Zero-oxygen in water killing fish, shellfish in Western Cape

Marine life along the West Coast has been dealt another blow, likely due to “icy and anoxic”, or zero-oxygen water conditions. 

As a result, large numbers of fish and shellfish in the shallows of Elands Bay, Olifants River estuary, port of Cape Town and False Bay have washed out to shore. 

Affected species include sardine, harder, white stumpnose, cuttlefish, octopus and evil-eye pufferfish. This time around, crayfish have not been affected. 

An evil-eye pufferfish (Amblyrhynchotes honckenii). Photo: iStock

The Department of Environmental Affairs, Forestry and Fisheries issued a statement on Thursday renewing the call for local communities not to touch, take home or eat any washed up fish or shellfish. 

So far, tests on phytoplankton at Elands Bay has proved not to be toxic and therefore “unlikely responsible for the fish and shellfish mortalities there”. 

No other bays have had non-toxic confirmation. 

A satellite composite map with red areas showing the highest phytoplankton densities and extent of the red tide on the West Coast. Photo: Deff

In January, a toxic red tide resulted in the deaths of around 1,000kg of crayfish in the Western Cape from St Helena Bay to the north of Lambert’s Bay and into the Berg River.

Earlier this month, a series of complex changes attributed to a “very large Agulhas Current meander” resulted in more fish and shellfish mortalities, this time along the east and south coasts of the country.

The department said the zero-oxygen deep water conditions “probably” drove fish and shellfish into warmer, shallow and more oxygen-rich waters near the shore. 

Some animals were too weak to avoid being washed out, which has resulted in an influx of fish and shellfish deaths. 

“This, and the possibility that some of the fish and shellfish may have been dead and rotting for longer than thought, makes it a wise choice to not feed any to family, friends or pets.” 

So far, most of the fish mortalities in False Bay are pufferfish, which at last count were at 300 to 400 dead fish per kilometre of shoreline. 

Pufferfish contain a deadly neurotoxin called tetrodotoxin. Dog walkers are urged to keep their pets far away from these fish.

If one is eaten, Deff advised that pet owners immediately induce vomiting and urgently consult a veterinary surgeon. 

Anoxic events explained

According to Australian online coastal information hub Oz Coasts, anoxic events are due to decomposing organic matter by bacteria that use oxygen. 

This occurs when the oxidation of organic matter by bacteria occurs more than the supply of dissolved oxygen.

In many cases, anoxia and hypoxia (algae overgrowth resulting in oxygen depletion) is due to an underlying problem of “excessive nutrient loads”. 

These nutrients come from a variety of sources, including aquaculture operation waste and sewage released into the ocean from yachts, boats and ships. 

In tropical regions, heavy rains after the dry season can also result in an uptick of rotting weeds, grasses and waste, which can also cause oxygen depletion. 

Estuaries and lagoons are particularly susceptible to anoxic and hypoxic events. 

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