Fish deaths in Hartenbos estuary not linked to red tide or sewage spill, says fisheries department

The mass fish deaths reported in the Hartenbos Estuary in Mossel Bay were not caused by a red tide or a sewage spillage, the forestry, fisheries and environment department said on Tuesday, pushing back against widespread speculation after similar marine die-offs along the west and south coasts. After engagements with the Mossel Bay municipality, the department said the incident was driven by a combination of environmental stressors, including elevated nutrient levels, reduced freshwater inflow due to water abstraction and dam development in catchments and the presence of harmful algal blooms. “These are some of the conditions that lead to eutrophication and ammonia toxicity, which the recently released National Biodiversity Assessment and earlier studies highlight as key contributors to fish mortalities in South African estuaries — especially in smaller and temporarily closed systems such as Hartenbos,” the department said.

Water quality monitors have since confirmed that oxygen levels in the estuary have normalised and the situation is under control. The clarification follows a series of unrelated marine mortality events along the west coast, where large numbers of white mussels, whelks and other shellfish were washed ashore at St Helena Bay and Elandsbaai due to active red tides. The red tide is bioluminescent, the phytoplankton producing spectacular flashes of colour at night.

Satellite imagery and field observations show visible red tide activity stretching from Elandsbaai northwards beyond the Olifants Estuary, as well as bioluminescent blooms in Walker Bay near Hermanus. While no fish or shellfish deaths have been linked to these red tides beyond the initial washouts, the department warned that the blooms carry a high risk of toxicity and advised that shellfish, whether washed up or collected, should not be eaten. The department said the Hartenbos incident should not be conflated with red tide-related events elsewhere. As a precaution, the department reiterated its warning to the public to avoid handling or consuming any washed-up marine organisms, regardless of the suspected cause of death, citing serious health risks.