A marine heatwave off the east and south coast of SA has caused a large fish and shellfish “walkout” – a natural phenomenon where sea species are beached as they try to escape ocean conditions.
The department of environment, forestry and fisheries said large numbers of fish and shellfish appeared to have died over the past week as a result of the marine heatwave.
The department cautioned coastal communities against collecting and eating the washed up fish and shellfish, as they may be toxic.
“Some of the fish may have been dead longer than thought, some of the unfamiliar ones may be toxic and it is not clear yet whether the anomaly also resulted in ‘red tides’ or Harmful Algal Blooms,” read a department statement.
The phenomenon coincided with a large Agulhas current meander which the department said was an offshore deviation of the Agulhas current.
“The Agulhas current meanders are associated with complex, and drastic changes at the coast, with changes in temperature, ocean currents, water level and biochemistry of the water, leading to fish mortalities,” said the department.
During the earlier stages of the marine heatwave, reports were received of fish swimming away from warm water and of seaweed bleaching on parts of the coast.
But as cold water welled up from the depths or intruded into the warm water currents, fish and shellfish suffered thermal shock and froze, with many lying stunned in the shallows and too weak to avoid being washed out on shore.
“The large anomaly or event observed in the Agulhas current along the coastline recently [February and March] resulted in a ‘marine heatwave’ with water temperatures of 24°C and above throughout much of the east and south coast. This was followed by upwelling of deep water with a 10-15°C difference between cold and warm water at the coast,” said the department.
These current meanders occur irregularly, four to five times a year in the northern Agulhas current system, but only once or twice a year near Port Elizabeth as they weaken on their way south.
BY Aron Hyman