Investigations by the Mossel Bay Local Municipality’s environmental department confirmed that the primary cause of the fish mortality was critically low dissolved oxygen levels, a condition known as hypoxia, resulting from a combination of natural ecological and physical factors.

A number of social media posts and news reports suggested that the recent fish mortality in the Hartenbos Estuary, which happened between January 1 and 5, 2026, was caused by pollution, sewage spills, or infrastructure failures.

The municipality emphasised that these claims were not supported by evidence and is working closely with the Department of Forestry, Fisheries and the Environment (DFFE) to ensure an accurate investigation.

In its statement, the municipality reassured residents that “this event was the result of natural ecological conditions and not due to pollution or infrastructure failure. No evidence was found of sewage spills, pump station breakdowns, or wastewater treatment malfunctions.”

Routine recreational water quality testing by the Garden Route District Municipality before the event confirmed that the estuary was safe for bathers, showing no signs of contamination.

The Municipality continued to clarify that the incident was driven by natural processes common to closed estuary systems during hot, stagnant summer periods, including algal blooms and oxygen depletion.

“By clarifying this, the municipality emphasises that the incident was not caused by human negligence or service delivery failures, but by natural factors beyond immediate control,” the statement added.

The hypoxic conditions developed as a result of multiple interacting natural factors. The estuary mouth was closed, preventing tidal flushing and oxygen replenishment. High summer temperatures and shallow waters accelerated oxygen loss, while a harmful algal bloom further consumed oxygen, especially at night. Minimal rainfall and freshwater inflow allowed the water to stagnate.

Together, these conditions caused oxygen levels to drop to a point incompatible with fish survival. The municipality said that it acted swiftly once reports of fish distress were received.

On the morning of January 3 at 06:30, an emergency breach of the estuary mouth was carried out, restoring tidal exchange and oxygenation. The removal of the dead fish began immediately and continued until January 5, with most of the cleanup completed by that date.

The municipal laboratory also initiated post-breach monitoring of dissolved oxygen and other water quality indicators to track the estuary’s recovery.

In its statement, the municipality noted: “Our priority was to act quickly to restore oxygen levels in the estuary and protect the ecosystem. The emergency breach was successful, and monitoring continues to ensure stability. We want to reassure residents that this event was caused by natural ecological conditions, not by pollution or infrastructure failure. Mossel Bay Municipality remains committed to transparent communication and responsible environmental management.”

The municipality acknowledged the concern this event has caused among residents and visitors, and “will continue to monitor the estuary closely, ensuring that factual information is shared with the public.”

lilita.gcwabe@inl.co.za

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