Tracking the predators of the Bight
Privacy is a thing of the past, even in the deep waters of the Great Australian Bight. Pygmy blue whales, white sharks, sea lions, southern bluefin tuna—details of their lives are being tracked and monitored.
Stretching from Cape Otway in Victoria to Albany in Western Australia, the Bight is thought to contain the greatest biomass of apex predators in all of Australia’s coastal marine waters, many of them migratory. Little has been known about the Bight but, now, following four years of research, one of the world’s largest tracking databases is revealing when these predators come and go, where they like to feed and breed, and whether their numbers are growing or declining.
The research was conducted as part of the $20 million Great Australian Bight Research Program—a collaboration between BP, CSIRO, the South Australian Research and Development Institute (SARDI), the University of Adelaide and Flinders University.
Professor Simon Goldsworthy, a principal scientist with SARDI, co-led the program’s research on apex predators and iconic species: “At SARDI, we’ve tracked and surveyed many species over the last 15 years,” he says. “We knew the region was important for them; what we wanted to understand was why.”
Cetacean sights and sounds
In the largest ever survey of cetaceans in the Bight, the research theme led by Simon and Dr Campbell Davies from the CSIRO conducted aerial surveys and ship-based acoustic surveys of whales and dolphins. Based on surveys from a fixed-wing plane, team members from Flinders University estimated that the area between Ceduna and Coffin Bay contains about 20,000 common dolphins. On offshore aerial surveys conducted along the shelf slope of the eastern Bight conducted by the Blue Whale Study Group recorded 58 cetacean sightings, including pygmy blue, fin, sperm, pilot, killer whales; and Risso’s, common and bottlenose dolphins.
In the first ever systematic acoustic survey of toothed whales in the waters of the central Bight, SARDI researchers spent a week at sea up to 150 km offshore of the shelf break in harsh conditions—the Bight has some of the biggest swell in the world. They recorded whale sounds with a hydrophone, to determine the diversity of species and their relative abundance.
“Toothed whales, such as sperm whales, use sonar to locate prey, so when they’re diving, which can last up to an hour, they’re continually producing loud clicks,” explains Simon Goldsworthy. “This means we can detect them acoustically, even though they are not visible at the surface. We can also identify different species of whales from acoustic recordings based on their different vocalisations.”
Hands on with sharks
Tracking sharks was a bit more ‘hands on’. Travelling offshore to the submarine canyons and continental shelf slope in the eastern and central Bight, SARDI researchers attached satellite tags to a range of pelagic (oceanic) species, including blue shark, shortfin mako, bigeye thresher and white sharks. Using a longline with circle hooks to catch the sharks, they first removed the hook before attaching the tag to the dorsal fin—except for the white sharks, which were tagged as they swam freely beside the ship.
The data collected shows that these highly mobile species spend a lot of time in the Bight, says Simon: “They do long, loopy migrations to the tropical Indian Ocean and Subtropical Front [south of the Bight], but most return to the Bight to forage for months each year.”
Fur seals up, sea lions slide
Sealing was Australia’s first commercial industry and in the early 1800s seals were indiscriminately hunted, resulting in a massive reduction in numbers and contraction in range, with local extinctions. Numbers remained low for the next 150 years until they were protected in the 1970s.
The long-nosed fur seal has made a huge recovery over the past 30 years and is once again abundant. The research found that 98 per cent of Australia’s population of about 100,000 long-nosed fur seals live in the Bight, following new surveys and the first ever comprehensive synthesis of data on pinnipeds in the Bight. To estimate the population of seals, SARDI researchers visited each breeding site to count the pups, which remain ashore after the breeding season.
It’s a very different story for the charismatic and endangered Australian sea lion which occurs only in South Australia and Western Australia—the total population is relatively small at about 10,000, and the research found that almost all of them (93 per cent) live in the Bight.
The population has been declining over the past few decades and this latest study found no change in that downward trend. Simon’s team estimates that populations are declining by about 3 per cent per year, with almost 40 per cent of individual breeding sites meeting the ‘Critically endangered’ criteria.
Big data reveals the secrets
All of the tracking data—including about 5000 individual tracks collected for nine important species over 15 years—has now been compiled into what is one of the world’s largest tracking databases.
Along with the data for the nine species—the Australian sea lion, Australian fur seal, long-nosed fur seal, little penguin, short-tailed shearwater, white shark, blue shark, shortfin mako and southern bluefin tuna—the database includes location data for the pygmy blue whale and the sperm whale, based on aerial survey and historic whaling data.
“Integrating the data for all species was an amazing opportunity,” says Simon. “For the first time, we have integrated tracking and distribution data for apex predators in the Bight. We are able to develop maps of distribution and habitat for many species, and identify important areas of shared use. We can also identify and resolve overlap between species and human activities, which will improve our capacity to manage any potential impacts.”
What may seem like an invasion of privacy has increased our knowledge so that future management of the Bight can be informed by the science of the Bight.