Slick New Coating More Than Doubles Coral Spat Survival on Turfy Inshore Reefs

On the inshore reefs of the Keppel Islands in the southern Great Barrier Reef, one of the deadliest threats to a newly settled coral is not a predator or a heatwave but a quiet, creeping carpet of competitors. Within months of settling, a coral spat barely a millimetre long can be smothered by crustose coralline algae, turf, bryozoans and sediment, and most restoration ecologists know that this early life-stage bottleneck is where larval-based restoration so often fails. Now, a year-long field trial has shown that borrowing a trick from the shipping industry, a non-toxic foul-release coating, can more than double the survival of seeded corals on these challenging reefs.

The new study, published in the journal Coral Reefs, was conducted by a team from the Australian Institute of Marine Science working across six macroalgal-influenced reef sites around the Keppel Islands, on Woppaburra sea Country in Queensland, Australia. The researchers tested whether a commercial, biocide-free foul-release coating, or FRC, applied to ceramic coral seeding devices could reduce fouling accumulation and improve the chances of survival for juvenile corals of the branching species Acropora millepora. The results were striking: devices treated with the coating carried roughly 24 percent fouling after 48 weeks in the water, compared with about 67 percent on untreated controls, and spat survival rose from roughly 22 percent to about 49 percent.

Foul-release coatings work very differently from the biocidal antifouling paints used on ships’ hulls. Rather than leaching toxic copper or organotin compounds, which are known to harm coral fertilisation and larval metamorphosis, these coatings rely on hydrophobic or amphiphilic surface chemistry that simply weakens the adhesion strength of fouling organisms. Algae and invertebrates struggle to grip the slick surface, and much of what does attach is easily sloughed off by water movement. That property has made FRCs increasingly popular in aquaculture and maritime applications, and earlier work by the same group had shown they reduced fouling on ceramic seeding devices by up to 70 percent at healthier, coral-dominated mid-shelf reefs, with a modest boost to spat survival of up to 12 percent.

What remained uncertain was whether such benefits would hold up under the far harsher conditions of degraded inshore reefs, where high macroalgal biomass, eutrophication and sedimentation create a fusillade of biological pressure on any artificial surface. To find out, the team collected gametes from 37 gravid Acropora millepora colonies around the Keppel Islands during the October 2022 spawning event, fertilised them on board an aquaculture-configured vessel, and reared the larvae in culture tanks. Once more than 70 percent of the larvae had reached settlement competency, roughly five days after fertilisation, they were induced to settle onto preconditioned concrete tabs carrying a crustose coralline algae biofilm, at a density of around 24,000 larvae per tank.

The settled spat, cut onto small 14 by 14 millimetre settlement tabs, were then slotted into alumina seeding devices, 216 in total, half coated with the commercial FRC Hempasil 77300 and half left as controls. Divers deployed the devices at six sites across the Keppels in December 2022 and retrieved them 48 weeks later. The sites spanned a gradient of benthic regimes: three were dominated by canopy-forming Sargassum macroalgae covering nearly two-thirds of the substrate, while the others were characterised by encrusting Lobophora and coral assemblages dominated by Montipora. At retrieval, divers recorded each device’s burial status, photographed the surrounding benthos, and brought the fouled devices back to the laboratory for detailed image analysis of fouling cover and spat survival.

The scale of the fouling difference was dramatic. The largest treatment effect appeared at Humpy, where control devices averaged nearly 72 percent fouling cover while coated devices carried only about 10 percent. Across all sites, the dominant foulers were crustose coralline algae and brown and red algae, both of which were substantially rarer on coated devices. Crucially, survival declined steeply with increasing fouling on the settlement tabs themselves: the model predicted survival of around 43 percent on completely clean tabs but under 4 percent where fouling approached total cover. Burial compounded the problem, with survival dropping to less than 1 percent on fully buried devices, whether they were buried by sediment, rubble, macroalgae or overgrowing coral.

Interestingly, the surrounding benthic community shaped outcomes in ways that were not always intuitive. Survival was generally higher at Sargassum-dominated sites than where understorey macroalgae such as Lobophora and Caulerpa, or encrusting corals, prevailed, possibly because these lower-growing competitors and sediment-retentive assemblages creep directly onto devices. Sediment accumulation of up to 25 percent on tabs at one site corresponded with sharply reduced survival regardless of treatment. Yet even across these environmental gradients, the FRC benefit remained consistent: coated devices maintained predicted survival above 30 percent across most benthic assemblages, while control survival fell to as little as 2 percent at coral-dominated locations. Posterior estimates suggested a 38 percent higher probability of finding live spat on FRC devices overall.

There was also a hint that the coating helped in an unexpected way, by discouraging burial. Fully buried devices were consistently less common among FRC treatments than controls, and the authors suggest that by limiting initial fouling accumulation, the slick surfaces may deny secondary colonisers, encrusting algae and corals the foothold they need to slowly overgrow and entomb the devices. Spat on coated devices also grew larger, with tissue covering up to about 24 percent of tabs compared with roughly 7 percent on controls, likely because reduced competitive pressure freed resources for growth, accelerating the recruits towards the size refuge at which they become far less vulnerable to overgrowth.

The findings carry practical weight for a restoration field that is racing against repeated mass bleaching. The 2024 bleaching event alone cut coral cover by more than 20 percent across the Great Barrier Reef and caused mortality approaching 90 percent on some reefs around Lizard Island, while inshore systems such as the Keppels face added pressure from flood plumes, turbidity and strong tidal extremes. Paradoxically, the same turbid conditions that buffer adult colonies against heat stress may worsen sediment deposition and fouling pressure at the millimetre scale where spat live. The Keppel Islands’ Acropora populations have repeatedly bounced back from six major bleaching events in three decades, but the authors caution that resilience at the colony level does not automatically protect the fragile post-settlement stages that determine whether a population can rebuild.

The team is quick to note that foul-release coatings are not a panacea. They mitigate one critical bottleneck, competition and overgrowth at the settlement surface, but they cannot override heavy sedimentation or poor site selection, and survival still fell sharply where tabs accumulated silt or were buried. The authors recommend pairing FRC-treated devices with site-prioritisation frameworks that minimise sediment and burial risk, testing coatings applied even closer to the settlement surface, trialling biologically inert settlement substrates, and refining device geometry to improve hydrodynamic performance. Longer-term monitoring will also be needed to capture seasonal swings in macroalgal cover. Still, as a scalable, non-toxic intervention, the approach offers something restoration has badly needed: a way to keep young corals alive long enough on degraded inshore reefs to give them a fighting chance of reaching adulthood.

Subject of Research: Testing biocide-free foul-release coatings on coral seeding devices to improve spat survival on macroalgal-dominated inshore reefs

Article Title: Coral spat survival on inshore reefs is enhanced by foul-release-coated seeding devices

Article References: Coral spat survival on inshore reefs is enhanced by foul-release-coated seeding devices. (n.d.). https://doi.org/10.1007/s00338-026-02953-5

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02953-5

Keywords: coral restoration, coral spat survival, foul-release coating, Great Barrier Reef, Keppel Islands, larval seeding, macroalgae, biofouling, Acropora millepora, sedimentation, reef rehabilitation, early life-stage bottleneck