The foraging ecology of white sharks (Carcharodon carcharias) in New South Wales coastal waters
Information on the diet and foraging patterns of white sharks in Australia is limited, especially for Eastern Australia and New South Wales (NSW).
Apex predators, such as white sharks often pose significant management challenges which arise due to a combination of factors including; 1) their populations are often under pressure from human activities, 2) they have a propensity for involvement in conflicts with humans, necessitating strategies to manage these conflicts, and 3) there is often a scarcity of data on aspects of their ecology that are relevant for their management.
White sharks are listed as vulnerable by the International Union for Conservation of Nature and are protected across much of their range, including Australia. Concurrently, white sharks are also the primary species of shark responsible for bites on humans in Australian waters. Despite this, information on the diet and foraging patterns of white sharks in Australia is limited, especially for Eastern Australia and New South Wales (NSW). Understanding more about diet, prey selection and the foraging patterns of white sharks is critical if we are to gain insights into broader aspects of their ecology, such as potential drivers behind their movement patterns (Huveneers et al., 2018), and subsequently their interactions with humans.
Therefore, the goal of this project is to investigate the foraging ecology of white sharks in NSW and examine the role of nutrition in prey selection by this apex marine predator. To do this, a range of complementary techniques are being used to address the following aims:
- Determine the diet and key prey species of white sharks in NSW coastal waters. At SIMS, the stomach contents of white sharks are being analysed. Stomach contents analysis provides a means to quantify the diet with high taxonomic resolution, helping to identify the key prey species of white sharks in NSW. This also represents the first diet assessment for this species in Eastern Australia.
- Examine the nutritional composition of the prey resources used by white sharks, and how this varies in time and space. Laboratory and field-based studies have revealed that many organisms (invertebrates and vertebrates, herbivores, omnivores and carnivores) select foods based on their composition of specific nutrients (e.g. proteins, fats, carbohydrates), and that the balance of these nutrients can impact various life-history traits and influence broader aspects of animal ecology (Simpson and Raubenheimer, 2012, Kohl et al., 2015), such as migration patterns (e.g. Nie et al., 2015). How might white sharks respond, not only to variation prey quantity, but also prey quality? Nutritional analyses are being conducted on samples of white shark prey species, collected over the latitudinal range of NSW, across multiple seasons, to provide important baseline data on spatial and temporal variability in the quality of the prey resources consumed.
- Examine the trophic and foraging habitat dynamics of white sharks using stable isotopes.In addition to nutritional analyses, the stable isotope signatures of prey items are also being investigated and combined with isotopic measurements of shark tissues. Stable isotopes are advantageous because they provide a time-averaged dietary signal and, when applied to tissues that are formed sequentially throughout the life of the shark (e.g. calcified tissues), can be used to examine foraging profiles at the individual level. Here, an isotopic approach is being employed to investigate potential variation in dietary inputs and foraging habitat use among individual white sharks (i.e. individual-level specialisation in foraging patterns).
- Examine the fine-scale behaviours and post-release responses of white sharks caught on SMART drumlines.Shark-Management-Alert-In-Real-Time (SMART) drumlines are a promising new, non-lethal technology for shark bite mitigation which are used to catch, tag, relocate and release sharks. This technology is currently being trialled as part of the NSW Government’s Shark Management Strategy, however there is a need to better quantify what sharks do immediately following release. Custom-built animal-borne “SharkCams”, comprising a video camera and other integrated data loggers, are being deployed on white sharks released from SMART drumlines to investigate their post-release behavioural responses, including potential foraging activities. This information will be highly valuable for better understanding the utility of SMART drumlines for non-lethal shark bite mitigation.
The new information on the diet, foraging patterns and behaviours of white sharks provided by this project is intended to help inform future decisions for the management of this species in NSW, including strategies for bather protection.
This study is led by Richard Grainger from the University of Sydney, with supervision from Prof. David Raubenheimer (USYD), Dr. Gabriel Machovsky-Capuska (USYD) and Dr. Vic Peddemors (NSW DPI Fisheries). The project is being run in collaboration with the NSW Department of Primary Industries Fisheries, with funding support from the NSW Shark Management Strategy.