There’s a new turn of phrase emerging among the scientific community across New South Wales that goes ‘you can lead a horse to water… but you can’t make a blue groper eat a long-spined sea urchin.’ No matter how much you may wish it to. 

Surprising new evidence has emerged this past week casting new doubt over the long-held belief that large predators such as the blue groper and pink snapper are the primary control species of the long-spined sea urchin – a species that has been held mostly responsible for the spread of ‘barrens’ outside the waters of New South Wales and into those of Victoria and more southerly regions of the Australian coast.

It’s an unexpected turn of events that could go some distance to shape new approaches to marine ecosystem understanding, presented in a study that has – since its publication in the scientific journal Estuaries and Coasts – done its best to subvert many an assumption.

So, let’s start at the beginning. ‘Barrens’ is the term now given to areas of low – or no – seaweed cover of a marine ecosystem in which it would be expected to flourish. As such, a ‘barren’ – arguably – has lower productivity and absorbs less carbon dioxide than the seaweed forest it replaces. This is, of course and in many regards, bad news.

Except where it isn’t. In New South Wales, ‘barrens’ are a natural part of the marine habitat created – as they are – by a native species, the long-spined sea urchin. These long-spined sea urchins were once believed to be the snack of choice for large predators like the blue groper and the pink snapper – two species once thought to play a significant role in controlling the spread of the urchins.

That is, at least, until Jeremy Day and his colleagues from the University of Newcastle decided to take a closer look at the stomach content of these two species of fish. Because what they discovered challenges everything we thought we knew about them. What they found “was these species are eating long-spined sea urchins far less in the wild than we previously believed.”

An unexpected turn of events. These findings could now prove critical to our understanding of the shifting food web and could certainly be used to inspire approaches to the way marine management strategies are thought through, particularly as ‘barrens’ emerge further south and into the waters around Victoria and Tasmania.

“We investigated the stomachs of blue groper and pink snapper from the Great Southern Reef and were surprised to find almost no evidence that these fish prey on long-spined urchins specifically,” said Jeremy Day, a University of Newcastle Ph.D candidate and lead author on the study. “However, they seem to consume other species of sea urchins in significant quantities.”

To make their discovery, Jeremy and the team examined and analysed the stomach contents of 39 blue groper and 111 pink snappers, all of which were donated for science by international fishers in 2021 and 2022. They were fished from an area spanning 1,250km along the Great Southern Reef, where long-spined sea urchins are a naturally abundant, native species.

Much less than first thought. While it has been observed that both the blue groper and the pink snapper do indeed snack on long-spined sea urchins, the aim of this study was to assess and shed light on just how regularly this predation occurs in the wild, without human interference. And according to the findings – it’s occurring much less than initially thought.

In fact, the results revealed a clear trend: in blue groper, long-spined sea urchins were detected in just 5.1% of fish, while other sea urchin species were present in 30%. Similarly, pink snapper consumed very few long-spined urchins (2.7% of fish), yet other urchins were present in 13%.

“This tells us it’s likely pink snappers and blue gropers help control other sea urchin populations to an extent. But their role in managing long-spined sea urchins in New South Wales appears far less significant than previously assumed,” said Day. 

The most obvious conclusion that can be drawn from this study is that the foodwebs across the waters of New South Wales are more complex than scientists may have accounted for. While it was thought that larger fish – due to their stronger jaws and feeding abilities – were key to controlling populations of long-spined sea urchins, it certainly seems – based on the limited evidence in this study – that this is no longer the case.

But it appears there may be more layers to the story here.

A contentious time. There’s no denying that the prevalence of long-spined sea urchins has become something of a contentious issue across the Great Southern Reef. In 2023, a Senate inquiry recommended a princely $55million be invested into the development of new management strategies for the emergence of ‘barrens’ left behind by this voracious species.

The issue is, in the waters of Victoria and further south towards Tasmania, these long-spined sea urchins are not native – raising concerns among conservationists that the habitats they form as they spread into these regions act against the local ecology. It’s understandable, therefore, that actions will be discussed on how best to manage their spread.

In New South Wales, however, where the urchin is native Day argues that the formation of ‘barrens’ is part of the natural order.

“The fact they are not being predated as much as once thought – even in protected areas where there are steady populations of these predator fish species – and the fact they are native species in their native waters, it presents an idea that this is a natural habitat and that this might be a natural shift.”

What does it mean for the future? Of course, there are many other factors that could be contributing to all of these findings – not least, what’s going on in the biology of long-spined sea urchins that has taken them off the menu for those predatory fish. And, with climate change driving changes across marine ecosystems, it;’s understandable that this will be an area keenly observed by scientists and researchers for time to come.

For Day, the next step in the process will be to conduct “further research using other methods of dietary analysis” of groper and snapper stomach contents to “help build a more complete picture of urchin predation.”

“At the moment, a study like this provides us with lots of answers,” he said. “The trouble is, we don’t really know what the questions are.”

The study was co-authored by Dr Megan Huggett, a senior lecturer in marine science and Associate Professor Troy Gaston, Associate Dean Engagement and Outreach with the School of Environmental and Life Sciences from the University of Newcastle.

Through its research, the University of Newcastle is committed to finding solutions to protect the next generation resources to bring the world close to a sustainable future. The study was published in the journal Estuaries and Coasts.

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