A new way of mapping the corridors that link populations in the ocean has found a patch of corals at Western Australia’s Houtman-Abrolhos Islands that is shouting out for attention.

The shallow reefs of the Abrolhos, 80 kilometres west of Geraldton, are a southern stronghold for Acropora spicifera, a coral species listed as vulnerable by the International Union for Conservation of Nature.

As with many other species across the archipelago, this tropical coral is like a fish out of water.

The Houtman-Abrolhos Islands are hot-linked to the tropics by the Leeuwin Current, a warm stream that brings larvae, eggs and fledgling marine life into latitudes normally too cold for comfort.

Hundreds of species of algae, invertebrates, corals and fish depend on the reefs and tidal pools of the archipelago, and the islands support breeding and nesting seabirds from around the world. Almost half the lobsters in the multi-million-dollar Western Rock Lobster Fishery begin life here.

Marine biologist Luke Thomas has spent three years studying corals at the Abrolhos as part of his PhD research with the University of Western Australia.

He and his colleagues have combined biological sampling, DNA profiling and ocean modelling to chart the level of genetic differences between adult populations at different reefs, (how closely they are related) and how they are spatially connected through larval dispersal.

The study, supported by the National Environmental Research Program (NERP) Marine Biodiversity Hub, was the first to combine these techniques to a complex island system and at a scale directly relevant to management.

Connections are a lifeline

‘The resilience of marine populations depends largely on how well they are connected,’ Luke says.

‘When connectivity is high, areas that have avoided being damaged by a disturbance event (such as coral bleaching) can act as a lifeline to damaged sites by providing a steady supply of larvae to help them recover.

‘When connectivity is low, reefs can be isolated and largely reliant on locally sourced larvae and so will recover much more slowly (or may be replaced by algae).’

Almost 400 coral samples were collected during expeditions to the islands in 2013─2015 in either October or March, when conditions are optimal for diving.

‘It was a great experience,’ Luke says. ‘This is what we marine biologists live for, but it is also stressful and a lot of work. Months of preparation went into each expedition, which involved a team of 10 researchers doing different studies.

‘The Abrolhos are often referred to as the 'edge of the earth’ and is a very remote place for field work: strong currents, challenging weather and an abundance of dangerous wildlife make for a unique experience.

‘I was the principal researcher on the trips and was in charge of everyone’s safety. So on top of worrying about my own research I had to keep track of everyone else.’

Modelling connectivity

The most genetically different corals were contained in a roughly 100 km² pocket at the Palsaert Island group, at the southern end of the archipelago. Relatively little variation occurred between the more distant sites.

To see if this pattern was driven by ocean currents, Luke worked with Johnathan Kool from Geoscience Australia (GA) who, has developed a model of marine connectivity. The modelling work was part of the NERP Marine Biodiversity Hub Theme on National Ecosystems Knowledge, led by Scott Nichol of GA.

‘The model uses “agent-based simulation” to predict how the larvae are transported,’ Johnathan says.

‘Agent-based simulation provides a powerful way to study complicated systems, and has been used to study a wide array of topics including stock markets, social networks and economies.

‘We create artificially intelligent agents and embed them in a virtual world based on realistic values, and see where they go.’

The model has been used in other Marine Biodiversity Hub research to identify submarine canyons off Albany and North-West Cape that contribute strongly to larval exchange in the deep sea.

Understanding such connections provides a context for assessing the impacts of development in deep sea environments which are becoming increasingly explored for oil and gas development and deep seabed mining.

For the Houtman-Abrolhos Islands, the model simulated the dispersal of 1000 coral larvae from each of the 15 coral sampling sites ─ on multiple dates (five) and multiple years (five) ─ based on what is known about the ocean currents.

Sure enough, the model showed that larvae from the Palsaert corals were cut off from others across the archipelago.

‘The model showed this pocket was isolated from the greater larval pool, possibly because it is shadowed from the Leeuwin Current by the northern island groups,’ Luke says.

‘This means it is likely to rely on locally sourced larvae to maintain healthy coral populations.’

Other species with a high-capacity for dispersal, such as the pulmonate limpet, also show significant genetic structure in the Palsaert group, but not elsewhere at the islands.

Isolation slows recovery

These findings have direct relevance for management and conservation planning, particularly in light of the marine heatwave off WA in 2011 that caused large-scale coral bleaching, with patchy effects at the Abrolhos.

Special management may be needed to conserve the genetically distinct coral populations of the Palsaert group because they are likely to recover slowly from disturbance.

In addition, the Palsaert Group hosts one of four fish-habitat protection areas at the islands. This lies adjacent to the unique genetic areas identified in this study and is likely to protect a valuable larval-source population for adjacent reefs.

‘Knowing how genes are exchanged can help to find the most important places for conservation, but most research has focussed along coastlines or at scales of 250 km or more, ’ Luke says.

‘Our study was one of the first to link genetics and oceanography to study an island archipelago at a scale of less than 100 km.

‘These smaller scales are generally more relevant to management, so linking gene flow with the physical environment at this scale should be a priority, particularly for habitat-forming species such as reef building corals which are in decline worldwide.’

See the 2011─2015 Final Report of the NERP Marine Biodiversity Hub for more research that supports biodiversity management in Australia.
 

Further reading

Thomas L, W. Kennington J, Stat M, Wilkinson SP, Kool JT, Kendrick GA. Isolation by resistance across a complex coral reef seascape. Proceedings of the Royal Society B: Biological Sciences
 

Captions for above thumbnails

• Hundreds of species of algae, invertebrates, corals and fish depend on the reefs and tidal pools of the Abrolhos.
• Luke Thomas has spent three years studying corals at the Abrolhos as part of is PhD research with the University of Western Australia.
• Almost 400 coral samples were collected during expeditions to the Abrolhos Islands in 2013─2015 in either October or March, when conditions are optimal for diving.
• Connectivity modelling identified an isolated pocket of coral, possibly shadowed from the Leeuwin Current, that is likely to rely on locally sourced larvae to maintain healthy populations.

Contact

Luke Thomas, University of Western Australia
Johnathan Kool, Geoscience Australia
Scott Nichol, Geoscience Australia