Sardine Investigation Advances

There is still no clear explanation for the mass mortality of sardines along the Western Cape coastline but some progress is being made to rule out possible causes and there is now a coordinated response plan.

The Department of Forestry, Fisheries and the Environment (DFFE) reports that no new mass-mortality incidents have been reported in the last week which suggests that the acute phase of the event may have eased. Monitoring nevertheless continues.

More than six dozen sardines samples have been included in the PHV testing programme to date. Initial testing by the South African Institute for Aquatic Biodiversity and Rhodes University detected pilchard herpesvirus (PHV) genetic material in all 15 fish examined. Of these, 10 fish collected from the mortality event showed consistently high viral levels. Viral levels varied among the five fish that were caught alive and appeared healthy at the time of collection. The viral sequence was a 99.7% match to an Australian PHV gene.

A further five dozen additional samples have been screened for PHV with preliminary results for about half of these indicating high PHV levels. Confirmatory testing is continuing, while pathological examinations of a further 40 samples are under way to determine whether the fish show the characteristic gill inflammation and damage associated with active PHV disease.

Investigations into other possible causes have not found evidence that harmful algae were the primary cause. Paralytic shellfish toxins were not detected, while domoic acid and yessotoxin occurred only at low levels, with the highest concentrations in an apparently healthy comparison sample. Examination of stomach contents and material rinsed from the gills also found no harmful algae at densities likely to have caused the mortality.

Unusually cold water and low-oxygen conditions were observed along parts of the West Coast. However, these conditions were broadly similar to those recorded during the recent pelagic recruitment survey prior to the event, when no dead or dying sardines were observed. Low oxygen also occurs periodically on the West Coast without previously causing sardine mortality on this scale. The apparent restriction of the mortality to sardines makes a general environmental cause less likely, although environmental conditions cannot yet be excluded as contributing stressors.

Taken together, the evidence increasingly points to PHV as the leading explanation, but its causal role will only be confirmed once the pathological and wider surveillance results are available. PHV is a fish virus and poses no known risk to human health. There is no evidence that it can infect or cause disease in people. Its detection does not, by itself, mean that healthy sardines caught commercially or canned sardine products are unsafe to eat.

A high-level task team comprising DFFE, affected municipalities, management authorities, the fishing industry and other relevant stakeholders has held its first meeting and agreed on a coordinated response plan. This includes expanded surveillance and testing, clearer guidance on the disposal of dead fish, public-health communication and an assessment of the possible impact on the sardine resource.

The public should continue not to collect, consume or feed dead or dying sardines to pets. DFFE will provide further updates as confirmed results become available.