· Scientists found pilchard herpesvirus in dead West Coast sardines, but say environmental conditions may also have played a role in the deaths.
· Authorities say healthy commercially caught sardines and canned products remain safe, with no evidence that the virus can infect people or animals.
Scientists have found a virus never before reported in South African sardines after large numbers of the fish died along the West Coast.
But there is still a mystery to solve.
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The Department of Forestry, Fisheries and the Environment says tests found pilchard herpesvirus in dead sardines collected along the coast.
Lower levels of the virus were also found in some fish that appeared healthy.
This means scientists cannot yet say the virus alone killed the sardines.
They are now looking at whether problems in the ocean may have triggered or worsened the mass deaths.
These could include harmful algae, low oxygen levels or unusual ocean conditions.
The department stressed that this remains a theory and more evidence is needed.
Laboratory sequencing confirmed genetic material from the virus in the sardines.
An apparently healthy sample from St Helena Bay was also tested as part of the investigation.
Scientists also looked for toxins linked to harmful algae.
None of the paralytic toxins linked to a harmful algal bloom in Saldanha Bay were found.
Two other algal toxins, domoic acid and yessotoxin, were found at low levels.
But the highest levels were found in the apparently healthy comparison sample.
"No other toxins included in the screening were detected," the department said.
The results suggest the toxins tested were unlikely to have been the main cause of the sardine deaths.
However, scientists have not completely ruled out harmful algae or other environmental conditions.
The discovery of pilchard herpesvirus is significant because it has never before been reported in South African sardines.
The virus has been linked to sardine deaths elsewhere.
It was associated with two large mass mortality events involving sardines in Australasia during the 1990s.
But it was later found in apparently healthy Australian sardines without causing continuous deaths.
The department is now increasing disease monitoring along South Africa's West, South and East coasts.
It has also called an urgent briefing with the small pelagic fishing industry.
A task team involving the department, fishing industry and other stakeholders will be formed to coordinate the response.
Scientists will continue studying the disease, the dead fish and environmental conditions.
A routine survey planned for October and November will give scientists their first opportunity to assess what effect the deaths may have had on South Africa's sardine population.
For people who eat sardines, authorities say there is currently no reason to panic.
There is no evidence that pilchard herpesvirus infects people or other animals.
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The discovery also does not mean healthy sardines caught commercially or canned sardine products are unsafe.
"The finding does not currently justify a general suspension of fishing or canning," the department said.
The toxin test results were also described as reassuring.
But authorities warned that these results should not be seen as a blanket safety clearance for every commercial catch.
Normal food safety checks and monitoring must continue.
People have also been warned not to pick up or eat sardines that are already dead or dying.
They should not feed the fish to their pets either.
The department says it will release more information as scientists receive further results.
For now, the virus is an important clue in the West Coast sardine deaths, but scientists are still trying to establish exactly what caused so many fish to die.
