Sharks have a fearsome reputation but most are skittish and swim away from strange noises, an underwater sound study has found.
However, the great white was not as skittish as other species, according to the study presented last week at a bioacoustics conference hosted by SA researchers.
Researchers used a baited underwater camera rig fitted with a speaker emitting low-frequency sounds of the type used by some shark-repellent devices.
Fieldwork was conducted at two locations, one in Australia targeting reef sharks and the other in Mossel Bay targeting great whites.
Whereas the reef sharks became markedly less inquisitive and aggressive when the speaker was playing, the great whites did not seem as bothered.
The research team tested shark reaction to two pre-recorded tracks — one artificially generated and the other an orca calling sequence.
Great white interaction was not significantly affected by either.
Orcas prey on sharks and have been linked to the recent decline in shark numbers in False Bay.
The level of interaction was ranked on a scale of one to six, with one being a “swim pass” and six a “bite on the bait”. Among great whites, researchers observed behaviour differences between sharks, whereas reef sharks behaved more uniformly.
“With great white sharks we could identify each shark — they have scars on their bodies you can recognise,” said lead researcher Lucille Chapuis in her presentation. “Some were behaving differently to others.”
Overall the findings — which have been published in the journal Nature — suggested sound could affect the behaviour of some shark species, but not all.
“Reef and coastal sharks spent less time in the vicinity of the presented sounds compared with white sharks and were also less likely to directly interact with the rig,” the scientists said.
“We also found interspecific differences in behavioural responses between the reef sharks tested; the ‘species’ factor was a strong predictor of time spent in the area, time of arrival and the total behavioural scores.
“White sharks spent less time around the baited camera rig when the artificial sound was presented, but showed no significant difference in behaviour in response to orca calls. The use of the presented acoustic stimuli alone is not an effective deterrent for Carcharodon carcharias (great whites).”
The sound impact on reef sharks was a cause for concern due to the increasing extent of anthropogenic (man-made) noise in the ocean, the study said. More research was needed to study these impacts, as well as research on sounds that might attract sharks rather than repel them.
In addition to being noisy, humans are increasingly listening in on animal sounds thanks to evermore sophisticated bioacoustic equipment. Last week’s international conference showcased many recent and ongoing research initiatives into geese honks, whale song and human baby cries.
Event organiser Dr Tess Gridley, founder of the African Bioacoustics Community and co-director of Sea Search, said: “Researchers in Africa have access to a wide range of behaviourally interesting and vocal species and unique environments which require better protection.
“Increasing industrialisation is resulting in noise pollution which can change behaviour and cause stress, so there are many reasons to grow the field of bioacoustic science in Africa.”
