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Species of the Month: The Trigger Fish


The majority of dive sites here on Koh Tao are lucky enough to have a resident triggerfish or two. Out of a total of 40 species of Trigger, the three most common to Koh Tao are the Yellow-margin Trigger Fish (Pseudobalistes flavimarginatus), the Titan Triggerfish (Balistoides viridecens) and very occasionally the Clown Trigger Fish (Balistoides conspicillum). These beautiful, exotic, colourful fish are some of our favourites for many reasons and have an important role in the marine ecosystem.


The Trigger Fish can grow up to 75 cm long, and gets its name from its first of three dorsal fins which when ‘triggered’ stands erect above its head like a horn. This is a sign of defensive aggression on behalf of the Trigger Fish whenever it feels threatened or provoked, often when a diver or predator enters its territory or ‘garden’. The Titan Trigger Fish is known to be especially territorial around mating season, and should therefore be viewed with caution from a safe distance. The anal and posterior fins rhythmically undulate from side to side to provide the main force of movement, whereas the sickle shaped tail fin, or caudal fin, is only used for speed when defending against or escaping potential predators. Other notable features include their small independently rotating eyes allowing them excellent 360 degree visibility and special awareness.


The majority of Trigger Fish species are solitary for their adult lives, yet meet at set timetables according to moons and tides at traditional mating grounds. This is typically when species become the most territorial and potentially aggressive, and dominate a territory in the shape of a cone rising vertically. Therefore if ever attacked or chased by a Trigger Fish, try to swim away low and horizontally to escape their cone shaped ‘garden’ and do not swim up further into the cone. Also try to swim backwards which allows you to keep an eye on the Trigger Fish, and places your fins in-between yourself and any potential attack. The main defence to a Trigger fish is not to provoke one in the first place by respecting its personal space and keeping a watchful eye on its trigger. These really are beautiful fish so please don’t worry or avoid them, just respect and enjoy!

Unsurprisingly, Trigger Fish are known to show a higher level of intelligence that is unusual among fish and actually have the ability to learn from previous experiences. This is apparent when observing the deliberate way they examine their surroundings, contemplate possible food sources and methods of feeding all while staying aware of potential threats. Trigger fish in aquariums are also thought to recognise individual keepers and further understand human behaviours such as those associated with feeding and tank cleaning.


Trigger Fish feed on a diet of bottom dwelling sea urchins, molluscs, crustaceans, and coral. You can often see them eating hard coral by clamping on with their strong jaws and ripping it apart by spinning and finning in various directions. This activity often stirs up nearby coral parts and small organisms, allowing other fish to feast on the leftovers. Trigger Fish play an important part on coral reefs by naturally pruning the coral, much like pruning roses.
You’re bound to see a friendly Trigger Fish or two while diving on Koh Tao, so admire their beauty, applaud their intelligence, but just remember to respect their garden!

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