Members of the SA Stranding Network spent the day on a remote beach near Woody Cape on Tuesday, taking samples from 15 common dolphins which were reported stranded there on Monday.
Bayworld marine biologist Dr Greg Hofmeyr, who led the team, said two of the carcasses found were old and were not linked to the latest incident – the seventh of its kind in the area in 40 years.
It was not clear why the animals had stranded, he said.
“There are various possibilities but the one that seems most likely here relates to the site, which consists of a gently sloping beach extending some way off shore.”
The thinking is that this topography delivered a poor return signal to the echolocation that the dolphins used to guide themselves, he explained.
“If they were an inshore species then they could probably have interpreted it but the common dolphin is an offshore species unused to this kind of challenge.”
Hofmeyr said there were similar repeat mass stranding spots around the world from New Zealand to SA’s west coast but this was the only stranding hotspot of this kind on SA’s east coast.
Each of the six previous Woody Cape events had also involved common dolphins and had occurred at or near the same site about 6km along the beach on the Sundays River side of Kransies, he said.
Kransies comprises a series of high cliffs which extends west from under the Alexandria Trail hut, opposite Bird Island, gradually dwindling to shoulder height and eventually a flat sandy beach.
The desolate site lies inside the park and is usually only frequented occasionally by conservation law enforcement officers and research scientists.
The other strandings had all taken place in March-April, Hofmeyr noted.
Among past incidents, in March 2000 28 animals stranded, in March 2017 38 animals stranded, and in April 2018 14 animals stranded.
Other theories for mass dolphin strandings range from pursuit by orcas to seismic activity by mineral exploration vessels interfering with dolphin echolocation, and plastic pollution.
Related to this, common dolphins are very sociable animals and the thinking is that if the lead animal is sick and swimming dangerously off course, the pod may follow it to its death.
Likewise related to their sociable behaviour, they may become disorientated after chasing fish into the unfamiliar surf zone.
Addo Elephant National Park marine senior section ranger Rob Milne, who is also a member of the stranding network, said it had been a grim spectacle when they drove out to the beach on Tuesday.
“Suddenly they were there. It was shocking.”
The dolphins were first spotted on Monday afternoon by emergency services helicopter pilot Andrew de Bruyn who was flying to East London from Port Elizabeth.
When the park was alerted on Monday evening conservation manager John Adendorff took his fixed-wing plane over the area and was able to confirm the stranding.