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 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

Jun 13, 2025

Glimpses of recovery: what the reef could be if we let it

Day 6 of this photo series from Norfolk Island coincides with the final day of the UN Ocean Conference in Nice. After a week of documenting decline, today’s post offers a different view – what reef recovery can look like when conditions improve. Drought in 2024 gave the reef a break, and the results were unmistakable: healthier corals, lower disease, and more fish. This is what’s possible if we act.

Jun 12, 2025

Warning signs: quiet and unnoticed collapse of two coral colonies

Day 5 of my blog series for the UN Ocean Conference: two long-lived coral colonies in Norfolk’s lagoon died quietly from disease. No drama – just slow collapse and overgrowth by algae. A reminder that not all reef losses are loud, but they are happening.

Jun 11, 2025

Warning signs: what Norfolk Island’s reef is telling us

Day 4 of a week-long photo series from Norfolk Island, shared during the UN Ocean Conference in Nice. Today’s post spotlights a Hydnophora pilosa colony where white syndrome appeared suddenly and spread quickly, taking out around a quarter of the coral. In the months that followed, algae quietly filled the gap – a subtle but telling shift from coral to algae that’s happening across the reef.

Jun 10, 2025

Warning signs: coral disease takes hold

In Day 3 of this blog post series, published while leaders gather at the UN Ocean Conference in Nice, we see Norfolk Island’s coral reef lagoon quietly delivering a stark warning: recurrent land-based pollution, coral disease, and delayed decisions are dismantling this ecosystem in real time.

Jun 9, 2025

Warning signs: coral growth anomalies – the slow cancers of the reef

Day 2’s post coinciding with the UN Ocean Conference looks at coral growth anomalies – sometimes called coral ‘cancers’. These slow-moving diseases quietly weaken coral colonies, making them far more vulnerable to storm damage and algal takeover. On Norfolk Island’s reef, I’ve watched this exact process play out over several years. This is how chronic stress silently dismantles coral ecosystems.

Jun 8, 2025

Warning signs: shifting baselines on Norfolk Island’s reef

Today is World Ocean Day — a timely moment to launch my week-long blog series on Norfolk Island’s reef. Each day this week, I’ll be sharing photo essays that document the slow but steady pressures reshaping this fragile reef. Today: how shifting baselines make us blind to what we’ve already lost.

Apr 29, 2025

The Governance–Government Vacuum: Norfolk Island’s Forgotten Ecology

A personal reflection on Norfolk Island’s coral reef environment, political denial, and what John Wyndham’s The Kraken Wakes can still teach us about slow-moving disasters — and why this election matters more than ever.

Feb 20, 2025

Cute as buttons – Astrea curta

Astrea curta corals are ‘small, moderately plocoid [flattened], distinct, and almost circular’ . Normally grey-green in colour, you can see from the images here, ours are often beautiful rich gold, although they do vary. They have a neat growth habit and button-like corallites, which can grow in columns, spherically or flattened. Large colonies of these can form gorgeous undulating bumps.

Jan 26, 2025

From 'Watch' to 'Warning'

Last week, the chance of coral bleaching in Norfolk Island’s inshore lagoons was raised from ‘Watch’ to ‘Warning’ and will more than likely rise to Alert levels one and two in coming weeks. So why do I worry about water quality all the time when bleaching seems inevitable these days and so the reef is probably doomed anyway? Read on to find out.

Jan 4, 2025

From little things – watching them grow

Small numbers of different fish species is not an unusual phenomenon on Norfolk Island’s reef, but it does demonstrate what a tiny, precious, coral reef ecosystem we have, when we can count individuals on one hand and watch each of them grow, like these little blackeye thicklips, a member of the wrasse family.