When it comes to corals, the first image most people think of is something like the Great Barrier Reef. They think of big, colourful, rocky structures in warm tropical waters, teeming with all kinds of life. The hard corals that build these structures might be the most famous, but they’re not the only important coral group. Soft corals, which don’t form lasting structures like their hard cousins, can be just as important for biodiversity, especially outside the tropics.
Dr Vincent Raoult, from the University of Newcastle, has spent time studying one of these lesser known coral species in collaboration with the NSW Department of Primary Industries and Central Coast Council. Dendronepthya australis, commonly called the cauliflower soft coral, is a threatened species that occurs in a few isolated populations along the central coast of NSW.
Vincent and his fellow researchers used drones to check how well some of these populations are doing.
Dendronepthya australis, commonly called the cauliflower soft coral. Source: Vincent Raoult
The importance of cauliflower coral
Soft corals are far less researched than hard corals. However, there is evidence that they play an important role in marine ecosystems, especially in temperate areas.
“There’s a lot more soft coral in around Sydney than there is hard coral,” Vincent said.
“Of course, it doesn’t make that persistent habitat, which hard corals do make. But it’s still a very unique habitat that does a lot of the similar things that hard corals do. So it provides actual habitat for things, it provides food and nutrition for a lot of things. And so it does definitely have that role to play that’s similar to the hard corals.”
Cauliflower coral in particular is quite important for many local marine organisms.
“A lot of commercially and recreationally important species of fish tend to live around that soft coral and feed on things that live on that soft coral,” Vincent said. The threatened White’s Seahorse also uses the coral as habitat.
Unfortunately, there’s a lot we still don’t know about this coral species, and it’s populations are disappearing.
Cauliflower coral. Source: Vincent Raoult
Why are soft corals understudied?
Part of the reason we know so little about soft corals is from a negative perception from historical studies.
“There was some evidence, historically, that showed that soft corals became more common in degraded reefs,” Vincent said.
“There was also studies that suggested that nothing ever really ate soft corals and so they didn’t have a role to play in the food webs. Whereas hard corals are fed on quite regularly by specialist fishes… there was a perception [soft corals] were toxic because there are some toxic species. And so they were seen kind of as a bad coral.”
While Vincent says this point of view has been progressively shown to be incorrect, the historical negative perception is reflected in the lack of interest in studying soft corals.
“There’s still very little known on soft corals relative to the hard corals.”
Threats to the cauliflower coral
Cauliflower corals grow in estuarine habitats, the places where salt and freshwater meet. But humans have had significant impacts on these habitats, especially on the heavily populated central coast of NSW.
“There’s lots of threats to these animals, already in the form of dredging, of sand provisioning on beaches…and boat anchor lines or mooring lines,” Vincent said.
“And then, unfortunately, with the flooding that we had over the last couple of years, we were concerned that basically they have been exposed to fresh water for too long.”
The relative ‘saltiness’ of an estuary is heavily influenced by openness to the sea and volume of freshwater inflow. Big floods, like the east coast has experienced over the past few years, have the potential to make estuaries much fresher than usual. Too much freshwater for too long can be devastating for salt adapted species like the cauliflower coral.
“It’s especially a problem around where they’re found in Brisbane Waters because the water basically comes from the Hawkesbury,” Vincent said.
“If the Hawkesbury is running really strong with a whole bunch of fresh water, then it prevents marine water from coming back up the system,” he said.
“There were probably conditions once or twice where there was essentially freshwater in Brisbane Waters and similar things happened in Port Stephens. And so there’s a lot of concern from local council as well as from New South Wales Fisheries that the population that was in Brisbane waters was basically decimated.”
Vincent was part of the research team that conducted a drone survey of Brisbane Waters to check on the status of the population.
Checking on the coral population with drones
The team had some idea of the coral population in the area prior to the flooding thanks to high-resolution satellite imagery from 2018. But satellite imagery wasn’t going to work for a post-flooding population assessment.
“Looking through the [satellite] data…if you don’t see them, it’s hard to say whether it’s because the conditions weren’t great or whether they’re not there,” Vincent said, “so we went out to try and verify this.”
The research team chose to use drones to conduct a comprehensive survey.
“I knew that we could see them with drones previously, because if the conditions are good, you can just see them from the surface.”
“The soft corals are bright white, so they come out pretty clearly if you can see them, and so here the visibility was about two metres from the surface. So we expected that we would see a lot of these soft corals, and unfortunately, we saw zero,” Vincent said.
“There’s a possibility there’s still some there in the deeper channels of the area, but it wasn’t particularly optimistic for many of us,” he said.
The outlook for the cauliflower coral isn’t promising. While their threatened status offers them some protections, most of the significant stressors that led to their decline still remain.
“It’s hard because the things that are threatening them are still occurring,” Vincent said.
“And you can imagine preventing what are deemed to be critical activities like dredging to allow vessels to pass are quite hard to stop,” he said.
“The mooring line problem has been something that’s been very well understood. You can install environmentally friendly mooring lines that prevent these sorts of issues. But the best majority of mooring lines don’t adhere to those requirements.”
It’s also difficult to know what steps to take when the species is so understudied.
“It’s quite sad, because this is a fairly unique species… We’d only just begun to study them and know what the species had done. So we’ve come out with four or five published scientific studies on this species, and now it’s basically almost gone.”
Fortunately, there’s a significant effort underway in Port Stephens to develop ways to rehabilitate the corals. For the Brisbane Waters population, the next step is to verify if any remnant colonies remain in the deeper channels where poor clarity might have hidden them in the drone images.
“The next step is to look a bit more deeply and try and see if we can find any remnant soft coral colonies in those deeper areas,” Vincent said.
“The good thing is, the drone data that we collected kind of forms the basis for tracking any recolonization that might happen in the future,” he added.
Explore more on GeoNadir
While Vincent captured these drone datasets to look for soft coral, they also have the potential to offer other valuable information, like the condition of seagrass in those locations. Because he uploaded these datasets to GeoNadir, other users can find and learn from these datasets.
Do you work with drone mapping? Consider uploading your datasets to GeoNadir so other people can learn from your work. Plus it’s an easier way to store and manage your data. You can get started for free today!