Melanotaenia lacustris (wild-caught male) - photo© Neil Armstrong

Melanotaenia lacustris


Munro, 1964
Lake Kutubu Rainbowfish

Species Summary
Depending on water conditions, captive environment and diet, Melanotaenia lacustris can display an array of different colours and patterns ranging from cobalt blue, steel blue, aquamarine through to lighter and darker shades of turquoise. When spawning the nape area changes to a bright orange or gold colouration. Males are more brightly coloured, larger, and with a body depth of 4~5 cm, much deeper bodied than females. They may reach a maximum size of 12 cm, but usually less than 10 cm.

Melanotaenia lacustris is closely related to Melanotaenia mubiensis. Large adults of the two species have very similar colouration. However, they differ in several features, including body depth, eye size, and modal fin ray counts. Adults of Melanotaenia mubiensis are much more slender than those of Melanotaenia lacustris; M. mubiensis had an average depth as percent of the SL of 38.4 compared to an average of 47.2 for M. lacustris. The eye diameter of M. lacustris is larger than the snout length, but in M. mubiensis it is shorter or equal to the snout length. Although the two species have overlapping counts for dorsal, anal, and pectoral fin rays, there are significant modal differences. M. mubiensis most frequently has 14 or 15 dorsal rays, 21 or 22 anal rays, and 14 pectoral rays compared to usual counts of 12 or 13, 18 or 19, and 15 respectively for M. lacustris.

Melanotaenia lacustris (wild-caught female) - photo© Neil Armstrong

Distribution & Habitat
Melanotaenia lacustris have been collected in Lake Kutubu and the Soro River, which is the only outlet stream of the lake. The Soro River eventually flows into the Kikori River system. In late 1983 Gerald Allen, John Paska, and Barry Crockford collected around 40 live specimens. Only 4 fish (1 male and 3 females) survived the journey back to Australia and a week after arriving in Melbourne the only male became infected with hook worm and subsequently died. The aquarium hobby had to wait another 3 years before Heiko Bleher was able to collect further live specimens. He returned in 1988 together with Gerald Allen and once more was able to bring back live specimens.

Lake Kutubu is a scenically beautiful crystal clear lake situated about 40 kilometres from Mendi, the main town of the Southern Highlands Province in central Papua New Guinea. Lake Kutubu was originally formed when debris and ash originating from a volcano, blocked a valley. It is the 2nd largest lake in PNG and its largest perched lake. Measuring approximately 19 km by 4 km at its widest point, it has a maximum depth of about 70 m. Water conditions recorded at the lake were a temperature range of 21~25°C and a pH of 8.5~9.0.

An exceptionally clear lake where, in contrast to most of PNG's inland waters. The Lake plays a significant role in the maintenance of biodiversity of the Kikori River basin and beyond. The lake's extraordinary level of fish endemicity (10 of the 14 fish species found within the Kikori drainage are endemic to the lake itself) exceeds that of any other lake in the entire New Guinea-Australian region. The Kikori drainage and the surrounding primary rainforest also support high levels of endemism and rare terrestrial fauna. Lake Kutubu provides the sole spawning, nursery and feeding grounds for the 10 species of endemic fish.

The water quality of the mainstream rivers of the Tagari-Hegigio and Lake Kutubu-Digimu-Mubi sub-basins are typical of other mainstream rivers in Papua New Guinea that are near neutral to mildly alkaline (pH 7.4~8.2) and calcium-bicarbonate dominated. These properties are indicative of water draining a limestone catchment area. The lower calcium concentration, alkalinity and hardness of the Ai'io River, which drains to the upper Hegigio River, probably reflect the predominantly volcanic and sedimentary terrain at this location. Water hardness in all rivers except the Ai'io River (30 mg/L CaCO3) is moderate to hard (60~180 mg/L CaCO3). Conductivity values are generally similar in all streams, with median values ranging between 167~267 µS/cm.

Melanotaenia lacustris - photo© Gunther Schmida

Remarks
Australian Patrol Officer T. Terrell first collected this species in Lake Kutubu during 1955. He sent preserved specimens to Australian ichthyologist Ian Munro, who in 1964 described them as Melanotaenia lacustris. Ian Munro (1919-1994) worked with the CSIRO Division of Fisheries Research and was an early pioneer in the identification of Australian and New Guinea Rainbowfishes. Not only was he a highly regarded ichthyologist but an accomplished aquarist as well and maintained many Australian and New Guinea rainbowfish species. During the 1960's he was maintaining New Guinea rainbowfish species such as Melanotaenia papuae, Melanotaenia sexlineata and Melanotaenia goldiei in captivity. Although at the time Melanotaenia papuae and Melanotaenia sexlineata had not been scientifically described. He later went on to publish "The Fishes of New Guinea" in 1967.

Melanotaenia lacustris - photo© Neil Armstrong


Literature
Allen G.R. (1991) Field guide to the freshwater fishes of New Guinea. Christensen Research Institute, Madang, Papua New Guinea.

Munro I.S.R. (1964). Additions to the fish fauna of New Guinea. Papua New Guinea Agricultural Journal 16 (4): 141-186.

Adrian R. Tappin
Updated December, 2008.


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