Moon jellyfish bloom at Hobart waterfront indicator of unbalanced marine ecosystem, scientist says

A mass of thousands of jellyfish at Hobart’s waterfront dock may look like a natural phenomenon, but a leading scientist has warned its presence indicates something awry in the marine habitat.

Biologist and jellyfish expert Lisa-ann Gershwin said Thursday’s spectacle was the most “gobsmacking” bloom of aurelia aurita — or moon jellyfish — she has ever seen.

“They actually bloom as a natural part of their life cycle, and they do bloom here at Constitution Dock every year, but not in these kind of numbers,” Dr Gershwin said.

Thousands of jellyish in the water
The jellyfish naturally occur in the river, but not usually in such large numbers. (ABC News: Scout Wallen)
Thousands of jellyish in the water
Moon jellyfish clone themselves and can quickly increase their population. (ABC News: Jonny McNee)

She said she has been receiving reports for some weeks of increasing jellyfish numbers in Storm Bay, east of Hobart, stretching to as far as halfway up Tasmania’s east coast.

The jellyfish put on a bioluminescent show for onlookers at Hobart’s waterfront on Thursday night.

blue bioluminescence and glowing jellyfish in water at night time
The jellyfish bloom and bioluminescence were visible at the Hobart docks on Thursday night. (Supplied: Mélina Lee)

While it may appear beautiful to onlookers, Dr Gershwin said the jellyfish bloom was not only a visible indicator that something was wrong in the ocean, but that jellyfish were also a driver of more damage.

Why are jellyfish numbers exploding? 

An “incredible sequence of events” has led to the mammoth population explosion, Dr Gershwin said.Gelatinous salps are swarming Tasmanian beaches

Photo shows A chain of mostly transparent bioluminescent organisms glowing blueA chain of mostly transparent bioluminescent organisms glowing blue

While they may not look like it, salps are more closely related to humans than they are to jellyfish. The extraordinary numbers of them accumulating at Tasmanian beaches have delighted biologists.

In mid-December, blooms of small jelly-like marine creatures known as salps appeared at beaches and bays over south-eastern Tasmania.

The salps were feeding on phytoplankton in the water, which are at a “tipping point” of abundance at the moment due to excess nutrients from aquaculture and agricultural runoff.

As the salps died off, they gave way to a bloom of noctiluca scintillans, or bioluminescent algae.

“That’s been lighting up shores all through Storm Bay and halfway up the east coast,” Dr Gershwin said.

Thousands of jellyish in the water
The population increase is a direct result from the river’s increasing nutrient load. (ABC News: Scout Wallen)

Dr Gershwin said as the noctiluca died off, the moon jellyfish has bloomed in numbers larger than she has ever seen.

“It’s a cascade of pest after pest after pest.”

But while the moon jellyfish is bad news for the marine environment, for people out swimming or paddling in the River Derwent a brush with the aurelia would not cause immense pain.

Dr Gershwin said that while the jellyfish have stinging cells that can penetrate delicate tissue, for most people “a sting would feel more like a warm blanket than a bee sting”.

a moon jellyfish
The jellyfish are washing up on Hobart beaches. (Supplied: Beth Duncan)

Waterways out of balance

Dr Gershwin, who has been studying jellyfish for 32 years, said these blooms were “red flags” the marine environment was off-kilter.

“This cascade of blooms of species … indicates that something’s out of balance in the ocean.”

She said the jellyfish population explosion was a direct result of an oversupply of nutrients in the water, from runoff, overfishing, and natural upwelling in the ocean.

Woman smiling for photos
Lisa-ann Gershwin is a jellyfish expert. (ABC News: Ebony ten Broeke)

She said “huge amounts” of nutrients come from salmon hatcheries upriver and fish farms.

“It’s too much. The ocean can’t take it … the ecosystem is teetering on collapse.”

Thousands of jellyish in the water
The sting of a moon jellyfish is mostly harmless. (ABC News: Scout Wallen)

Salmon Tasmania chief executive Luke Martin said it was “drawing a long bow” to associate the jellyfish blooms with salmon farming.

“Tasmania’s salmon industry is highly regulated, informed by some of the best marine scientists in the world, and uses the most advanced technology available,” Mr Martin said.

“Following one of the wettest Decembers in the last 20 years, perhaps commentators need to broaden their scope, turn their attention to the multitude of other potential impacts on our waterways.”

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‘Battle’ between healthy and unhealthy ecosystem

Dr Gershwin said as the ocean became more unstable, the jellyfish would continue to thrive.

“We see this battle between healthy ecosystem and unhealthy ecosystem,” she said.

“It goes back and forth and increases the numbers of jellyfish until the jellyfish reach a critical mass that they actually eat all the eggs and larvae of everything else, and the other species just can’t come back.

“I think we’re getting pretty close to that, if not already there.”

blue bioluminescence and glowing jellyfish in water at night time
Lisa-ann Gershwin says the cascade of blooms of species “indicates that something’s out of balance in the ocean”. (Supplied: Mélina Lee )

Jellyfish clone ‘squillions’ of themselves

Dr Gershwin said adult moon jellyfish could live up to 13 years, although most survive for anywhere from three to six months.

In the polyp stage, reached soon after hatching, jellyfish young can survive for a surprising 60 years or more and “clone squillions of replicas of themselves”.

Thousands of jellyish in the water
The jellyfish swarm at Hobart has coincided with a bloom of bioluminescent algae. (ABC News: Scout Wallen)

While adult jellyfish might only live for a few months, each female can additionally lay “squillions” of eggs.

She said it was also unusual to see jellyfish swimming in and around the bioluminescence algae, as it is now in Hobart.

“I’ve only seen this during the day, but at night it’s going to be one of the most magical experiences that any of us will ever see.”

How long the jellyfish stay will depend on weather and their food supply.

“Could be days to perhaps March or April. They could live the rest of their lives here, or get blown out by currents,” Dr Gershwin said.

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