Every year, Port Jackson sharks find their way home after a mammoth post-winter migration. Sherrie Chambers is piecing together how they do it.
Ethograms is a monthly column published in collaboration with the Australasian Society for the Study of Animal Behaviour (ASSAB), showcasing the work of early-career researchers. Andrew Katsis is a PhD candidate at Deakin University, and an outreach officer for ASSAB.
Port Jackson sharks aren’t particularly large—a bit over a metre in length—but at the end of each winter they undertake an impressive migration. Sharks at Jervis Bay in New South Wales travel down the mainland coast, swimming as far south as Tasmania’s Cape Barren Island, more than 600km away. Nine months later, they return to Jervis Bay to breed.
Sherrie Chambers, a PhD student at the Behaviour, Ecology and Evolution of Fishes Lab at Macquarie University, is trying to understand how these sharks find their way home. “They come back to exactly the same reef year after year after year,” she says, “so there’s got to be some way by which they can identify which reef they’re going to, and how to get there”.
Chambers has been studying the Port Jackson shark for over a year now. The species is a benthic predator, spending the bulk of its time near the ocean floor, where it uses its flat, grinding back teeth to feed on molluscs and crustaceans. Like many animals, individual Port Jacksons have distinct and consistent personalities. They are also a common, recognisable species in an otherwise understudied category of sharks, which makes them ripe for research.
“I think it’s really important that we get a better picture of what benthic sharks are doing—a lot of them are endangered or relevant to fisheries,” says Chambers. “And Port Jackson sharks make a really good model species, because they’re highly abundant, super easy to catch, and we get to go to some really awesome field sites like Jervis Bay, which is beautiful.”
Despite several seasons of tracking, there’s still uncertainty about where many of the Port Jackson sharks disappear to every year. Chambers hopes to use stable isotope analysis to piece together their movements. In this technique, the ratio of carbon and nitrogen isotopes inside different body tissues can help determine where a fish was, and what it was eating, at different times in the recent past.
“The carbon isotopes vary on a longitudinal gradient,” says Chambers, “so they can tell us where in the world the shark was—longitudinally, anyway—which makes it easy for us to see how far down the coast the sharks can go. And nitrogen isotopes basically tell us at what trophic level an animal is feeding. Nitrogen actually accumulates up the food web, so you’ll find different levels of nitrogen isotopes in predators versus herbivores, for example.”
Once we know where the sharks are going each year, a new question emerges: How do they get there? Chambers suspects that they navigate the Australian coast using a combination of olfactory and magnetic cues.
Magnetic navigation, or magnetoreception, is not a new concept in biology, having been shown in a variety of migratory and non-migratory species. For instance, homing pigeons are thought to navigate by detecting the earth’s magnetic field, although exactly how they do this is still up for debate. Similarly, cattle and deer tend to align their bodies in a north-south direction during grazing, as do dogs when they urinate and defecate. Even captive carp, when placed in large circular tubs at the Czech Christmas Fish Market, aligned their bodies north-south.
In an experiment conducted this year at the Sydney Institute for Marine Science, Chambers placed juvenile Port Jackson sharks, about 30cm in length, inside a small circular tank and photographed them every minute for a whole week. The sharks, she predicted, would tend to orientate themselves along the north-south axis. Chambers then repeated the experiment, this time with strong magnets buried beneath the substrate of the tank to disrupt the surrounding magnetic field. Preliminary data from these photographs suggest that the sharks can sense magnetic fields, although they’re not necessarily using them to orientate themselves.
Is this sensory ability used in navigation? To find out, Chambers and her colleagues also conducted a displacement experiment: Wild Port Jackson sharks were captured at their home reef in Jervis Bay and released 8km away. Each shark was fitted with an acoustic tag to track how quickly it returned home, and some also had small magnets attached to their bodies. If magnetic cues are involved in navigation, then these 'magnetised' individuals should take longer to find their way home.
Chambers is yet to analyse the data from this experiment, but she hopes her results will show that there is more to sharks than meets the eye. Across the world, sharks tend to get a bad name in the popular media, with most of the attention on larger species like great whites and tiger sharks. Port Jacksons, on the other hand, are a friendly, charismatic species that pose no serious threat to humans. They can “give a bit of a nip,” says Chambers, but their teeth are unable to penetrate the skin; the only reported 'attacks' have been incredibly mild.
“I feel like sharks are quite misunderstood in general,” says Chambers. “They’re often seen as these mindless killing machines, and people are very afraid of them, but they’re actually really beautiful animals and are probably more intelligent than we give them credit for. I think it would be really cool to change that perception.”
Edited by Ellie Michaelides