Cape Town -Two hundred tons of West Coast rock lobster – starved of oxygen because of the red tide – have died at Elands Bay after a mass “walkout”.
On Wednesday morning 80 tons of lobster were stranded in Elands Bay, the latest casualties since the start of the red tide about 10 days ago.
About 30 tons of lobster walked out on Monday and more than 70 tons on Tuesday.
The fisheries branch of DAFF said on Wednesday about six tons of live lobster had been rescued on Monday, but the bulk of the stranded lobsters had died and were being collected and dumped at a municipal dump site.
Rescued lobsters were removed and released in areas on the West Coast unaffected by red tide.
These mass strandings come at a time when the lobster resource is already under severe pressure from over-fishing – particularly illegal fishing – which has caused stocks to be reduced to less than 3 percent of what they were before they were commercially exploited.
While staff are clearing up the dead lobster, the beaches and roads leading to them have been closed to the public. Police and municipal officials are patrolling the area.
The algal bloom that caused the lobster walkouts is now moving south from Elands Bay. It is around 80km long and about 3km wide, stretching from Dwarskersbos on the West Coast to north of the Oliphant’s River
Officials from municipalities and DAFF, as well as volunteers, are helping to clean the beaches.
Laura Blamey, of UCT’s Marine Research Institute, said losing 200 tons of lobster to the red tide was significant.
“For a resource that is so overfished, that’s a lot – especially on the West Coast where catches have dropped. The lobster resource is already just 2.5 percent of what it was in its pristine condition, so losing 200 tons is a lot,” Blamey said.
Algal blooms happen after south-easterly winds cause upwelling from the sea bottom, bringing nutrients to the surface.
This creates an abundance of food for plankton, which results in a population explosion of these tiny organisms, known as an algal bloom. The density of the algae turns the sea surface red or brown.
If the upwelling is followed by a long period of calm with little wind to disperse the algal bloom, the plankton are not dispersed. When the algae die and decompose, the decomposition bacteria deplete the oxygen in the water. If the oxygen becomes too low, marine animals leave the area – or die.
Lobster move into the surf zone where there is likely to be more oxygen because of the wave action. When the tide recedes, the animals are stranded, too weak to move back with the tide, and suffocate.
Blamey said there was a lot of variability with upwelling, which depended on the wind. If there were a lot of south-easterlies, and the winds were constant, the upwelling would move offshore.
If the upwelling was followed by periods of calm, the algal bloom did not disperse and led to a red tide.
“There’s been a lot of wind in December and January, but not the strong south-easters for days on end. They’ve switched to south-westerlies or northerlies.”
In 1997, South Africa experienced its worst-ever rock lobster mortality from red tides when about 2 000 tons died from lack of oxygen on the West Coast.