What emergency intervention to save the world’s only mass-breeding cuttlefish population from a toxic bloom might look like

The Biodiversity Council is pleased to see the South Australian Government rapidly pulling together an expert advisory group to investigate the potential for an emergency intervention. The move aligns with one of the most urgent recommendations of the Biodiversity Council report released last week.

We spoke to two experts about the population, the risk posed by the algal bloom and what an emergency intervention might involve. A key takeaway is that decisions will need to be made quickly while the window to safely collect and transport eggs is open.

A globally unique population at risk

Each winter, tens of thousands of Giant Australian Cuttlefish gather to breed in the shallow waters of Point Lowly, near Whyalla. This is the only population of any species of cuttlefish in the world that aggregates to breed in a spectacular annual mass event along a short strip of coastline less than 8km long.

“It's a kaleidoscope of colour. They're here, all communicating, interacting, and mating, and it's here that the females lay their eggs. This happens nowhere else in the world that we know of,” said marine ecologist Dr Zoe Doubleday from The University of South Australia.

Most years, mating and egg laying come to an end around mid July, and the adults begin to die, leaving the next generation of eggs to develop alone for several months before hatching and dispersing.

"Cuttlefish, like most cephalopods, are short-lived and typically die once they breed. As such, if the offspring are wiped out, there are no parents around to replenish and repopulate the population.”

"If the toxic algal bloom, which is devastating marine life in the St Vincent Gulf and lower part of the Spencer Gulf, were to reach and kill all of these eggs, or the newly hatched young, it is likely to mean the end of this amazing aggregating cuttlefish.”

"We’ve got to think of ways to protect them,” said Doubleday.

A Whyalla treasure

Marine scientist Stefan Andrews from the Great Southern Reef Foundation has spent hundreds of hours underwater observing the cuttlefish breeding aggregation over the last 15 years. Stefan notes that the aggregation was delayed a little this year, giving scientists a little more time before eggs mature.

“Early in the season, cuttlefish numbers were lower than usual, and there were more predators like snapper, so they were more timid. Last week, there was still a lot of action, with the most mating pairs I’ve seen all season.”

Over the years, he has witnessed firsthand how important the event is for Whyalla. “This is internationally recognised. Divers, photographers and tourists come from all over the world, stay for a few days, spend money in town. You see cuttlefish postcards and souvenirs everywhere – the town really comes alive in winter,” he says.

The site is recognised as a place of outstanding natural significance, protected under Australia’s National Heritage list because of its global uniqueness. Stefan also highlights the protections put in place in recent years: a permanent ban on cuttlefish fishing in the upper Spencer Gulf and a Cuttlefish Coast Sanctuary Zone. “These measures were critical to protect the population, and it would be tragic if the bloom undid all that work."

What might emergency intervention look like?

As the South Australian Government brings scientists and managers together today, the survival of this population could hinge on swift, precautionary action. The key option that will be on the table is the collection and captive rearing of a portion of the eggs.

Unfortunately, managers cannot track the bloom for months and collect eggs at the last minute. If eggs are going to be collected, it needs to be while they are immature, as they are right now. As the months pass, they risk hatching prematurely if disturbed. Newly hatched young cannot be collected at all because they swim away and disperse.

Once transferred to tanks, the water conditions of Spencer Gulf’s unique environment need to be matched to ensure the eggs develop normally. The shallow northern part of the gulf experiences strong warming and evaporation, creating salinity levels as high as 42 parts per thousand, compared with 35 in the open ocean.

Captive rearing of collected eggs has been successfully done in the past on small scales during research. In the wild the females lay eggs suspended from the underside of rocks, so in tanks, eggs can be stitched to floating styrofoam, allowing them to hang freely to maximise water flow and oxygenation.

The biggest challenge of captive rearing would come months later if the eggs hatch and the young still cannot be released due to the bloom. Hatchlings are harder to keep than eggs as they require large volumes of live food, usually tiny shrimp, which would need to be cultured or sourced.

Scaling this up to provide a safety net for a meaningful proportion of the population would require identifying appropriate aquaculture facilities with the water-handling systems and expertise to raise much larger numbers of cuttlefish hatchlings.

Preparing for an uncertain future

This year’s bloom has already caused widespread marine mortality in other parts of South Australia and its future behaviour is uncertain.

For the sake of the cuttlefish and other marine life already impacted or under threat, we hope that the bloom will dissipate soon. But, until it does, conservation managers need to rapidly prepare for the worst.