Phase shifts and biodiversity — Norfolk Island's Reef

Out on A Swim

‘Out on a swim’ is a coral reef blog that tells the stories of the characters who live under the waves and what has caught my eye when ‘out on a swim’ in the lagoons of Norfolk Island. It is also a record of the difficulties Norfolk Island’s reef faces, like many others around the world, as a result of the poor water quality that has been allowed to flow onto it.

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Day 9 – March focus on Norfolk Island’s reef

‘Bok, bok bok.’

That’s the noise a banded scalyfin (Parma polylepis) makes as it shoos away anyone or anything that strays into its territory. Its territory is quite large. And they guard it quite fiercely.

And while we are at it, yes, fish do have voices, although they don’t have vocal cords. We just don’t get to hear them that often because of where they live.

Known here by Norfolk Islanders as aatuti, these fish aren’t the colourful show ponies of our reef, by any means. They grow to around 22 cm in length. The bands after which they are named gradually fade as they mature and they develop an orangey face with bony knobs that get bigger as they grow.

Aatuti mainly eat algae, although I have seen them scavenging on leftover sea urchins and having a good go at nibbling on the exposed bodies of tun snails. But where they excel is as great little gardeners, grooming and nibbling on their algal patch, which they aggressively guard all year round.

So why am I featuring them during my March-long focus on Norfolk Island’s reef?

It is because they are one species that is doing remarkably well in our bays, particularly as the reef transitions from coral-dominated to algal-dominated, which is unsurprising really as that is their main food source. The downside is they harass and bully all the other species that come anywhere near their territory, which, because they are so plentiful, is almost everywhere. All other fish – whether they share the same food source or whether they target a completely different one – are fair game. No fish is too large, no school is too numerous – the aatuti are uncompromising when it comes to protecting their turf. And everything, but everything, is their turf!

Above: Life stages of the aatuti

My concern is that there is a phase shift* taking place on our reef that is allowing aatuti to become the dominant species, to the detriment of some of our other inhabitants, thereby reducing the reef’s biodiversity and, ergo, its resilience.

Incidentally, I have noticed the aatuti moving away from their individual territories to spots where they are hoping to be fed by visitors. Like the aggressive monkeys in a Balinese temple, they can become very pushy and bitey, so please don’t take titbits out with you to feed to them.

Further reading: Underwater wars! Aatuti versus the elegant wrasse

*A phase shift on a coral reef is when the cover of the coral substrate is reduced in favour of algal dominance (McManus, Polsenberg, 2004)

Aatuti are normally spaced along the reef in their own territories. These ones are congregating at a spot where they think they may get fed. They can get quite aggressive towards passing snorkellers

July 24, 2026

From ridge to reef: unpicking the Waterway and Wetland Strategy

Norfolk Island’s draft Waterway and Wetland Strategy will guide work for the next ten years. I support its intent, but it needs to be clear, practical and measurable – joining the dots from ridge to reef so we know whether public money is improving water quality and helping the reef.

June 28, 2026

The red seaweed behind low-methane beef

A small red seaweed on Norfolk’s reef has become part of a much bigger story. Asparagopsis taxiformis can look like a delicate red feather duster or, at another stage of its life cycle, like a tiny cottony pom-pom. It is beautiful, easily overlooked, and now being used in the cattle industry to help reduce methane emissions. This post looks at the reef oddity behind the low-methane beef story – and why repeated local observation can be more useful than it first appears.

June 20, 2026

From coral scar to aatuti farm

Aatuti are bold little algae farmers, but how does one of their farms begin? Over the past year, I have been following several coral patches as small white scars became algal footholds, then larger defended patches. I still cannot say what caused the first wounds, but the photo sequences show something fascinating: on a reef where algae is already gaining ground, even tiny changes on the coral surface can become part of a much bigger story.

June 15, 2026

Norfolk’s water quality – when action is reported as outcome

A recent Australian Government media release presents investment, monitoring and catchment works as progress on Norfolk Island’s water quality. Some of that work is useful, and some of it was badly needed. But activity is not the same as proven improvement. This post looks at Kingston sewerage, wetlands, cattle, acid sulfate soils, groundwater and reef health, and asks whether Emily Bay and Slaughter Bay are actually being better protected.

June 14, 2026

How surgeonfishes got their name

Surgeonfish are named for the sharp little scalpels near their tails, but on Norfolk’s reef their more useful work happens at the other end. Pencil surgeonfish, bluespine unicornfish and their relatives help browse algae across the reef – a small daily job that becomes very valuable on an algae-rich lagoon reef like ours.

May 28, 2026

A shrimp storm

While setting my research cams last week, I swam into what looked like an underwater snowstorm. It appeared to be the aftermath of a mass moulting event, with large numbers of tiny, translucent shrimp-like exoskeletons drifting together near the surface.

Kingston dredging: what happens when a reef does not fit the framework

This correspondence with DCCEEW is about more than one dredging proposal. It is about what happens when an ecologically distinctive place is assessed through standard tools that do not always make its most important values easy to see. I am publishing it here because that is something we need to be aware of, both on Norfolk Island and more broadly in Australia.

May 24, 2026

Kingston dredging: the project advances, the questions remain

Kingston dredging is edging closer, and the paper trail is growing. This post brings together earlier correspondence with the Department and the latest media release so readers can see what has been asked, what has been answered, and what still remains unclear about the project, its rationale, and the protections proposed for the reef.

May 17, 2026

The lime-green coral in Slaughter Bay – a 40-year paper trail

Green Mountain – the name I give this coral in my database – is a coral I’ve photographed for years as I swim past. Then I found its backstory in the Norfolk Island National Parks archives: a rough map, reused paper, a note in the margin – ‘still thriving’. That’s how baselines begin.