A rare purple cauliflower coral off NSW coast may be extinct within 10 years

An ovulid on a cauliflower coral colony. Such coral may be extinct within a decade. Credit: Author supplied

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Coral species are struggling including a rare type known as cauliflower soft coral which going on the brink of extinction. This species, Dendronephthya australis, looks like a purple cauliflower due to its pink-lilac stems and branches, crowned with white polyps. The coral primarily occurs at only a few sites in Port Stephens, New South Wales, and is a magnet for divers and underwater photographers. But sand movements, boating, and fishing have reduced the species population dramatically.

Recent flooding in NSW compounded the problem, in fact, it may have reduced the remaining coral population by 90%. The recent research found cauliflower soft coral may become extinct in the next decade unless we urgently protect and restore it.

Cauliflower soft corals are predominantly found in estuarine environments on sandy seabeds with high current flow. They rely on tidal currents to transport plankton on which they feed. The species is most commonly found in the Port Stephens estuary, about 200 kilometers north of Sydney. It's also found in the Brisbane Water estuary in NSW and has been found sporadically in other locations south of Jervis Bay.

The coral colonies form aggregations or gardens. At Port Stephens, these gardens are the preferred habitat for the endangered White seahorse and protected species of pipefish. They also support juvenile Australasian snapper, an important species for commercial and recreational fishers. In recent months, the cauliflower soft coral has been listed as endangered in NSW and nationally.

Scientists first mapped the distribution of the cauliflower soft coral in 2011. They found none of the biggest colonies in the Port Stephens estuary were protected by no-take zones areas where fishing and other extractive activities are banned. In research in 2016, found a sharp decline in the extent and distribution of cauliflower soft coral.

The recent study examined the problem in more detail. It involved mapping the southern shoreline of Port Stephens, using an underwater camera towed by a vessel. Researchers found the cauliflower soft coral in the Port Stephens estuary has declined by almost 70% over just eight years. It now occurs over 9,300 square meters down from 28,600 square meters in 2011.

Researchers subsequent modeling sought to identify what was driving the coral's decline. We found a correlation between coral loss and sand movements over the last decade. Human changes to shorelines, such as marina developments, have changed the dynamic of currents across the estuary. For example, previous research found a large influx of sand from the western end of Shoal Bay smothered cauliflower soft coral colonies at two nearby locations. As of 2018, those colonies had disappeared completely.

While diving as part of the project, we identified other causes of damage to the coral. Dropped boat anchors and the installation of moorings had damaged some colonies. Others were injured after becoming entangled in the fishing line. It is possible that disease, and pollution, or other water quality issues, may also be contributing to the species decline.