Six months of major marine events in south-east Tasmania
Hobart's River Derwent and other south-east Tasmanian waterways have experienced some extraordinary marine events since December: jellyfish population explosions, toxic algal blooms, wild shellfish health alerts, and mass salmon and sardine deaths.
Water quality scientist Christine Coughanowr said it was not uncommon to see one or two such events in a typical summer, but "rarely would you see this many events over such a short period of time".
"I think it is quite plausible that at least some of these events are related."
A spokesperson for the Department of Natural Resources and Environment Tasmania (NRE Tas) said the incidents were "being closely examined".
"While these events can occur independently, it is uncommon to see several in close succession," the spokesperson said.
"It is important to note that marine events such as algae blooms are typically driven by a range of environmental factors such as water temperature, nutrient levels, and runoff from various sources, not solely agriculture and aquaculture.
"However, the potential for larger toxic algal blooms exists and the ShellMAP program continues to monitor local algal species."
NRE Tas and the Environmental Protection Agency (EPA) have engaged the CSIRO to help better understand why the events occurred.
"This work will inform the development of state-of-the-art tools that can be used to investigate the contributing factors of events like those experienced over the past summer," an EPA spokesperson said.
"And to establish near real-time modelling tools to foster understanding and support more informed decision-making around policy, regulation, planning and incident response."
The presence of warmer than usual waters is "likely to be an important factor contributing to the proliferation of marine organisms along the east and south-east coast of Tasmania", according to an EPA spokesperson.
Ms Coughanowr, co-chair of Tasmania's Independent Science Council and former CEO of the Derwent Estuary program, said "increasing pollution from human activities, combined in some cases with increasing water temperatures and changes in ocean circulation" were key causal factors.
"Excess nutrients, in particular, stimulate microscopic algal blooms in near-shore coastal waters, which can set off the whole cascade above," she said.
December 12, 2024 — An unprecedented number of small jelly-like creatures called "salps" are seen at beaches and bays across south-east Tasmania.
Lisa-ann Gershwin, a jellyfish and bioluminescence expert, told ABC Radio Hobart an abundance of phytoplankton caused by excess nutrients from aquaculture and agricultural runoff had attracted the salps, which feed on phytoplankton.
Dr Gershwin cautioned that salps "literally wiped out" the phytoplankton wherever they occurred, so everything else starved.
December 19, 2024 — People warn against swimming at many of Hobart's beaches after industrial waste from the Cadbury factory knocked out a wastewater treatment plant, resulting in sewage flowing into the River Derwent next to the Museum of Old and New Art (Mona).
Public health alerts include "do not swim" advice applied to the River Derwent between Austins Ferry and Old Beach in the north, and Sandy Bay across to Howrah in the south.
December 20, 2024 — Some residents in Hobart and surrounds post photos to social media showing a pink substance in the water.
Dr Gershwin told ABC Radio Hobart the pink hue was a kind of bioluminescent algae called Noctiluca scintillans or "sea sparkles", caused by an increase of nutrients coinciding with the salp die-off.
While not toxic for humans, the blooms can be a problem for fish farms, due to the fish being unable to escape should a pen be impacted by the algal bloom.
December 23, 2024 – Public health mostly lifts warnings against swimming in parts of the River Derwent due to untreated sewage, except for the western end of Bellerive Beach due to a localised issue.
January 1, 2025 – There are more widespread reports that Tasmania's south-east coast was aglow with the bioluminescent algae Noctiluca scintillans.
It is the biggest "bloom" in almost a decade.
Dr Gershwin told ABC Radio Hobart the algal bloom could lead to a spike in jellyfish populations across the state's south.
She described it as an environmental red flag, which could be fatal for other sea life.
January 23, 2025 – Large blooms of Aurelia aurita – moon jellyfish – are seen in the River Derwent and halfway up Tasmania's east coast.
The jellyfish naturally occur in the river, but not usually in such large numbers.
Dr Gershwin said the bloom had been caused by an oversupply of nutrients in the water, from runoff, overfishing, and a natural upwelling in the ocean.
She said "huge amounts" of nutrients came from salmon hatcheries upriver and fish farms.
