Yellowback Fusilier

The Yellowback Fusilier (Caesio teres) is a vibrant and common sight across the Indo-West Pacific, captivating divers with its striking coloration and dynamic schooling behavior. Belonging to the family Caesionidae, these slender, fast-swswimming fish are a key component of reef ecosystems, often encountered in immense aggregations.

This species is classified as Least Concern by the IUCN, indicating a stable population. Yellowback Fusiliers are diurnal, actively feeding on zooplankton in the water column above coral reefs, lagoons, and coastal waters. They are frequently observed at depths of 5–60 m, though often seen between 10–40 m, where they utilize their protrusible mouths to efficiently snatch plankton. Divers frequently encounter abundant schools of Yellowback Fusiliers at dive sites such as Kudarah Thila and HP Reef (Rainbow Reef).

Identification

• Size: The Yellowback Fusilier typically reaches a maximum size of 30 cm (12 in), though some individuals can grow up to 40 cm (16 in).
• Body Shape: They possess a moderately deep, fusiform (spindle-shaped), and laterally compressed body, indicative of their fast-swimming nature.
• Coloration: This species is easily identifiable by its brilliant yellow upper back, which extends from just anterior to the dorsal fin origin, covering the posterior part of the dorsal fin, the caudal peduncle, and the deeply forked caudal fin. The remainder of the upper body is a bright blue, transitioning to white on the belly. In some larger individuals, the yellow coloration may not extend as far forward.
• Fins: The dorsal fin features 10 spines and typically 14–16 soft rays. The anal fin has 3 spines and usually 11–13 soft rays. Their pectoral fins are white, distinguished by a prominent black spot at their upper base. The pelvic and anal fins are also white.
• Mouth: The mouth is small, terminal, and highly protrusible, adapted for capturing small planktonic prey.

The Yellowback Fusilier (Caesio teres) is a vibrant and commonly encountered species across the tropical Indo-West Pacific, known for forming impressive schools that are a highlight for divers. This pelagic, reef-associated fish thrives in a variety of coral reef environments, playing an integral role in these dynamic ecosystems.

• Habitat and Depth: Yellowback Fusiliers are primarily found around coral reefs, including outer reef slopes, deep lagoons, and coastal waters. They are often observed swimming in mid-water, particularly in areas with strong currents such as the entry and exit ways of tidal channels, which are rich in zooplankton. Divers can typically spot these fusiliers at depths ranging from 5–60 m, though they are frequently encountered between 5–50 m. They prefer warmer waters, generally found in temperatures of 22–28°C.
• Substrate Preference: While they are pelagic, Yellowback Fusiliers are closely associated with hard substrates, including rocky and sub-coral formations within their reef habitats.
• Ecosystem Associations: As a crucial component of coral reef ecosystems, Caesio teres feeds on zooplankton, converting microscopic organisms into energy that supports the broader reef food web. They are highly social fish, forming large, rapidly moving schools, often schooling alongside other fusilier species such as the Yellowback Fusilier (Caesio xanthonota). These synchronized schools are a common sight on many dive sites.
• Geographic Distribution: The distribution of the Yellowback Fusilier spans a vast area of the tropical Indo-West Pacific. Their range extends from the East African coast, encompassing regions from Somalia to South Africa, eastward across the Indian Ocean to the Pacific, reaching as far as the Line and Tuamotu Islands. Northward, they are found up to southern Japan, and southward to the Great Barrier Reef in Australia. Notably, they are absent from the Red Sea and the Persian Gulf.

Dive sites like Kudarah Thila and HP Reef (Rainbow Reef) are known for abundant populations of Yellowback Fusiliers, offering spectacular viewing opportunities for divers.

The Yellowback Fusilier (Caesio teres) is a captivating species for divers, known for its vibrant schooling behavior and crucial role in reef ecosystems.

Underwater Encounter Behavior

Divers frequently encounter Yellowback Fusiliers in large, organized schools, often comprising hundreds or even thousands of individuals. These schools exhibit fluid, synchronized movements, creating a mesmerizing spectacle as they navigate the mid-water column. They are diurnal, meaning they are active during the day, and their agile swimming allows them to dart rapidly, primarily as a defense mechanism against predators. When approached by divers, these schools typically maintain a respectful distance but can be observed moving together as a single, cohesive unit. They are commonly found around coral reefs, in lagoons, and along coastal waters, typically encountered at 5–50 m.

Feeding Habits

Yellowback Fusiliers are planktivores, with their diet consisting predominantly of zooplankton and small crustaceans. They are visual feeders, actively hunting for food in the water column. Divers can often observe them feeding just below the surface or in mid-water, particularly in areas where currents are strong, such as the entry and exit points of tidal channels in lagoons. These currents deliver a rich supply of zooplankton, including copepods, cladocerans, and the eggs and larvae of various benthic animals, which constitute their primary food source. Their small, protusible mouths are well-adapted for efficiently capturing these tiny organisms.

Social Structure

The social structure of the Yellowback Fusilier is characterized by strong schooling behavior. They live in groups and frequently form mixed schools with other fusilier species, such as the Yellowback Fusilier (Caesio xanthonota) and the Yellowtail Fusilier (Caesio cuning). This collaborative schooling enhances their survival rates by providing safety in numbers against predators. For optimal well-being, especially in controlled environments, it is recommended they be kept in groups of at least five or more, highlighting their inherent need for social interaction.

Notable Seasonal Patterns

While Yellowback Fusiliers are not considered migratory in the traditional sense of seasonal movements to different home ranges, they do exhibit distinct spawning patterns. Mature adults undertake migrations to specific areas near the reef for reproduction. Spawning typically occurs close to the surface, often in the openings of deep channels during ebbing tides. This reproductive timing is governed by lunar cycles, with couplings frequently observed shortly after sunset around the full moon. The eggs produced are numerous and pelagic, floating offshore with the currents. Their vibrant colors, which are prominent during the day, tend to subdue at night.

This species is abundant at dive sites such as Kudarah Thila and HP Reef (Rainbow Reef), offering divers excellent opportunities to witness their impressive schooling behavior in waters typically around 21.7–31.4°C.