Taxonomy & naming
Melanotaenia fredericki was described by the American ichthyologist Henry Weed Fowler in 1939, on the basis of material from the Sorong region of what was then Netherlands New Guinea and is today the Indonesian province of Southwest Papua. Eschmeyer's Catalog of Fishes governs the valid name; the species is now placed in Melanotaenia, and because it was not originally described in that genus the authority is conventionally cited in parentheses as (Fowler, 1939).
The genus Melanotaenia is the largest in the family Melanotaeniidae, the true rainbowfishes, which radiate across New Guinea and northern and eastern Australia within the order Atheriniformes. The Sorong rainbowfish belongs to the cluster of Bird's Head species, where meristic and morphometric characters overlap broadly between forms. That overlap has made the boundaries of M. fredericki a moving target. Material once attributed to it from Salawati Island has since figured in the description of the closely allied Melanotaenia salawati, which differs in a longer snout, larger interorbital width, a shorter second dorsal spine, and a longer anal-fin base with more rays.
Meristically, FishBase records the species with 5–7 dorsal spines and 13–14 dorsal soft rays, and a single anal spine with 25–28 anal soft rays — counts that situate it comfortably within the genus rather than diagnosing it sharply against its relatives.
Morphology
Melanotaenia fredericki is a small rainbowfish, reaching a maximum recorded length of about 3.5 in SL (Allen, 1991). Examined specimens cited in the taxonomic literature run a little smaller in practice — a female of 3 in SL from Salawati Island and a male of 3 in SL from the Samson River, West Papua, are among the reference material. As in other Melanotaenia, the body is laterally compressed and moderately deep, carrying the family's characteristic divided dorsal fin: a short anterior dorsal followed by a longer posterior dorsal, set above an elongate anal fin.
Life-colour notes for this species are thin in the accessible literature, and the FishBase reference figure is a line drawing rather than a colour photograph — so treat any account of its precise hues cautiously. As in the rest of the genus, mature males are likely deeper-bodied and more intensely coloured than the plainer, silvery females, with iridescence that shifts with mood and light. Beyond the general form and the meristic counts above, fine detail on its coloration is data sparse.
Habitat
The Sorong rainbowfish is endemic to the Bird's Head (Vogelkop) Peninsula of West Papua, Indonesia. According to the IUCN assessment, it is known from streams on the outskirts of the city of Sorong and from the Samson River system roughly 43 mi farther afield, with additional records attributed to Salawati Island. FishBase characterises it as a freshwater, benthopelagic species of the Sorong region and the wider Vogelkop.
It lives in inland wetlands and streams — small watercourses on a peninsula that has seen rapid human development. FishBase puts it in warm, near-neutral water: a tropical range of about 75–82 °F and pH 6.5–7.5. It was once common in the streams around Sorong, but the IUCN notes it has grown scarce there as the city expanded, leaving most of the population dependent on the Samson River system and other waters outside the urban footprint.
Feeding
No species-specific diet study is available in the cached sources, and FishBase gives only a modelled trophic level of about 2.9, consistent with a small omnivore feeding low on the food chain. By analogy with the rest of the genus, expect an opportunistic omnivore: small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, and surface-fallen insects — plus algae and fine plant material grazed from submerged surfaces.
In the tank, rainbowfishes of this kind are undemanding. They take good flake and small pellets alongside frozen and live foods such as Daphnia, brine shrimp, and bloodworm, and a varied diet improves both condition and colour. With no firm feeding data for M. fredericki itself, these expectations come from the well-documented genus rather than from records of this fish.
Mating
Direct observations of courtship in Melanotaenia fredericki are not present in the cached sources; FishBase records its reproduction simply as involving distinct pairing. The wider taxonomic literature notes that the species shows the egg-scattering, plant-spawning reproduction typical of melanotaeniids, with adhesive eggs deposited on fine-leaved plants or, in captivity, on spawning mops.
Across the genus, courtship follows a consistent script. Dominant males stake out display areas and court females with intensified colour and flared fins, often flashing a pale 'courtship band' along the head and nape, with activity peaking in the early hours after first light. A receptive female follows a displaying male into vegetation, where the pair release a few eggs at a time. With no dedicated field observations for M. fredericki, these are the genus-typical behaviours it is expected to share rather than anything documented for it directly.
Breeding
The Sorong rainbowfish is a continuous, egg-scattering plant spawner with no parental care, in line with the rest of Melanotaenia; FishBase resilience figures (a population doubling time of 1.4–4.4 years, rated medium resilience) imply a reasonably fecund small fish. The adhesive eggs hang by fine filaments from fine-leaved plants or spawning mops, and a female releases a few eggs at a time over an extended spawning period rather than a single clutch.
Dedicated captive-breeding accounts for this particular species are scarce. An ANGFA Queensland species profile is referenced by hobbyists as the standard care-and-breeding source, but the host domain did not resolve at the time the research cache was built, so its specific husbandry notes could not be retrieved. Following the genus pattern, a breeder would condition a group, let them spawn over moss or mops, then move the egg-laden medium to a separate rearing container, since the adults eat their own eggs. The tiny fry need very small first foods — infusoria or a liquid fry food — before they graduate to newly hatched brine shrimp. Specifics such as egg counts and incubation time for M. fredericki are data sparse.
In the aquarium
Melanotaenia fredericki is rarely seen in the trade, and far less commonly kept than its celebrated neighbour, Boeseman's rainbowfish. Where it does turn up, treat it as a typical small-to-mid-sized rainbowfish: active, peaceful, and happiest in a shoal, in a tank with a long open swimming lane and stable water.
FishBase parameters point to warm, near-neutral conditions — roughly 75–82 °F and pH around 6.5–7.5 — which is a softer, more neutral profile than the hard, alkaline water favoured by Ayamaru-lakes species. A planted tank with open central swimming space and gentle current suits the genus, and the fish is described as harmless. With firm husbandry literature thin, a keeper is best guided by general rainbowfish practice and by sourcing tank-bred stock where it can be found — species-specific care guides are, for now, largely unavailable.
Conservation
Melanotaenia fredericki is assessed as Near Threatened (NT) on the IUCN Red List, under criterion B1ab(iii), with the assessment dated 4 December 2019. The listing reflects a small extent of occurrence — on the order of 2,0 mi² — combined with a continuing decline in habitat quality that brings the species close to the threshold for Vulnerable under criterion B1. The population trend is reported as decreasing.
The pressures are those of a small-range fish on a fast-developing peninsula. The IUCN documents urban expansion around Sorong, residential, commercial, and industrial development, agriculture including oil-palm plantations, fishing and harvesting, and stream pollution from stone-crushing plants. Once common in the streams immediately around Sorong, the species has grown scarce there as the city has spread, tying its persistence more and more to the Samson River system and other waters beyond the urban edge.
For a narrow-range endemic, that mix of a small footprint and intensifying habitat pressure is just what a Near Threatened listing is meant to flag: not yet imperilled, but with little margin if current trends hold.