A humpack whale sighted off Sleaford Bay at Port Lincoln on Monday, May 3 has been recorded as the first sighting in SA for the 2020 whale season.

The south coast whale season meanwhile kicked off early, with two humpbacks spotted in Encounter Bay waters on May 5, spotted by whale watcher Elizabeth Steele-Collins from the Waitpinga Cliffs.

While some might joke these whales paid little attention to the State's coronavirus border controls to enter SA waters, most probably don't realise that marine mammals carry their own viruses.

There was in fact a mysterious viral outbreak in South Australian waters in 2013 when at least 27 dolphins, all juveniles, died in Gulf St Vincent waters.

Autopsies determined a virus was responsible for the death of at least six of the dolphins.

Founder of the citizen science group Kangaroo Island / Victor Harbor Dolphin Watch, Tony Bartram recalls the deaths.

"Interestingly whales, especially dolphins, suffer from a virus akin to COVID-19, called morbillivirus, which kills large numbers around the world regularly," he said.

"We last had an outbreak here in 2013 and lost animals in both KI and Victor Harbor. The first recorded victim was a young calf we named Indigo.

"It is endemic in the SA dolphin populations and tends to kill the very young and the very old. Not surprising really considering they are mammals just like us."

There have been no whale sightings off Kangaroo Island yet, but KI / VH Dolphin Watch and the AusOcean marine research group have placed listening devices at Smith Bay to detect any arrivals.

The 2013 morbillivirus outbreak was investigated by Primary Industries and Resources SA, which released a report later that year.

The onset of the dolphin deaths that year coincided with high water temperatures and an algal bloom thought to be responsible for widespread fish kills, mostly leatherjackets, in SA's gulf waters.

There was no direct connection made at the time between the fish kills and the warming waters, so we may or may not see more fish kills and dolphin deaths as the climate warms.

"Major epidemics can occur when cetaceans not previously exposed to the disease come into contact with it," the PIRSA report reads.

"Not much is known about dolphin Morbillivirus in cetaceans but the literature suggests it causes lethargy, respiratory distress, sometimes abnormal behaviour patterns (such as striking their heads against rocks) or skin lesions; and can predispose the animals to secondary infections through compromised immune systems.

"Lethargy or certain abnormal behaviours may explain the bruising found on necropsied animals. The virus spreads through close contact between animals, including between mothers and newborns. There is no known risk to humans from the virus. Humans have their own morbillivirus, measles."