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)