Movement of Megafauna in Shark Bay

Main Club, 2 September 2022

Dr Ana Sequeira spoke to us about the movements of megafauna in Shark Bay, known as Gathaaguda, by the traditional owners, the Malgana people.

Ana moved to Western Australia from Portugal to do a PhD on Whale Sharks. Through that work, she developed a global database for Whale Sharks using tuna fishing data. This directly resulted in changing their conservation status from Vulnerable (2005-2015) to Endangered (2015+). Most of us know that the ocean is under constant pressure from industrialisation, with almost no region unaffected by human influence. Megafaunas are impacted by the effects that cascade down the food chain.

Shark Bay is a huge area but is a closed system compared to the open ocean. Impacts on a single component of the ecosystem, by their very nature, flow onto the rest. The research looks at the movements of the top predator, the Tiger Shark, which feeds on turtles and dugongs, which in turn feed on seagrasses. The seagrasses also provide buffering for the area’s famous stromatolites.

Tracking the movements of megafauna provides data on individual variability, the scale of movement, drivers of behaviour, space utilisation, spatio-temporal patterns, changes in distribution and connection between ecosystems. The tracking is performed by a number of different mechanisms, each providing differing levels of information.

Tracking devices Image by Tanya Marwood

ID tags are good for marking and recapturing but give no data on diving depths and movements between recaptures. Satellite tags are good for the large-scale oceanographic movement. Camera and ‘diary’ tags are great for fine-scale movement but do not store much data and must be retrieved to get the information. To deploy the tags, the animals must be caught – sharks via drumline. Ana shared videos of the capture of turtles which need to be visually sighted and then caught by diving off the boat and bringing them aboard!

Turtles in the recent study had ID tags (20), and satellite tags (17) deployed, which provided data on home range estimation and was linked to seagrass cover. Nineteen Dugongs (Wuthaga) were satellite-tagged, and the tracking showed seasonal movement patterns. They are massive mammals and can only be captured in shallow water by diving off the boat and guiding them alongside while the tags are attached. Research on sharks (Thaacka) focused on Tiger Sharks and several other medium-large shark species, with ten tigers ID-tagged, seven satellite-tagged and six deployments of camera tags (which pop off after ~6hrs). We saw footage of ‘Boof’ (the formerly captive shark) swimming through the water, which gives insight into movements and behaviour.

To summarise, Shark Bay is a pristine environment that is World Heritage listed but is under catastrophic threat from climate change. Globally there are a plethora of threats to the seas, and we saw visualisations of the impacts and overlap of fishing, noise, plastic pollution and shipping. Hardly any region is not affected by one or more of these.

Ana ended her presentation by talking about the Megamove organisation advancing the long-term conservation of marine megafauna through strategic mitigation of global threats. She is looking for researchers to participate in this and contribute their movement data.

Steve Lofthouse