population is closely connected to their behaviour, ecology, and evolution.”
To do so, the team are using proximity receivers, a new, acoustic technology that records and stores information about encounters with other acoustically tagged sharks. By recording the date, time and an identification number, research on interactions can take place without human interference, and over a continuous, extended period of time. For this relatively small and slow-moving species the recording distance is set at four body lengths (four metres), but it is possible to adjust the distance at which proximal receivers are detected and information is exchanged, to suit larger and more mobile species.