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.

This page shows the most recent blog posts. For the complete catalogue, visit the ‘Out on a swim index’ page.

This blog is rated in the Top 20 Coral Reef Blogs in the world.

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

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.

April 5, 2026

What Norfolk Island’s reef tells us about environmental blind spots

The Kingston dredging proposal on Norfolk Island raises a bigger question than dredging alone: how well do standard environmental assessment tools capture the real significance of a remote and unusual reef system like Norfolk Island’s?

March 30, 2026

Hammer coral time!

Hammer corals have unique tentacles that are large, fleshy, and tubular; these terminate in a ‘T’-shaped, hammer-head or anchor. Beneath all these softly waving tentacles is an extraordinary skeleton structure, which helps define them as a large polyp stony coral.

March 24, 2026

Norfolk Island’s fishes: drifters, residents and the ones still missing

Norfolk Island’s fish fauna reflects both connection and isolation. Some species may arrive from elsewhere as drifting larvae, some populations appear to persist locally, and some fishes known from islands on either side of Norfolk have still not been recorded here. This post looks at what old survey work, regional checklists and genetic studies suggest about that more complicated picture.

March 7, 2026

Alveopora or flowerpot coral – how to tell the difference

They look alike at first glance, but Alveopora and flowerpot corals are not the same. The easiest way to tell them apart is to count the tentacles.

February 27, 2026

Norfolk’s lagoonal reef – the 2025 report, in plain English

We now have the 2025 Norfolk Island reef health report, so I’m taking the opportunity to translate it into plain English here. Sadly, it’s more of the same story in Emily and Slaughter Bays – a reef that can cope with some stress, but is being asked to cope with too much, too often.

February 20, 2026

Halimeda’s night shift – why this reef algae changes colour

Halimeda is a calcareous green reef alga that forms new segments overnight, shifts from white to bright green by dawn, then pales again as calcification begins. A quick look at one of the reef’s smartest algae.