Milkfish

The milkfish (Chanos chanos) is a prolific species of ray-finned fish that is prevalent across the Indo-Pacific region. It holds the distinction of being the solitary extant species within the family Chanidae and the sole surviving member of the genus Chanos. The tautonym derives from the Greek word khanos (χάνος, meaning 'mouth'). Taxonomically, milkfish belong to the order Gonorhynchiformes and share close evolutionary ties with the Ostariophysi group, which includes freshwater species such as carps, catfish, and loaches.

The species is known by various vernacular names: in Hawaii (🇺🇸 United States), it is called "awa"; in Tahitian, "ava"; in the 🇵🇭 Philippines, it is referred to as "bangús," although not officially recognized as the national fish due to the lack of legal endorsement by the National Commission for Culture and the Arts. In the Nauruan language, it is termed "ibiya"; meanwhile, in 🇮🇩 Indonesia, it is known as "bandeng" or "bolu."

Geographically, Chanos chanos is found in both the 🇮🇳 Indian and 🌊 Pacific Oceans, with its range extending from 🇿🇦 South Africa to Hawaii (🇺🇸 United States) and the Marquesas, as well as from California to the Galapagos, extending north to 🇯🇵 Japan and south to 🇦🇺 Australia. An isolated sighting in 2012 was noted in the eastern Mediterranean Sea. The California Fish and Game Commission previously attempted to establish a population in Solano County, California, in 1877 by introducing specimens from Hawaii; however, this population did not become permanent, and its ecological impacts remain undetermined. Typically, milkfish inhabit tropical offshore marine waters around islands and along continental shelves, residing at depths of 1 to 30 meters. They frequently traverse into estuaries and rivers as well.

The genus Chanos is the solitary extant representative of the ancient Chanidae family, traceable back to the Early Cretaceous. Several fossilized species of Chanos have been identified within Paleogene strata of the Tethys and North Seas, dating to the early Eocene epoch. Initial fossil records indicate freshwater origins in Europe and North America, inferring that the species emerged in freshwater approximately 40-50 million years ago before transitioning to marine environments during high sea-level or flooding events post-Eocene. Subsequent global cooling in the Mid-Eocene likely eradicated milkfish from the Atlantic, although they persisted in the Indo-Pacific region.

Adult milkfish can attain lengths of up to 1.80 meters, though they are typically less than 1 meter, and can weigh approximately 14 kilograms, with a potential lifespan of 15 years. Their body is elongated and compressed, symmetrically streamlined with one dorsal fin, falcate pectoral fins, and a pronounced forked caudal fin. Notably, the head is disproportionately small relative to the body with a small, toothless mouth. The exterior features an olive green hue with silvery flanks and dark-bordered fins. Anatomically, they possess 13–17 dorsal soft rays, 8–10 anal soft rays, and 31 caudal fin rays, alongside numerous fine intramuscular bones which may impede human consumption.

Morphologically, milkfish are often mistaken for Clupeidae, Megalops cyprinoides, Gonorhynchus gonorhynchus, and Elops machnata. Distinguishing characteristics include size, absence of scutes, a tubercle on the lower jaw, fin positioning, lack of a gular plate between jaw arms, and only four branchiostegal rays.

On occasion, variant body types have been recorded, such as the 'goldfish-type' milkfish in the 🇵🇭 Philippines, exhibiting elongated dorsal, pelvic, and anal fins, alongside a caudal fin commensurate with its body length. Dwarf or hunchback ‘shad-type’ variants are known from Hawaii (🇺🇸 United States), 🇮🇩 Indonesia, and 🇦🇺 Australia. Additionally, a specimen with a red head, red fins, and blue dorsal color has been documented in Northern 🇦🇺 Australia.

Reproductive behavior in natural habitats is less documented than in captivity. In the wild, milkfish reach sexual maturity within 3-5 years, with females capable of producing 0.5-6 million eggs, spawning multiple times annually. Spawning typically occurs nocturnally, potentially in lunar cycles and is markedly seasonal. Eggs are pelagic, spanning 1.1-1.25 mm in diameter, with preferred spawning sites being clear, warm, shallow waters over sand or coral reefs to mitigate predation by benthic planktivores.

Milkfish are omnivorous and opportunistic feeders. Juveniles consume diverse edibles, including phytoplankton, zooplankton, filamentous green algae, and small invertebrates. Adults continue to eat benthic invertebrates, additionally partaking of planktonic and nektonic organisms, such as juvenile clupeids.

Mainly, adults form schools around coasts and islands with coral reefs. The juvenile phase involves living at sea for approximately two to three weeks before migrating to mangrove swamps, estuaries, and occasionally lakes, subsequently returning to the sea for sexual maturation and reproduction. Favoring undisturbed coastal ecosystems for settlement, juveniles seek semi-enclosed, tranquil, shallow predator-free environs saturated with aquatic vegetation. Milkfish demonstrate significant adaptability to environmental changes and dietary conditions, as evidenced by their capacity for osmoregulation and euryhalinity.

The expansive geographical distribution across the Indo-Pacific facilitates genetic diversity among milkfish populations, which show differentiation between the Hawaiian islands, central Pacific islands, 🇹🇴 Tonga, Tahiti, the 🇵🇭 Philippines-🇹🇼 Taiwan-🇮🇩 Indonesia cluster, 🇹🇭 Thailand-🇲🇾 Malaysia, 🇮🇳 India, and Africa. Despite this variation, all populations interbreed, maintaining low genetic diversity across the species. However, reproductive, migratory, and survival strategies may vary among different regional populations.