Laboratory testing has confirmed the presence of pilchard herpesvirus (PHV) genetic material in sardines linked to the recent mass mortality event along South Africa’s West Coast.

However, the Department of Forestry, Fisheries and the Environment (DFFE) says scientists have not yet established that the virus is responsible for the deaths.

PHV was detected in dead sardines and at lower levels in some apparently healthy fish collected from the West Coast. This suggests that the presence of the virus alone may not be sufficient to cause widespread mortality.

Scientists investigating environmental factors

Researchers are now investigating whether environmental conditions may have triggered or worsened the deaths.

Possible factors include harmful algal blooms, low oxygen levels and unusual oceanographic conditions.

These remain hypotheses and further evidence is needed before scientists can determine what caused the mortality event.

Testing of sardines collected from Saldanha Bay, the Cape Canyon and Gansbaai, as well as an apparently healthy comparison sample from St Helena Bay, found none of the paralytic toxins associated with harmful algal blooms in Saldanha Bay.

Low levels of two other algal toxins, domoic acid and yessotoxin, were detected. However, the highest concentrations were found in the apparently healthy comparison sample.

The department says the findings suggest the tested toxins were unlikely to have been the primary cause of the sardine deaths.

No evidence of risk to people

The department stressed that PHV is associated with sardines and there is currently no evidence that it infects humans or other animals.

The discovery of the virus also does not mean that commercially caught healthy sardines or canned sardine products are unsafe.

There is currently no general suspension of fishing or canning, although normal food-safety controls and monitoring will continue.

The public has nevertheless been warned not to collect or eat dead or dying sardines, or feed them to pets.

Urgent response planned

DFFE has convened an urgent briefing with representatives of the small pelagic fishing industry.

A task team involving the department, fishing industry and other stakeholders will be established to develop a coordinated response to the PHV-associated mortality.

Disease surveillance is also being expanded along South Africa’s West, South and East coasts, while additional disease, pathology and environmental investigations continue.

The department’s routine October/November hydroacoustic biomass survey will provide the first opportunity to assess whether the mortality event has had a measurable impact on the country’s sardine population.

PHV has not previously been reported in South African sardines, although it was associated with major sardine mortality events in Australasia during the 1990s. The virus was later detected in apparently healthy Australian sardine populations without causing ongoing mortality.

DFFE says it will provide further updates as the investigation progresses.