Large fish and shellfish mortalities and washouts have been observed along South Africa’s east and south coast in the last week. These mortalities appear to have been caused by a marine heatwave experienced in recent weeks.
The Department of Environment, Forestry and Fisheries cautions coastal communities against collecting and consuming the washed up fish and shellfish. 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 (HABS).
The marine heatwave and the marine species walkouts are coincidental with a very large Agulhas Current meander (offshore deviation of the 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, subsequently leading to fish mortalities.
During the earlier stages of the “marine heatwave,” reports were received of fish swimming away from warm water as well as of seaweed bleaching on parts of the coast. With cold-water intrusion, fish and invertebrates (shellfish) suffered thermal shock (froze) with many lying stunned in the shallows, some too weak to avoid being washed out on the 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. These meanders occur four to five times per year in the northern Agulhas Current system but only one to two times per year near Port Elizabeth as they weaken on their way south. Their occurrence is irregular.
