Lacustrine Rainbowfish

The Lacustrine Rainbowfish (Melanotaenia lacustris), also widely known as the Lake Kutubu Rainbowfish or Turquoise Rainbowfish, is a freshwater fish in the family Melanotaeniidae, endemic to Lake Kutubu and its outlet in the Southern Highlands of Papua New Guinea. It is prized in the aquarium trade for its brilliant, uniform turquoise-blue coloration, among the most vivid of any rainbowfish species. Because it is restricted to a single lake system, the species is entirely dependent on the health of Lake Kutubu's waters. It is currently assessed as Endangered by the IUCN, with pollution associated with natural gas extraction around the lake identified as a significant ongoing threat to its wild population.

Lacustrine Rainbowfish reach about 10-13 cm and are known for their striking, uniform coloration.

• Body: slender, laterally compressed
• Coloration: brilliant iridescent turquoise-blue across most of the body
• Markings: a darker blue-black horizontal stripe runs along the midline
• Fins: two separate dorsal fins; translucent fins with a yellowish tinge
• Tail: forked The intense, solid turquoise-blue body sets this species apart from most other Melanotaenia rainbowfish, which typically display red, orange, or more mixed striping and spotting patterns rather than a single dominant blue tone.

This species is restricted entirely to Lake Kutubu and its outlet stream in the Southern Highlands Province of Papua New Guinea, a large, clear freshwater lake at moderate elevation. It favors clear, still to slow-flowing water with aquatic vegetation, submerged roots, and logs, typically schooling in the mid to upper water column. Because Lake Kutubu is its only habitat, the species has an inherently limited natural range compared to rainbowfish distributed across multiple river systems. Its dependence on a single lake makes it especially sensitive to changes in water quality within that system.

Lacustrine Rainbowfish are peaceful, social fish that form schools and move through open and vegetated areas of Lake Kutubu, generally avoiding aggressive interactions with other fish. They feed omnivorously on small invertebrates, algae, and organic matter found within the lake's productive shallows and vegetated zones. Breeding follows the typical rainbowfish pattern, with courting males displaying intensified turquoise coloration and eggs scattered among aquatic plants, left without parental care. As an endemic species facing habitat pressure from gas extraction-related pollution around Lake Kutubu, its population trends are closely tied to ongoing water quality conditions within this single lake ecosystem.