ʻŌmilu: The Bluefin Trevally of Hawaiian Reefs
Pronunciation: ʻŌmilu, approximately “OH-mee-loo,” holding the opening “ō” slightly longer.
Meet the ʻŌmilu
A flash of blue fins moving across the reef may belong to an ʻōmilu, the bluefin trevally (Caranx melampygus). This strong-swimming member of the jack family is a predator with a distinctive look: mature fish develop blue coloration and bluish-black spots that help separate them from other Hawaiian jacks.
Where some reef fish spend their days grazing algae, ʻōmilu spend theirs searching for prey. They may travel alone or in small groups, moving over reefs and using channels that bring them close to shore.
How to Identify a Bluefin Trevally
Adult ʻōmilu are known for their blue fins and spotted bodies, but juveniles do not look exactly like smaller adults. Hawaiʻi’s Division of Aquatic Resources describes young fish as silvery blue above and silver below, with yellow pectoral fins. As they grow, bluish-black spots appear and blue coloration becomes more pronounced.
- Adult markings: Bluish-black spots across the body.
- Fins: Distinctive blue coloration as the fish matures.
- Juveniles: A more silvery appearance, with yellow pectoral fins.
- Movement: Active swimming over reefs, alone or with other fish.
Look at several features together rather than relying on a single flash of color. Age makes a real difference to this fish’s appearance, and a silvery juvenile may not yet have the markings that make adults so recognizable.
ʻŌmilu, Ulua, and Pāpio
Hawaiian jack names can describe both species and stages of growth. Smaller, juvenile jacks are commonly called pāpio, while ulua is used for larger fish of several species. ʻŌmilu identifies the bluefin trevally specifically; other names recorded for it include blue ulua and hoshi ulua.
The bluefin trevally is a different species from Ulua aukea, the giant trevally. The giant trevally’s steep forehead and substantial build contrast with the bluefin’s characteristic adult spots and blue fins. Learning the species names helps untangle the broader names used for Hawaiian jacks.
Where ʻŌmilu Live
Young ʻōmilu use shallow bays and estuaries, which serve as nursery habitat. As they grow, they use reef habitats and more exposed waters. Adults can follow channels through coral reefs toward shore and swim over reefs during the day.
Although closely associated with Hawaiian reef life, bluefin trevally are not found only in Hawaiʻi. Their distribution extends widely across the Indo-Pacific and into parts of the eastern Pacific.
What Do Bluefin Trevally Eat?
ʻŌmilu feed primarily on smaller fish. Studies summarized in Hawaiʻi’s life-history compendium also record crustaceans in their diets, though fish make up the larger share of prey by volume in the studies described.
They are mainly daytime hunters, with feeding activity also noted around dawn and dusk. Rather than treating the reef as a place to settle, they move through it looking for opportunities to catch prey.
A Hunter That Follows Other Foragers
One particularly interesting behavior is the ʻōmilu’s association with other feeding animals. Research summarized by Hawaiʻi’s Division of Aquatic Resources records feeding associations with goatfishes, sharks, barracuda, and rays.
They most often forage alone or in pairs, but may occasionally feed in larger groups. This variety makes them worth watching: a single fish crossing the reef and a pair following another forager can offer very different glimpses of the same species’ behavior.
Getting to Know the Reef’s Different Diners
Comparing ʻōmilu with algae grazers such as Manini or Lauʻipala reveals how differently reef fish find food. Some crop algae from surfaces; others pursue smaller animals. Both are part of the reef’s food web.
Observe fish without feeding, chasing, or handling them. Giving them room is a simple way to enjoy their natural movements and feeding behavior.
Meet the Fish Behind the Cards
Keep discovering Hawaiʻi’s marine life with our Hawaiian Reef Fish Playing Cards. Watercolor illustrations paired with Hawaiian and English names bring reef discovery to the table, one hand at a time. Explore the full deck to meet more of the fish behind the cards.
Sources
Hawaiʻi Division of Aquatic Resources: Fishing in Hawaiʻi, A Student Manual, for identification, juvenile appearance, habitat, and feeding.
Hawaiʻi Division of Aquatic Resources: Life History Compendium of Exploited Hawaiian Fishes, for names, distribution, nursery habitat, diet, and feeding associations.