Sharks leave their marks

Dolphins and porpoises are welcome features of Australia’s coastline however little is known about the predation risk they face from sharks in their habitat.

In one of the first studies of its kind in Queensland, marine mammal experts from Flinders University and around Australia found about one-quarter of Australian snubfin and humpback dolphins studied near Gladstone, Bowen and Townsville featured shark-inflicted wounds. 

This study provides a baseline to monitor the effects of predation pressure on habitat use, behaviour and group living of these dolphin species living among bull, tiger and white sharks in coastal waters of south, central and north Queensland, says Caitlin Nicholls, from the Cetacean Ecology, Behaviour and Evolution Lab (CEBEL) at Flinders University.   

Flinders University researcher Caitlin Nicholls is doing more field work in Queensland this month.

The study assessed thousands of photos taken from boat-based surveys, with 33% of the 56 snubfin and 24% of 36 humpback dolphins identified and with greater photographic coverage of their dorsal bodies showing evidence of shark bites to gain new insights into failed predation attempts rather than fatality rates. 

“Overall, the study also found increased predation pressure on dolphins in northern Queensland, increased predation pressure closer to the coast and equal predation pressure between the two species,” says Ms Nicholls. 

“This study is the first to assess the occurrence of shark-bite scarring on snubfin and humpback dolphins in Queensland across different group sizes, habitat features, and locations, and serves as a baseline for shark-dolphin interactions in the area.  

“These results highlight the importance that habitat features could have in predation pressure of dolphins, as well as the importance of considering photographic coverage when assessing bite injuries on animals.” 

CEBEL team leader Associate Professor Guido J Parra is monitoring dolphin populations in northern Australia.

The research by CEBEL on Australian snubfin and humpback dolphins, led by Flinders University researcher Associate Professor Guido J Parra, is focused on monitoring both species which are listed as Vulnerable by the Queensland Nature Conservation Act and by the IUCN Red List of threatened species.  

The presence of shark-inflicted wounds and scars on live individuals can provide an indirect measure of predation pressure, representing failed predation attempts, and provide an estimate of the frequency of shark attacks, says Associate Professor Parra. 

“Predation and predation risk have a large influence on the ecology and evolution of both predator and prey species; therefore, to decipher how communities are structured and function, we need to understand how predators and prey interact,” he says. 

Snubfin and humpback dolphins in northern Australia are characterised by small population sizes, low genetic diversity and slow reproduction. Their ability to avoid predation from sharks is aided by altering their decisions regarding food, mates, groupings and habitat.  

An injured snubfin dolphin. Photo CEBEL (Flinders University).

The new article, Incidence of shark-inflicted bite injuries on Australian snubfin (Orcaella heinsohni) and Australian humpback (Sousa sahulensis) dolphins in coastal waters off east Queensland, Australia (2023), by Caitlin R. Nicholls, Katharina J. Peters, Daniele Cagnazzi, Daniella Hanf and Guido J. Parra has been published in Ecology and Evolution DOI: 10.1002/ece3.10026

Acknowledgements: Funding for the study was provided by the Port of Townsville, Gladstone Ports Corporation, Fitzroy River Basin Association, the Queensland Department of Environment, Flinders University and Southern Cross University.  

Photo CEBEL, Flinders University
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