Australian white sharks follow pathways etched in the seafloor
The granite bluffs of Neptune Islands stand just outside the mouth of Spencer Gulf, lashed by some of the wildest waves in South Australia. Early mariners bound for the Port of Adelaide came upon them by accident, until the South Neptune lighthouse beamed its first warning in 1901.
These four small islands are part of the Neptune Islands Group (Ron and Valerie Taylor) Marine Park. Breeding colonies of sea lions, fur seals, seabirds and shorebirds line their cliffs and coves, and the waters abound with abalone, rock lobster, sardines, snapper, tuna . . . and sharks.
This is prime habitat for white sharks. You might expect them to be residents, but actually they’re just visiting. They belong to Australia’s southern-western white shark population, and after a short stopover they may head for western Victoria or the North West Shelf, or possibly out to sea.
On the trail of white sharks
White shark tagging at The Neptunes helped scientists draw the boundary between Australia’s two white shark populations. The breakthrough in understanding led to the first reliable estimates of white shark population abundance, and the ability to monitor the rise or fall in numbers. This is essential to evaluating the national recovery plan for white sharks, which are listed as Vulnerable under Australia’s Environmental Protection and Biodiversity Conservation Act 1999.
Now the tagging program has mapped less conspicuous, more remote white shark habitat, and pathways that connect them. A paper published earlier this month in Marine Biology presents the combined tracks of 40 white sharks tagged over 15 years (2003–2017) at The Neptunes and at Doubtful Islands near Bremer Bay, Western Australia.
Several of the sharks were tagged by the paper’s lead author, Russ Bradford, a Senior Experimental Scientist at CSIRO. Russ tagged his first white shark at The Neptunes in 2003 alongside his colleague and mentor Barry Bruce.
“It was an exciting time because marine science was taking the next big leap in technology with the wider use of electronic tags,” Russ says. “We tagged a 3.8 metre female nicknamed Columba. I remember having to stretch out across her back to reach her dorsal fin and being amazed at how girthy she was.”
On the move
The pop-up satellite archival tags and satellite-linked radio tags that contributed to this study collected 3,663 days and 109,900 km of tracking data, for individual swims of up to 381 days. The tracks provide the best picture yet of how white sharks use coastal and ocean habitat off southern and western Australia.
Twenty-nine of the sharks covered more than 1000 kilometres each while tagged, and the females in particular ventured further offshore than has previously been recorded in Australia. The females also covered broader longitudinal ranges, dived deeper – sometimes beyond 1000 metres – and tended to occupy slightly cooler waters.
“We weren’t surprised by the differences because we have seen sex-differentiated behaviour in earlier research at The Neptunes, with females visiting almost exclusively during winter, and males visiting year-round,” Russ says.
“The use of different habitats by males and females may be a mechanism to maintain sexual segregation during spring–summer, when the sharks are away from important aggregation sites.
“The reason for sexual segregation is unknown. It could be because of competition, it could be different energy requirements, or could be that pregnant females don’t want to be harassed by the males.”
Two of the tagged adult females took epic offshore excursions, during which they spent time above major seafloor features.
One followed the shelf slope edge westwards from the Neptune Islands to the Recherche Archipelago off Western Australia in winter–spring, headed 1700 km south to the Heemskerk Fracture Zone, then west parallel to the Southeast Indian Ridge for 1000 km.
The other covered 12,240 km in five months, going eastwards from South Australia into the central Tasman Sea in summer, then south to sub-Antarctic waters off Macquarie Island in the Southwestern Pacific Ocean, before returning via the Ninene Trough south-east of Tasmania. This is the first record of a white shark visiting Macquarie Island, although their presence had been suspected from wounds observed on sea lions and fur seals.
Similar journeys have been recorded in New Zealand and the mid-Pacific, but the attraction of the fracture zones is unknown. It’s possible that the sharks use the distinctive magnetic fields to navigate to foraging areas such as canyons and seamounts. Sharks of both sexes made autumn–winter visits to The Neptunes and to productive canyons in the eastern Great Australian Bight such as the Murray Canyons Group south of Kangaroo Island.
Collective effort to guide conservation
Many strands of research came together to make this study possible, from protocols developed by Russ and Barry for catching and handling white sharks, to advances in interpreting location data. Also essential is the network of researchers who collaborate to address components of the national recovery plan.
“Given the time, expense and technical challenges involved in white shark research, it would be very difficult to gain traction without this collaboration,” Russ says. “In this study we worked with the South Australian Research and Development Institute; the Department of Primary Industries and Regional Development, Western Australia; Flinders University; Fox Shark Research Foundation; and Kent Stannard (Tag for Life).”
The study’s findings add to the body of knowledge that supports effective conservation management policy, ecological risk assessments for fisheries, monitoring and management of state and national marine protected areas, and the identification of potential risks of human-shark interactions.
“We still have so much to learn about this (southern-western) population,” Russ says. “Our abundance estimate for the adult population is 2,500, which is few and far between when you consider the vast territory these sharks occupy.
“To count the full population, we need to tag and genetically sample more juveniles. This will give us survival rates and family relationships required to build a population model, and ultimately the monitoring capacity to assess the efficacy of recovery actions.
“My present focus is on finding where pupping occurs. We have tagged mature females and juveniles to try to get them to lead us to those areas. Unfortunately, we have not pinpointed a specific region.”
This research was supported by the Australian Government’s National Environmental Science Program Marine Biodiversity Hub and CSIRO.
January 10, 2021 at 11:43 pm