Scientists investigating the mass deaths of sardines along South Africa’s West Coast have confirmed the presence of pilchard herpesvirus (PHV), but authorities say it is still too early to conclude that the virus caused the deaths.
The Department of Forestry, Fisheries and the Environment (DFFE) said laboratory sequencing detected PHV genetic material in dead sardines collected during the investigation. The virus was also detected at lower levels in some apparently healthy fish.
The finding has added an important piece of evidence to the investigation, but has not provided a definitive answer about what caused the mortality event.
According to the department, the fact that the virus was also found in apparently healthy sardines suggests that its presence alone may not be enough to cause widespread deaths.
Scientists are therefore investigating whether environmental conditions may have triggered or worsened the outbreak. Possible factors include harmful algal blooms, low oxygen levels and unusual oceanographic conditions.
“These remain hypotheses and further evidence is needed,” the department said.
The investigation began after large numbers of dead sardines were reported at several locations along the Western Cape coast, including areas stretching from Saldanha Bay towards Gansbaai.
Testing was also conducted for toxins associated with harmful algal blooms. While a harmful algal bloom had been recorded in Saldanha Bay, testing of sardines from Saldanha Bay, the Cape Canyon and Gansbaai found no paralytic toxins associated with that particular bloom.
Other marine toxins, including domoic acid and yessotoxin, were detected at low levels. The department said the results did not point to the tested toxins as the primary cause of the deaths, although scientists have not ruled out environmental stress as a contributing factor.
The discovery of PHV is significant because the virus has not previously been reported in South African sardines. It has, however, been associated with major sardine mortality events in Australasia in the 1990s.
Despite the concerns surrounding the discovery, the DFFE said there is currently “no evidence” that the virus infects people or other animals.
The department also stressed that the detection of PHV does not mean that commercially caught sardines or canned sardine products are unsafe, and it does not currently justify a general suspension of fishing or canning.
An urgent briefing with stakeholders in the small pelagic fishing industry has been convened, while the department plans to establish a task team involving government, industry representatives and scientists.
Surveillance is also being expanded along South Africa’s West, South and East coasts to determine whether similar fish mortalities or evidence of the virus are occurring elsewhere.
The country’s routine hydroacoustic biomass survey, scheduled for October and November, is expected to provide an opportunity to assess whether the mortality event has affected the wider sardine population.
For now, scientists are continuing to investigate whether PHV was the main driver of the deaths, a contributing factor, or part of a wider combination of disease and environmental pressures.
The DFFE has previously advised members of the public not to collect or consume dead fish found along beaches while investigations continue.
