A risk assessment of anthropogenic activities on cetaceans in South Africa in light of the expanding oceans economy

Abstract

The unique oceanographic characteristics of the South African marine environment gives rise to high biological productivity and a wide variety of habitat types. These in turn are able to support a high diversity of marine life, including a variety of cetacean species. In addition to this, the South African marine environment provides biotic and abiotic resources which can be utilised by humans. In an effort to boost the South African economy by expanding the utilisation of its marine environment, Oceans Phakisa was launched in 2014. In light of this expansion, the interaction between marine life and the anthropogenic activities utilising ocean resources must be investigated, and potential risks must be determined. Consequently, a risk assessment of anthropogenic activities on cetaceans in the South African Exclusive Economic Zone (EEZ) was developed. In first instance, the impact of anthropogenic activities on cetacean mortalities was determined.

This was done through an analysis of cetacean mortality data collated between 1969 and 2023. Most cetacean mortality records (62%) were not assigned a cause of death. However, at least 38% of cetacean mortalities were associated with an anthropogenic activity. Entanglements in bather protection nets were most frequently identified as the cause of mortalities, followed by bycatch, ship strikes, and entanglement in fishing gear. Midwater trawls were most frequently involved in bycatch incidents, while octopus and rock lobster trap fisheries were most frequently involved in fatal entanglements in fishing gear. Indo-Pacific bottlenose dolphins, Bryde’s whales, humpback whales, and dusky dolphins were frequently identified in anthropogenic related mortalities. Spatial analysis of the mortality data identified the west and south coast of South Africa as hotspots of cetacean mortalities, especially during summer months (October to March). Secondly, the risk that marine anthropogenic activities may pose to cetacean species was assessed. This was done by conducting a spatiotemporal analysis of the distribution overlap of 18 cetacean species and the spatial footprint of 9anthropogenic activities in the South African EEZ. Results indicated that all modelled cetacean distributions in the South African EEZ overlapped with at least one anthropogenic activity. Shipping activity overlapped with all cetacean distributions and may therefore pose the highest overall risk based on spatial overlap. Risso’s dolphins, minke whales, sperm whales and common dolphins were identified as the species with distributions that have a higher than 80% overlap with shipping activity. Results indicated that most cetaceans in the South African EEZ may be at risk from the effects associated with cumulative anthropogenic pressures. The west and south coasts of South Africa were identified as areas were risk to cetacean species from the effects of anthropogenic activities was highest. The results of this thesis provide a baseline analysis of risk which can be used during the development of mitigation and management strategies as well as to inform Marine Spatial Planning processes.