Sticky sediments and low sperm increase risk to coral fertilisation

Coral fertilisation is most sensitive to sticky inshore sediments according to researchers working to define the effects of dredging-related pressures and natural high turbidity events.

The Western Australian Marine Science Institution Dredging Science Node study, published in Marine Pollution Bulletin, involved a team of scientists from the Australian Institute of Marine Science (AIMS), The University of Western Australia and James Cook University. It analysed a range of realistic mineral and organic compositions to try to determine what combination is most likely to affect coral fertilisation success.

The research, which spanned a five-year period and eight mass spawning events, identified various characteristics that make sediment ‘bind’ to coral sperm, preventing contact and fertilisation of the eggs.

Lead researcher Dr Gerard Ricardo (AIMS) explained that collecting data to assess the risk of many sediment types to coral fertilisation was difficult because corals only spawn a few times per year.

“We needed to work long into the night whenever the opportunity arose that the corals spawned,” Dr Ricardo said. “It was a massive effort from everyone who helped and volunteered, and we have benefited greatly from having access to incredible facilities like the National Sea Simulator (Seasim) at AIMS in Townsville.”

The researchers found that sediments high in ‘sticky’ components, such as mineral clays and mucous-like products of microorganisms, had the greatest impact on coral fertilisation, whereas less cohesive sediments had relatively lower impacts.


Coral sperm (blue) bound to fine sediment grains are incapable of fertilising eggs (Image: Gerard Ricardo)


The study also discovered that fertilisation success was most vulnerable to sediments when sperm concentrations were low. Lower sperm concentrations are likely to occur as coral populations become more degraded.

“Healthy reefs are at a lower risk of sediment impacting fertilisation success because even if some sperm are lost, there should be sufficient sperm to fertilise the eggs,” Dr Ricardo said. “As reefs become more degraded and there are fewer reproducing adults, this leads to lower sperm concentrations which may increase the risk of fertilisation impacts by sediments.”

The study provides a number of sediment-specific ‘thresholds’ that can be used by regulators and industry to assess whether dredging at a given location poses a risk to coral fertilisation and the ongoing health of nearby coral populations.


Ricardo G, Jones R, Clode P, Humanes A, Giofre N, Negri A. (2018) Sediment characteristics influence the fertilisation success of the corals Acropora tenuis and Acropora millepora. Marine Pollution Bulletin

WAMSI Dredging Science Node – Dredging Pressures on Coral project page.


The WAMSI Dredging Science Node is made possible through $9.5 million invested by Woodside, Chevron and BHP as environmental offsets. A further $9.5 million has been co-invested by the WAMSI Joint Venture partners, adding significantly more value to this initial industry investment. The node is also supported through critical data provided by Chevron, Woodside and Rio Tinto Iron Ore.


Dredging Science