Trigger Fish

Watch out, watch out there’s a TrIgger Fish about... Before many dives you go on your Divemaster will say watch out for the trigger fish!!! These feisty creatures can be very territorial and protective of their young, here on Koh Tao we have to keep an eye on them especially at Green Rock and White Rock.

There are about 40 species of Trigger Fish and each type has distinctive colouring, being easily identified. Here on Koh Tao get a lot of the Yellow-margin Trigger Fish - Pseudobalistes flavimarginatus, the Titan Triggerfish - Balistoides viridecens and very occasionally the Clown Trigger Fish - Balistoides conspicillum.

They are a large-headed fish with a flattened body and have a first dorsal fin with 3 spines, 1st of which can be locked in the upright position by the 2nd smaller spine, this will then stay erect until the ‘trigger” is released. This is what they do when they feel threatened hence the term “trigger is up”. Its snout comprises about a third of its total length and its mouth is small with chisel-like teeth and a very strong jaw.

Trigger Fish do not normally use their tail for swimming, but instead undulate their dorsal and anal fins, keeping the body rigid. This gives the impression almost that they are flying through the water rather than swimming. Their territory is an inverted cone shape above the nest so remember to swim outwards rather than upwards if you need to take evasive action.


The Clown Trigger Fish can grow up to about 50cm and has beautiful colouring and is the least aggressive of the family. They are rare though and very camera shy, but if you’re lucky enough to encounter one they do make a perfect photo. The lower half of the body is dark with large white spots with the upper half, below and around its dorsal fin, being yellow with small black freckles. The eyes are generally masked with a white or yellow stripe crossing over the snout. The mouth of the clown trigger fish is bright yellow with a white line behind it. They are quite beautiful....


The Yellow-margin Trigger Fish can grow up to 60cm and the adults are pinkish to grey with a cross-hatched pattern on the sides of the body and yellow fin margins. There is a deep groove in front of the eyes and five or six rows of distinct spines on the scales on the tail base. They dominate Green Rock Dive site with a multitude of nests out in the sand. It is an amazing sight to see, but if you get too close and they feel that their young are threatened they will attack snapping at your fins!


Titan Trigger Fish can accelerate for short distances at a surprising speed, now while some triggerfish are merely reacting to what they perceive as threats to their nesting grounds - definitely a lesson for divers to respect the habitat of these fish - Titans seem to do so for the fun of it! The male stands guard over its nest and will charge at any divers or fish that cross into its territory- the zone in a full circle directly above its nest. If you’re lucky you will return with holes in your fins more misfortunate divers have been knocked out cold or suffered from bite wounds requiring medical attention.

Bites should be taken seriously as they are ciguatoxic. Ciguatoxin is a natural poison which is found on some tropical reefs. It infects fish which feed on marine algae. While harmless to the fish, even small amounts can have a harmful effect on humans. In cases of extreme poisoning, it may cause heart attacks, paralysis or death. Make sure if you are bitten you clean the wound well and apply antiseptic.

Saying that the Titan Trigger Fish won't always resort to violence though, just occasionally swimming at an intruder in order to provide them with an escort out of the nesting territory. However,should a colourful male titan happen to charge in your direction, it's best to quickly swim horizontally away on your back, using your fins as a barrier between you and it.

These really are magnificent looking creatures though and as long as you keep your distance are wonderful to watch in their natural environment.