Shark communication: Port Jackson case study
THE SHARKS LIE motionless in gutters etched into the floor of the rocky reef. Some are piled under overhangs, others rest in the open. Beneath a waving frond of kelp, an individual about 1m long raises its head. With a few beats of its tail it propels itself upwards to reveal a pencil-sized cylindrical device attached to its flank.
Quickly and purposefully, a diver fins forward to grab the shark, which, typical of its species, has a harness-like patterning on its back, a blunt head and spines on the leading edge of its two dorsal fins. He catches it by placing one hand on the back of its head and the other around the tail. It struggles and twists before it finally accepts defeat and allows the diver to guide it to the canoe 6m above.
“This is the recapture we were after!” yells Nathan Bass as he passes the shark to the team in the canoe. Nathan is a Macquarie University PhD candidate studying the social networks of Port Jackson sharks.
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Testing shark DNA
The team then gathers data; they’ll keep the animal for 10 minutes. They measure its length and put it in a bag to hoist it on a field scale for weighing. On average, male Port Jacksons are 95cm in length and 6kg in weight; females typically measure 102cm and 14kg.
A tissue sample is collected for DNA analysis and the shark’s identification number is read from the orange tag on its tail – she is a 110cm female. Finally, they remove the receiving device that drew Nathan to this shark and the fish is released at the Dent Rock survey site, about 3.5km south-east of Huskisson, NSW.
The device – a marvel of acoustic technology – records and stores information about a shark’s encounters with other sharks. When two acoustically tagged sharks come within a set distance of each other, their devices record the date, time and the other shark’s ID.
For Port Jackson sharks – a relatively small and slow-moving species – the distance is set at 4m, but it’s possible to increase the distance to suit larger or more mobile species. Four times the length of the shark is the rule of thumb for such studies, so a 12m whale shark would require a 48m recording distance.
“We are trying to learn how the sharks interact,” Nathan says. “The social structure of a population is closely connected to their behaviour, ecology and evolution.” By combining the proximity data with other tried and tested acoustic technologies – such as continuous acoustic tags together with receiving stations placed strategically around the survey sites – Nathan aims to demonstrate that the sharks form social relationships and to find out why they do so.
Studying Port Jackson’s sharks
It’s Nathans’s second winter studying the sharks, which are endemic to southern Australia and gather in NSW waters between June and September. His fieldwork is carried out at three study sites: Dent Rock, in Jervis Bay, on the NSW south coast; and two in Sydney, Bare Island in Botany Bay and Oak Park at Cronulla.
Port Jackson sharks are very suitable for trialling the receivers: “There are plenty of them, they are easy to handle and really hardy,” Nathan says.
Working with the sharks is “an amazing experience,” says Jo Wiszniewski, who is Nathan’s co-supervisor and a researcher with Taronga Conservation Society Australia, based in Sydney. “They all have different personalities, some come quietly to the surface, while others are feisty and whip around,” she says.
Jo has studied dolphin social networks and brings some of that methodology to the shark study. But to document dolphin interaction, she used photo and tissue sample IDs.
Preliminary results from the first year of the shark study suggest complex social behaviour and that individuals form stable bonds. During the breeding season, they have a limited home range and tend to group by size rather than gender. Much like some Saturday nights on the town, female sharks tend to interact with each other and avoid males, but males direct their attention towards females. Nathan says this suggests the males are looking to breed and females are trying to avoid harassment.
The major aim of the first year was to test the receivers, which proved effective. “A key potential of using this technology is to determine the connectivity between social structure, segregation and habitat use, which can ultimately contribute to their management,” says Nathan. In general, there’s a lack of knowledge about shark biology and behaviour.
Port Jacksons tend to forage at night when their prey – sea urchins, molluscs, crustaceans and fish – are active. Unlike other species, they can pump water into one gill slit and out through its other four and do not need to move to breathe. This allows them to lie on the sea bottom for a long time – as they do at breeding time.
Port Jackson sharks: common bycatch
Although Port Jackson sharks are not targeted by commercial and recreational fishermen, they are a common bycatch. They’re abundant and are “least concern” on the IUCN Red List, but habitat destruction and their habit of gathering in groups puts them at risk.
All shark species are susceptible to overfishing due to their slow growth, late maturity and few offspring. And sharks have a top-down influence on ecosystems, so overfishing can affect other species’ survival.
“By understanding the formation of shark aggregations and shark society, we’ll learn how to better manage shark populations and maintain healthy marine ecosystems,” Nathan says.
The full story can be found in Australian Geographic #117.