The 8000km Great Southern Reef is in peril

Even though it fringes more than 8,000 kilometres of densely populated coastline and is a global biodiversity hotspot on par with the Great Barrier Reef, few people have heard of Australia’s other great reef.

Comprised of thousands of temperate rocky reefs that are interconnected through oceanographic, ecological and evolutionary processes, the Great Southern Reef stretches from northern New South Wales, down to Tasmania and around the rugged southern coast, and up to Kalbarri in Western Australia.  

It is dominated by vast kelp forests which are home to hundreds of species of fish, sponges, crustaceans, chordates, bryozoans, echinoderms and molluscs – at least 70 percent of which are endemic to the region and found nowhere else on Earth.

As the team of scientists who, in 2016, first defined the Great Southern Reef as an entity, write, a “remarkable feature” of its biodiversity is the “the high rate of endemism within many taxa” – a result of the reef’s geographical isolation along with the “stable climatic and geological history it has experienced over the past 50 million years.”

But a new study led by Professor Graham Edgar, Senior Research Fellow at the Institute for Marine and Antarctic Studies at the University of Tasmania, has shown that the unique biodiversity of the Great Southern Reef is in even more serious peril than its more famous tropical counterpart – in large part due to the unstable climatic conditions that are now gripping the globe.

The unique biodiversity of the Great Southern Reef is in even more serious peril than its more famous tropical counterpart.

Published in Nature, the study – the “most comprehensive assessment of marine species population trends to date” – examined population trends of more than 1,000 marine species from 1,636 sites around Australia in the decade to 2021. In order to complete such a mammoth undertaking, Edgar and his colleagues relied on the help of a vast network of volunteer recreational divers trained in standardised scientific data collection as part of the Reef Life Survey.

This project, which Edgar helped establish in 2007, is one of the largest-long-term reef monitoring programmes worldwide. In the last fifteen years, its participants have counted more than 3,000 species of fishes, corals and other invertebrates at more than  2,500 dive sites around Australia and an additional 1,500 sites in 53 other countries worldwide. According to Edgar, it is also markedly different from many other citizen science projects, in that its emphasis “is on the quality of the data, not necessarily public engagement.”

For this study, data collected as part of Reef Life Survey was combined with data collected by the Australian Temperate Reef Collaboration and the Australian Institute of Marine Science in Queensland. It showed that while a handful of species had increased slightly in number, the populations of more than half of all shallow-reef species around Australia had decreased – with as many as 138 decreasing so much that they are now eligible for Endangered and Critically Endangered listing on the IUCN Red List.

“It is likely that some species, such as the smooth handfish (Sympterichthys unipennis), which hasn’t been observed since the early nineteenth century, is likely already extinct.”

Graham Edgar

Although Edgar had previously observed similar declines at the local scale, he says he was surprised by “the consistency of the declines at the continental scale.”

The steepest declines were detected in cool-temperate species that live within the Great Southern Reef, such as the iconic weedy sea dragon (Phyllopteryx taeniolatus) which suffered a 59 percent decline in numbers from 2011 to 2021. Other examples include the critically endangered red handfish (Thymichthys politus) and spotted handfish (Brachionichthys hirsutus) which have declined to tiny populations of, respectively, around 100 and 5000 individuals living in bays around Tasmania.

In fact, more than 30 percent of shallow invertebrate species in cool latitudes exhibited “high extinction risk” – and Edgar says it is likely that some species, such as the smooth handfish (Sympterichthys unipennis), which hasn’t been observed since the early nineteenth century, is likely already extinct.

While overfishing has contributed to the continent-wide decline of shallow reef species, the primary driver is increasing ocean temperature associated with human-induced climate change. But the reason temperate-species are being disproportionately impacted by this is more nuanced.

Part of the problem is that the area of the ocean in which the Great Southern Reef is located is a climate change hotspot: in recent years, the rate of ocean warming there has been in the top 10 per cent globally. This warming is particularly acute in the waters surrounding Tasmania, which have warmed by around 1.5 degrees Celsius in the past century.

This means that the temperate species that inhabit these rapidly warming waters need to migrate poleward, where the water is cooler, in order to survive. But this isn’t possible, because there is no more suitable habitat to colonise; there is only, as Edgar says, “a cliff at the Southern Ocean barrier”.

While overfishing has contributed to the continent-wide decline of shallow reef species, the primary driver is increasing ocean temperature associated with human-induced climate change.

“If the Australian continent extended another 200 kilometres south, those species would probably be doing fine at the bottom end of the range,” he says. “But that, of course, isn’t the case.”

Compounding this is the fact that species more typically found in warmer waters further north are moving south to escape the heat – especially following marine heatwaves, like the one that occurred in 2011 off south-western Australia, when the Leeuwin current heated up by 4 degrees Celsius. This increasing abundance of warm water species further south is having a squeezing effect on populations of cold-water species.

Even though limited data exists about population trends of marine species in other temperate waters of the world, there are indications that what’s happening in Australia is part of a larger pattern. For example, among Californian inshore and pelagic fish species, there have been marked population declines since the 1970s – especially among colder water taxa. 

And it seems as though the problem is only going to get worse, with the recent ‘triple’ La Nìna event which had helped limit ocean warming now over, and preliminary data from the National Oceanic Atmosphere Administration showing the average temperature of the world’s ocean surface is now at an all-time high of 21.1 degrees Celsius.

There are indications that what’s happening in Australia is part of a larger pattern.

In order to help mitigate the worsening impact of climate change on temperate marine species, Edgar says it is necessary to implement more effective conservation strategies that better address additional local stressors like overfishing.

Although he welcomes – “as an aspirational target” – the recent United Nations High Seas Treaty, which aims to place 30 percent of the ocean beyond national jurisdictions into protected areas by 2030, he says “a lot more focus needs to be put on location and habitat type and the quality of protection rather than just a blanket area.”

Specifically, Edgar wants to see a significant expansion of no-fishing marine reserves – “the most effective protected areas in the ocean,” according to one 2017 study – in cool-temperate waters that have, up until now, largely been neglected. For example, in Australia, ‘no-fishing’ marine reserves cover 14 percent of tropical waters off Queensland but only 1 percent of cool-temperate coastal waters around Tasmania.

But in order for this to happen, Edgar knows that people must first recognise “what an important treasure the Great Southern Reef is.” Because only then will they realise what is at stake if nothing more is done to protect it.

The Great Southern Reef Foundation is currently seeking donations on their website.