Fish and dolphin deaths explained
3 June 2013
An extensive, multi-agency investigation has confirmed that the spate of fish and dolphin deaths around South Australia in March/April was caused by a combination of high water temperatures, algal blooms and dolphin Morbillivirus.
- pathology observations in fish showed high water temperature and harmful algae as the primary cause, which led to some weaker fish becoming susceptible to lethal bacterial infection.
- morbillivirus was the underlying cause of the death of the dolphins – in some cases it was the primary cause of death.
The report was compiled by the multi-agency government team investigating the spate of fish deaths in March and April of this year and dolphin deaths over the same period.
The team comprised members from Primary Industries and Regions South Australia (PIRSA), the Department of Environment, Water and Natural Resources (DEWNR), SA Water and the Environment Protection Authority (EPA).
No single water quality or pollution point source was found responsible for such a geographically extensive series of events.
The EPA categorically ruled out any link to the desalination plant following an extensive review of monitoring data.
Rather, water samples collected along the Adelaide metropolitan coastline during the fish deaths confirmed the presence of the spiny diatom.
The spiny diatom (Chaetoceros coarctatus) is type of a micro-algae. It has barbs that lodge in the gills of fish leading to inflammation and eventual death.
Its growth was encouraged by a strong upwelling event of deeper, colder waters in early March helped.
During this time, higher water temperatures were detected across both Spencer and St Vincent gulfs and much of the ocean waters, as far west as Fowlers Bay and reaching east to Bass Strait.
Ongoing warm weather supported the growth of the micro-algae.
Weather conditions changed around 20 March with strong winds blowing surface waters to the east and along the metropolitan coast of Adelaide.
The majority of fish affected were small-bodied, bottom-dwelling species, with leatherjackets the most numerous reported.
The high proportion of small-bodied fish affected, abnormal behaviour and inflamed gills are all consistent with the effects of harmful algae.
Regarding the dolphin deaths, the report found that six Indo-Pacific Bottlenose Dolphins died following an outbreak of dolphin Morbillivirus, with secondary infections such as fungal and parasite infestations (due to compromised immune systems, probably caused by the Morbillivirus) also implicated.
It was the conclusion of the investigation that the Morbillivirus caused the immune suppression which allowed the fungal and parasite infections to thrive.
Similarities in post-mortem observations for a larger number of animals (where tests are still pending) indicate that further positive results for dolphin Morbillivirus can be expected.
Dolphin Morbillivirus is one of a group of viruses, which includes the viruses that cause measles in humans, distemper in dogs and rinderpest in ruminants.
Stomach contents of dolphins were analysed by the Cawthron Institute in New Zealand, which is a world leader in algal toxicity, and no algal toxins were detected.
The prevalence of young animals that died is not surprising, as the very young (and the very old) are generally more susceptible to infection by a wide variety of pathogens.
The report can be viewed at http://www.pir.sa.gov.au/fishmortalities