Cape Town – The Department of Forestry, Fisheries, and the Environment (DFFE) says the mass fish mortality observed in the Hartenbos Estuary in Mossel Bay is not linked to the red tide.
The DFFE has also rejected suggestions that the fish mortality was due to a sewerage spillage.
Through engagements with the Mossel Bay Municipality, it has been determined that this particular incident can be attributed to a combination of interacting factors, including, but not limited to:
- Elevated nutrient inputs from multiple sources, leading to eutrophication;
- Reduced freshwater inflow due to water abstraction and dam development within catchments, resulting in slower water movement and increased susceptibility to algal blooms; and
- Harmful algal bloom and high phytoplankton biomass.
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 DFFE cautions members of the public against handling or consuming any washed-up marine organisms.
Such organisms may pose serious health risks and are not safe for consumption, regardless of the suspected cause of mortality.
Furthermore, the DFFE’s Water Quality Monitors, with other relevant local authorities and stakeholders, have since confirmed that the oxygen levels in the water have normalised and the situation is under control.
At the weekend, the Mossel Bay Municipality announced that the emergency mechanical breaching of the Hartenbos Estuary on 3 January 2026 confirmed the cause of the fish mortality.
“Critically low dissolved oxygen levels were recorded in the system, triggering an intervention in accordance with the provisions of the Approved Estuary Mouth Management Plan, which allows for emergency breaching when ecological functioning is threatened,” explained the Mossel Bay Municipality.
A contractor was deployed onsite, and clean-up operations were conducted following the fish mortality event.
“Scientific observations have confirmed that the fish kill resulted from severe oxygen depletion within the estuary,” the municipality said.
“The oxygen deficit was caused by an algal bloom, exacerbated by a combination of low water levels, elevated temperatures, and nutrient‑rich conditions – all factors known to accelerate oxygen consumption and create hypoxic conditions.”
The Garden Route District Municipality, through its regular recreational water quality sampling programme, has verified that all recent water samples for the Hartenbos Estuary were compliant with applicable standards.
