Understanding shark movements: The spatiotemporal distribution of white sharks has implications for interactions with fisheries and spatial management
One-third of the world’s sharks, rays, and chimaeras are threatened with extinction according to IUCN Red List criteria, primarily due to fishing pressure. These iconic species are essential for maintaining the structure and function of marine ecosystems, have high cultural and societal value and contribute significantly to the global economy, mainly through tourism.
By combining animal movement modelling and spatiotemporal analyses of risk exposure from fisheries and protection from MPAs, we have shown that shark conservation would benefit from shifts to non-lethal solutions to beach safety, more observers in fisheries, expansion of the MPA network and continued monitoring of shark movements.
We satellite-tagged 33 male and female white sharks ranging from 2.5 – 5.0 metres with SPOT5 satellite transmitters (Wildlife Computers). White sharks were temporarily removed from the water onto a research vessel using a specialised hydraulic lift system. Best-practice capture, sampling and tagging procedures took 12 minutes and were supervised by a veterinarian. Photo by Ocearch
White sharks (Carcharodon carcharias) are the largest predatory sharks with a circumglobal distribution. In South Africa, their population size is small and estimated to be 500-2000 individuals. Unfortunately, protecting threatened mobile species across their range is impractical given the limited resources for surveillance and enforcement, especially in southern Africa. Therefore, directing conservation and management attention to focus areas is a promising solution. However, designing these interventions requires an understanding of the spatiotemporal overlap of shark movement patterns with threats.
Satellite tracks of 33 white sharks tagged at five locations along the South African coast in 2012.
White sharks spent 73% of their time inside South Africa’s exclusive economic zone (EEZ). Juvenile and subadult males stayed almost exclusively coastal. In contrast, subadult and adult females roamed vast areas of the southwest Indian Ocean, emphasising the need for a regional management plan. Only three sharks swam westwards up the productive and prey-abundant west coast of South Africa, possibly due to consistent low water temperature and oxygen levels.
Predicted shark locations coloured by behavioural state.
To identify focus areas for conservation and management interventions, we used an animal movement model (i.e. hidden Markov model) to identify ‘resident’ and ‘transient’ movement states and the effect of independent variables on the transition probabilities between states. Sex, size and season influenced the spatiotemporal movement of white sharks. The resident state was characterised by slow, directed movement likely representative of patrolling or feeding behaviour. However, the transient state was characterised by fast, directed movement, likely representing travelling or migratory behaviour. Tagged male and female sharks were more likely to be in a resident state near the coast and a transient state away from the coast. However, larger subadults and adults were more likely to be in a transient state. Larger sharks can expand their foraging range offshore to take advantage of open ocean resources and avoid competition with conspecifics and other large predators. Mature sharks may need to travel further to find suitable mates.
The average percentage of time white sharks spent in 29 MPAs they visited (MPAs in existence during tracking are indicated with a *; others were declared in 2019 based on systematic conservation planning).
White sharks of both sexes and all size classes spent significantly more time and travelled greater distances inside South Africa’s MPAs than expected by chance, suggesting a preference for protected areas. This is likely due to increased prey abundance or less disturbance, suggesting that MPAs can benefit large, mobile marine megafauna. In the context of SANParks managed MPAs, Table Mountain, Tsitsikamma and Addo Elephant National Park MPAs were extensively visited by white sharks. We also identified Gansbaai, Struisbaai and Mossel Bay, not currently in MPAs, as focus areas for conservation and management. The tracking data has been included in the Critical Biodiversity Areas and Ecological or Biological Significant Areas processes and will be used in the MPA expansion process.
White sharks overlapped with longline and gillnet fisheries within 25% of South Africa’s EEZ and spent 15% of their time exposed to these fisheries during the study period. The demersal shark longline fishery had the highest relative spatial and temporal overlap, followed by the pelagic longline fishery. However, few white shark catches were reported in these sectors. Although the KZN shark nets and drumlines had the lowest spatiotemporal overlap (1%), they reported the highest white shark catches (average 32 per year). The KZN shark nets and drumlines use a combination of gillnets and large, baited hooks with steel traces to target large, potentially dangerous sharks, explaining the high catches.
Our results emphasise the need to combine shark movement and fishing effort with reliable catch records to accurately assess populations. White shark exposure to shark nets and drumlines, by movement state, sex and size, corresponded with catch composition (mostly juvenile females) of the fishery, providing support for a meaningful exposure risk estimate. The KZN shark nets and drumlines pose the largest relative risk to white sharks. Therefore, the KZN Sharks Board should continue to reduce the number of gillnets and drumlines and eventually replace lethal methods with non-lethal methods. Increased fisheries observer coverage in the fishing sectors would provide more confidence in bycatch statistics of threatened and protected species.
This article was originally published in the 2021/2022 Research Report.
The article is based on a publication in the journal Frontiers in Marine Science.
Kock, A.A., Lombard, A.T., Daly, R., Goodall, V., Meÿer, M., Johnson, R., Fischer, C., Koen, P., Irion, D., Gennari, E. and Towner, A., 2022. Sex and size influence the spatiotemporal distribution of white sharks, with implications for interactions with fisheries and spatial management in the southwest Indian ocean. Frontiers in Marine Science, 9, p.443.
