Experts puzzled by mass clam stranding
Scientists are still getting to grips with the extraordinary marine stranding at Plettenberg Bay in which nearly 12million clams washed up on Robberg Beach.
Hundreds of kelp gulls and oyster-catchers are tucking into the shellfish platter served up by the sea on March 11, but the clam meat is now rotting and wafting a strong fishy aroma across the whole of Plett.
Nelson Mandela University marine zoologist Dr Gwen Penry of the Nature’s Valley Trust said the trust was helping SANParks marine biologist Kyle Smith to quantify the stranding and assess what occurred.
“We have taken samples that will be tested for possible red tide, but I think if it was that then there would be other species that stranded.
“At the moment, we believe that the stranding was related to strong easterly winds and the upwelling of cold water.”
There had been three days of strong onshore easterly winds before the washout of the Venus clams, which live burrowed into sandy sections of the seabed, she said.
“The winds would have churned up this habitat and we think the exposed clams would have been stunned by the cold water,” Penry said.
“Unable to burrow back into the sand, they would then have been washed ashore.”
Penry said the mass stranding had been extraordinary.
“We are still busy assessing the size of it, measuring with a tape measure and then multiplying and scaling up from a 1m² quadrant we constructed.
“We estimate that by the time they had all stranded, there were about 11.5-million clams spread over 6km, adding up to 127 tons of clam meat.”
The mass stranding of clams at Plettenberg Bay had begun suddenly three to four years ago, with the main mass always washing up in the southern corner of the beach where it intersected with the Robberg peninsular, she said.
“We have no proof yet as to why it has suddenly started happening, but my thinking is it could be related to the increased nutrient load of droppings from the growing populations of seals and cormorants on the peninsular.”
The increased nutrient load could have resulted in a population explosion of the filterfeeding clams, with too many to burrow securely into the adjacent sand seabed.
Alternatively, the stranding could be related to the reduced flow of sand to Robberg Beach.
“The sand used to be brought in from a large dune on the west side of the peninsular, but that system seems to be faltering as the dune becomes more vegetated.
“The reduced sand inflow could be affecting the depth of the sand on the seabed and the security of the clam burrows.”
Penry said the upcoming spring tide on March 21 would likely clear many of the clams.