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

September 13, 2026

No seagrass, but plenty of turtles

Norfolk Island does not have seagrass, although one of our most conspicuous green algae is often mistaken for it. Feather and grape Caulerpa can both form striking patches across the lagoon. I take a closer look at what Caulerpa actually is, why it is so easily confused with seagrass, and where our green turtles fit into the story.

September 6, 2026

Watching El Niño from Norfolk Island

El Niño is strengthening across the Pacific, but that does not tell us exactly what the coming months will bring to Norfolk Island. After a long wet spell, I am watching our rainfall and water supplies more closely again – and, underwater, keeping an eye on sea temperature, algae, coral recruits and the fish that turn up on the reef.

September 2, 2026

What the world’s reefs are telling Norfolk Island

Two major new coral reef reports carry a clear message for Norfolk Island: reefs can recover when pressures ease, but the time available for recovery is shrinking. Norfolk Islanders were raising concerns about water quality, runoff and the health of Emily and Slaughter Bays nearly forty years ago. Today we have much better science, a changing climate and a far clearer understanding of how closely the catchment and reef are connected. This article looks at what that means for water quality, long-term monitoring, dredging and how we manage Kingston from land to sea.

August 25, 2026

When the map changes

A fish once thought to belong only to Norfolk turns up elsewhere; another appears here hundreds of kilometres beyond its known range. Cardinalfishes and a Sabre squirrelfish show how our maps of species distributions are revised.

August 24, 2026

Endemic, native, undescribed – what do these words actually mean?

A Norfolk chromis that turned out not to be Chromis fumea, a blenny found nowhere else, and a tiny goby that may not yet have a name. A look at what we mean by native, endemic, undescribed and newly described – and why scientific names sometimes change.

August 16, 2026

What’s grazing in the green stuff?

A proliferation of Bryopsis on the reef has brought some beautiful little underwater grazers with it. I follow the trail from sap-sucking sea slugs and egg masses to a bigger question – why is there so much Bryopsis around at the moment?

August 8, 2026

The clam beneath Emily Bay

I’ve seen the beautiful shells of Laciolina chloroleuca in Emily Bay for decades, but I’d never really stopped to wonder where the living animals were. It turns out they may be buried half a metre beneath the sand. A closer look at an easily overlooked clam, its extraordinary shell markings, and the hidden life beneath our lagoon.

August 1, 2026

Reflections of an accidental marine scientist

What I thought would be a steady plod towards retirement, with plenty of time spent nursing grandchildren, was abruptly upended exactly six years ago today. This is the story of how an environmental catastrophe changed the course of my life – and led me to reinvent myself. It is a reflection on how, and why, I became an accidental marine scientist.

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.

July 13, 2026

Reef reality in pictures

Let the pictures do the talking. This is the reality on Norfolk Island’s reef today, laid out in a two-minute video.