Laboratory sequencing has confirmed the presence of pilchard herpesvirus (PHV) genetic material in sardines sampled following recent mass mortalities along South Africa’s West Coast.
The Department of Forestry, Fisheries and the Environment (DFFE) said PHV was detected in both dead sardines and, at lower levels, in some apparently healthy fish.
The finding suggests that the presence of the virus alone may not be sufficient to cause mass mortality, and scientists are investigating whether environmental stressors may have triggered or intensified the event.
These include harmful algae, low oxygen levels and unusual oceanographic conditions. However, the department stressed that this remains a hypothesis requiring further evidence.
Tests conducted on sardines collected off Saldanha Bay, the Cape Canyon and Gansbaai found no paralytic toxins associated with a harmful algal bloom in Saldanha Bay.
Two other algal toxins, domoic acid and yessotoxin, were detected at low levels. The highest levels were found in an apparently healthy comparison sample collected from St Helena Bay.
No other toxins included in the screening were detected.
The department said the findings indicate that the toxins tested for were unlikely to have been the primary cause of the sardine deaths. However, harmful algae or other environmental conditions cannot be entirely ruled out as factors that may have placed additional stress on the fish.
The DFFE has convened an urgent briefing with small pelagic fishing industry stakeholders. A task team involving the department, fishing industry and other relevant stakeholders will also be established to formulate a coordinated response plan for PHV-associated sardine mortality in South African waters.
Disease surveillance is being expanded along the West, South and East coasts, while further disease, pathology and environmental investigations continue.
The department’s routine hydroacoustic biomass survey in October and November will provide the first opportunity to assess the possible impact of the mortality event on the sardine population.
PHV has not previously been reported in South African sardines. The virus was associated with two large-scale sardine mortality events in Australasia during the 1990s, but was later detected in apparently healthy Australian sardine populations without causing continuous mortality.
The virus is associated with sardines, also known as pilchards. There is currently no evidence that PHV infects people or other animals.
The department said the detection of PHV alone does not indicate that healthy sardines caught commercially or canned sardine products are unsafe. It also does not currently justify a general suspension of fishing or canning.
The biotoxin results are considered reassuring, although they do not constitute a general clearance of all commercial catches. Monitoring and normal food-safety controls should therefore continue.
The public has been urged not to collect or consume dead or dying fish, or feed them to pets.
The department said it will provide further information as additional results become available.
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