Pod of 38 beached by accident, Bayworld expert believes
The 38 common dolphins that were spotted stranded on March 31 near Sundays River probably died lost and disoriented, but there is no evidence of foul play, Bayworld marine mammals curator Dr Greg Hofmeyr said yesterday.
All marine mammals are protected by law and it was initially thought that human action could somehow have caused the incident.
But Hofmeyr, who led a probe into the matter, said there had been no evidence to this effect.
Hofmeyr and his team – which included NMMU marine biologist Dr Stephanie Plön who is studying disease in marine mammals, SANParks vet Dr Dave Zimmerman and Bayworld intern Mthoko Msele – visited the site the day after helicopter pilot Noel Greyling spotted the dolphin carcasses from the air.
“The first line of evidence was that all 38 of the dolphins were lying within a bit less than 2km, which led us to believe that they had swum ashore together. If they had died in the water they would likely have been washed up over a much wider area,” Hofmeyr said.
The remoteness of the site – 18km east of Sundays River within the Woody Cape section of the Addo Elephant National Park – meant the carcasses had lain there four to five days before Greyling’s alert.
“So they were slightly decayed, meaning the tissue was not ideal for accurate analysis. But we found no bullet wounds and no scarring or sub-cutaneous bruising from nets.
“They all had a good blubber layer so there was no evidence of starvation and there was a mix of young and old, males and females, indicating nothing unnatural prior to them stranding.”
The Bayworld database of marine mammal strandings established by Graham Ross and maintained by Bayworld marine mammalogists like Dr Vic Cockcroft had further guided their finding, he said.
“The database shows there have been four strandings of common dolphins in the past 40 years in Algoa Bay – all at this same site. “So this was the fifth. “There are similar repeat stranding spots around the world in places like New Zealand, and in South Africa on the Western Cape coast. But this is the only one on South Africa’s east coast.”
The common denominator seemed to be their topography, Hofmeyr explained.
“Like with our site here, the factors seem to include a bay with a bit of a headland and a gently sloping sandy seabed.”
He said it was not clear how this topography triggered problems but the normal offshore range of the common dolphin – most famous for its role in the sardine run – clearly made it vulnerable inshore.
“These ones may have been pursuing fish or were chased in by killer whales and, once in, the noise of the waves might have interfered with the echo location they use to navigate.
“They are very social animals, so if one stranded, the rest would have followed.”
The stranding team dissected all the animals on site and various tissue samples were taken including skin samples to check the genetics of the different animals, part of a project to track the movement of different common dolphin populations.
The skulls were also removed to study as part of the same project and they are presently being “macerated” – left to lie in a pool of water for several months to allow bacteria to clean the flesh off them, he said.