… good news for the reef

For the last several weeks, a long-time diver in the Marine Park has been reporting the return of a sight he hasn’t seen in twenty years – a big school of doctor fish.  The return of these drab little fish in large numbers could be a lot of help to corals being smothered by algae.

The doctor fish is a member of a large family that also includes tangs, surgeon fish and unicorn fish, and lives in all the world’s warm oceans.  What all these fish have in common is a sharp spine on both sides of their bodies, just in front of the tail.  The spines, shaped like the blade of a surgeon’s scalpel, usually lie flat against the body.  When the fish is threatened it thrashes its tail and raises the spines, using them like knives.

Doctor fish aren’t much to look at.  Their bodies are narrow ovals, with small beak-like mouths.  They can grow to about a foot long, though most of the ones seen locally are between six and ten inches.  Their basic body colour ranges from pale beige to dark brown, with darker vertical stripes.  They can change colour within this range to blend in with the background.  (In surgeon fish, the doctors’ slightly larger cousins, the stripes are very faint or absent.)

Doctor fish are grazers.  Their main food is the algae found on rocks and hard corals, though they will also pick up plant and animal scraps (and the occasional small snail) off the bottom.  They’re not fussy and they eat all day.  Their hard, slightly saw-toothed jaws are well suited for tearing and scraping algae.  Food is swallowed whole, and ground up by bits of swallowed sand and rock in a gizzard that lies in front of the main stomach.  (Doctors, like parrotfish, are producers of beach sand.)

Their algae-eating makes doctor fish useful in salt-water aquariums, but in a confined space they can have problems getting along with each other.  In the wild they are often seen in groups.  The schools have two purposes, breeding (doctors spawn in groups) and defense.  A large school offers safety from predators, and discourages attacks from fierce little damselfish guarding territories where the doctors want to graze.

The school of doctors in the Marine Park had between 80 and 100 fish on the first sighting.  About three weeks later it had grown to at least 150, and included other grazers like tangs (colourful cousins that are bright yellow as babies and turn blue as they mature) and striped parrotfish.  Watching a big school of doctors is much like watching a flock of sheep cross a road.  The line moves slowly in the same general direction, but each fish is doing his own thing. Some are looking for food, others are stopping for a mouthful but none strays far from the group.

Doctor fish grow quickly, reaching breeding size in less than a year.  Many of the fish in this school were born in the protected area of the Marine Park and survived because fishing pressure has been reduced.  Now they are producing areas of clean bottom where existing corals can grow and new ones can get established.  It’s a small step towards restoring balance, but a welcome and useful one.