February 16, 2025 – Chunks of biological waste are discovered on a southern Tasmanian beach, prompting community calls for testing and an investigation into the nature of the waste.
EPA later confirms the waste, which washes up at other beaches in the D'Entrecasteaux Channel over the following weeks, is congealed fish oil from dead salmon.
February 20-21, 2025 – The Bob Brown Foundation shares images and videos of dead salmon floating on the edges of fish pens, and piled up in industrial skip bins.
There are reports the salmon industry in south-east Tasmania is grappling with a widespread bacterial disease outbreak that is causing mass mortality events.
March 20, 2025 – Tasmania's Chief Veterinary Officer Kevin de Witte makes a statement about the salmon deaths, saying the mortality event in the lower D'Entrecasteaux Channel is largely due to an endemic bacterium known as Piscirickettsia salmonis (P. salmonis).
The bacterium did not present a human or animal health, or food safety risk, Mr de Witte said.
"The bacterium also does not pose a risk to freshwater fish nor is it known to pose a threat to other marine [saltwater] species."
May 5, 2025 – Tasmania's Environment Protection Authority (EPA) reports a decline in fish deaths after the mass salmon die-off and says water-quality monitoring no longer detects the presence of antibiotics used to treat the fish infection.
May 8, 2025 – Public health issues a wild shellfish alert for Boomer Bay in south-east Tasmania. It warns toxic algal blooms (also known as harmful algal blooms or HABs) are present in Tasmania and that elevated levels of algal toxins have been detected in shellfish in the area.
May 16, 2025 – Public health expands its wild shellfish alert to include the Mercury Passage in Eastern Tasmania. It advises not to eat "recreationally harvested mussels, oysters, clams, pipis, cockles, wedge shells abalone and scallop roe".
May 17, 2025 – Hundreds of small fish, later confirmed to be sardines, are found dead along the shoreline of the River Derwent. A few days later, Kevin de Witte, tells ABC Radio Hobart the sardines most likely died "due to a toxic algae" called Heterosigma.
In Tasmania, the EPA is the primary statutory regulator for water quality and its management, alongside NRE Tas.
Both were involved in testing and reporting during and following the recent salmon mortality event.
The Derwent Estuary Program also plays an important role coordinating a range of projects aimed at restoring and promoting the estuary and reducing pollution.
Following the salmon mortality event, the EPA conducted an internal debrief to "capture lessons from the mortality event" and participated in an inter-agency debrief that extended to consultations with the three salmon companies, an EPA spokesperson said.
"Mortalities are a known aspect of salmon farming worldwide. From an EPA perspective, the critical issue is timely collection and management of mortality waste in an approved manner."
Ms Coughanowr said more "robust monitoring" of both the condition of, and many activities affecting the health of, our rivers and coastal waters was required.
"Often people just call up and report what they are seeing, but we need a more systematic approach than that.
Ms Coughanowr said a marine event of the scale of South Australia's toxic algal bloom, which is bringing unprecedented destruction to the state's marine environment, "would be a terrible outcome for Tasmania".
"It is important that we don't just blunder into this with our eyes closed," she said.
A spokesperson for NRE Tasmania said Biosecurity Tasmania was continuing to look into the recent deaths of the sardines found along the shores of the River Derwent.
"At this stage, investigations indicate this mortality event is not linked to the salmon mortality incident in south-east Tasmania from January to April," the spokesperson said.
"Biosecurity Tasmania understands it is not pollution-related and the events are likely due to different infectious causes. From ongoing testing, P.salmonis and POMV have been ruled out as the cause of the sardine deaths.
"Initial results of testing conducted of the sardines indicate it is likely to be a result of Heterosigma, a type of algae. It is currently not believed to be a bloom, however further testing continues."
Ms Coughanowr said whatever the cause of the most recent fish deaths, the health of our waterways needed to be a priority for Tasmania.
"We need to reduce pollution and other pressures on our rivers and coastal waters to make them as resilient as possible," she said.
Members of the public who observe dead fish, pollution events or suspected biological material can call the Emergency Animal Disease Hotline on 1800 675 888 or the EPA Incident Response Hotline on 1800 005 171.
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