Sergeant Major Fish
Scientific name Abudefduf vaigiensis
The Sergeant Major is one of the most recognisable and abundant reef fish around Koh Tao. Instantly identifiable by its bold black vertical stripes against a pale silver or yellow body, its pattern resembles military insignia — giving rise to its name.
Usually growing to around 15–20 cm, these damselfish are energetic, territorial and often seen darting in loose schools above shallow reefs. They are especially common in areas between 1–15 metres, making them familiar to snorkellers as well as divers.
During breeding season, males become highly territorial and may appear more yellow in colour. They guard patches of eggs attached to rocks or coral rubble and can be surprisingly bold — occasionally nipping at passing divers who venture too close to their nest. The behaviour is harmless but can be unexpected!
Sergeant Majors are hardy and adaptable, thriving in both calm and moderately current-exposed sites. Their abundance makes them an important part of the reef food web.
Find Sergeant Major Fish at...
Divemaster's tip
If one is circling you persistently in shallow water, check below — you’re probably near a nest. Simply move a few metres away and the behaviour will stop.
Look just above coral heads and rocky outcrops in shallow reef areas for large, active groups.
What do they eat?
Sergeant Majors feed on plankton, algae and small invertebrates.
Their flexible diet and tolerance for varying conditions make them one of the most successful and widespread reef fish species around Koh Tao.
