As part of a research project for studying the ecology of the Great Barrier Reef, researchers from James Cook University (JCU) and the Australian Institute of Marine Science have tracked bull sharks from their habitat on the reef for hundreds of kilometres down the East coast of Australia.
“In the context of this project they were just one of the sharks that appear on the Great Barrier Reef,” explained co-author of the study, shark ecologist Professor Colin Simpfendorfer from JCU. “Being an important shark on the reef meant they're important for us to understand the dynamics of that system.”
New knowledge on a classic marine predator
Bull sharks (Carcharhinus leucas) are quite possibly the prototypical animal we associate with the idea of a shark. At adulthood they reach 2-3.4 metres in length, and have often been quoted as some of the most dangerous sharks in the world.
Because they are a wide-ranging predator, bull sharks can come near coastal areas where they have interactions with humans, potentially leading to injury. As the researchers were studying the Great Barrier Reef, they were surprised to find a large population of bull sharks there.
Despite the widespread occurrence of bull sharks in the world’s oceans, there is limited knowledge of their movements and behavioural patterns. Recent studies have revealed complex migration patterns, and this Australian study is the latest work contributing to that research.
The researchers captured 33 adult bull sharks at various reefs in the Great Barrier Reef area, and equipped them with harmless 5-centimetre long acoustic transmitters, using a well-tested and safe procedure.
“Essentially we fished them out of the reefs with heavy gear,” said Prof Simpfendorfer. “We'd bring them up to the boat and surgically implant an acoustic transmitter in the body cavity, quickly stitch them up and release.”
“The tag transmits a signal every couple of minutes - we have equipment moored all along the East coast of Australia that listens to these specific tags, and we can reconstruct where they've moved based on the actions of these animals.”
Bull sharks not the meanies they're made out to be
Unlike GPS, where you can potentially track a signal in real time, data from these acoustic tags was collected over a longer period, by detecting when animals with tags swam past the tracking equipment. Overall sharks were monitored between February 2012 and October 2014, and the results were published on Thursday in the open-access journal PLOS ONE.
The scientists were surprised to discover that adult bull sharks are very common in the Great Barrier Reef throughout the year - it was previously thought these animals mainly forage in coastal areas, rather than spend time offshore.
Female sharks would also travel as far as 1,400 kilometres to give birth in the New South Wales coastal area, before returning to the Great Barrier Reef. The migration pattern was seasonal, with most sharks making an appearance in the summer months, between September and December.
All this knowledge is valuable for marine conservation purposes, explained Prof Simpfendorfer.
“Making sure we understand the ecology of these animals - how they move, where they go, the habitats that are important to them - helps us understand how these ecosystems function and make sure that we can manage them to the best of our ability.”
Even though bull sharks are feared and have a reputation for being aggressive and grumpy, Prof Simpfendorfer disagrees.
“We actually found them very easy to work with. They're very calm, and I think they get a very bad rep,” he said. “They do occur in areas that bring them in contact with people much more than most other shark species, and that's where some of that bad reputation comes from.”
“Bull sharks need to be respected as the wild animals they are, but they should be enjoyed and observed, and people should not get too worried about them.”