Caesio teres
Blue-backed, yellow-flanked schooler seen over reefs in Sabah, Koh Tao and Raja Ampat in warm 25-29C water year-round.
Slim, torpedo-shaped fish around 28cm with a sharply divided colour split: electric blue along the back, bright yellow from the midline down to the tail. Usually seen in loose schools cruising the reef edge in open water rather than tight against coral. The forked tail is often yellow too, which helps separate it from similarly schooling reef fish at a glance.
Typically spotted mid-water in loose, drifting schools over reef slopes and drop-offs, often mixing with other fusiliers as they pick plankton out of the current. They pay divers little attention and will often part around you and reform rather than bolt for cover. Most sightings are in open water just off the reef rather than tucked into structure, and they seem active through the day.
Fusiliers feed by picking zooplankton straight out of the water column in open water, which is why they school in the blue rather than hugging the reef the way many snappers do.
No clear seasonal pattern shows up in the sightings, and with a comfortable range of about 25 to 29C on tropical reefs it turns up year-round rather than in a particular window.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most reliably logged in Malaysia, especially Sabah and the Peninsular Malaysia east coast, with strong numbers also in Koh Tao and Koh Pha Ngan in Thailand. Raja Ampat in Indonesia and Queensland in Australia are also solid bets, with smaller but consistent sightings in Fiji's Taveuni, Timor-Leste and Mauritius.
Mapping where Yellow And Blueback Fusilier is logged…
Sabah and the east coast of Peninsular Malaysia are the strongest areas, alongside Koh Tao and Koh Pha Ngan in Thailand. Raja Ampat in Indonesia and Queensland in Australia also produce regular sightings.
There's no strong season in the sightings data, and the species sits comfortably in 25 to 29C water, so it's a fair bet on tropical reefs any month of the year.
Look for a slim, streamlined fish about 28cm long with a hard colour split: blue on top, yellow below, often with a yellow tail. They're usually seen schooling in open water off the reef rather than sitting in coral.
They grow to around 28cm, a slim, fast-moving reef fish rather than a bulky one.
No, they're harmless plankton feeders that school in open water and show no aggression towards divers.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Emperor
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