Melanotaenia lacustris - The Turquoise Rainbow
by Alan Ford
First published in Tank Talk, Canberra and District Aquarium Society, Australia
Aquarticles
Melanotaenia lacustris is found only in Lake Kutubu, and its outlet the Soro
River which forms part of the Upper Kikori system in the southern Highlands of New Guinea.
Lake Kutubu in approx. 220 km from the Gulf of Papua and has an altitude of 800 metres
above sea level. Australia is only 65 kilometres away from New Guinea across the Torres
Strait. The lake contains 13 recorded species of fish of which 11 are endemic.
The Turquoise Rainbow is aptly named, as its colour suggests a bluish/turquoise which
varies in intensity according to lighting conditions. The top half of the fish is blue and
the lower a silvery white. These colours are then divided mid laterally by a dark blue
lateral band.
The fish attains a 'standard length' (tip of nose to base of tail) of 100 mm (approx. 4
inches), although they are capable of reproduction at the much smaller size of 40 mm. The
male, as in most Rainbowfishes, is a little larger than the female. The largest I have
seen, very old specimens, were approx 125 mm or 5 inches in length. A dominant or spawning
male is a sight to behold and a true wonder of nature. His forehead turns a really bright
orange in colour and then flashes various shades of blue, green, purple and violet. These
colour changes may last up to about 1 second for each shade but continually change whilst
the male is spawning which in turn can last for 30 minutes or more. These colours are
almost as bright and attractive as a series of neon lights. Spawning may take place at any
time of the day but mine spawn mostly during the evening.
Sexing these fish by finnage can be rather difficult and I find it easier to sort by
body shape with the female's body being more torpedo styled, and the male's head is of
slightly different appearance. Once they are spawning the difference becomes very obvious.
These particular Rainbows are placid by nature and are suitable for community tanks.
However, they should not be spawned in a tank with other Rainbows present as cross
spawning is highly possible and totally undesirable.
Once I have conditioned my breeding fish (5 in all) with extra titbits of live food
etc. I introduce a synthetic wool mop which is suspended in the water at the top of the
tank. Rainbows will deposit their eggs in this mop in preference to an assortment of
living plants! After much chasing of the females by the males the females will inspect the
mop and then quiver over and around the mop whilst expelling their eggs. This will usually
happen within 24 hours of introducing the mop and will be immediately followed by the male
quivering and shaking his head over the mop, expelling sperm to fertilise the trapped
eggs. He will be wearing his brightest and best colours at this time. As soon as they have
finished spawning I remove the mop (New Guinean Rainbows are avid egg eaters) to a
separate small tank 250 mm x 200 mm x 200 mm to which I have added aged water, 1/2 a
teaspoon of rock salt and about 1 teaspoon of shellgrit and a small airstone, bubbling
gently. Filtration is not necessary at this stage, nor is gravel, but a small amount of
duckweed is optional.
Once you have removed the mop replace it with a clean one if desired. I repeat this
process for about 5 days, always placing the egg-laden mop in with the previous one.
Hatching takes about 5 days at 27 C and up to 15 days at around 20 C.
Once the fry have hatched and are free-nwiming, feeding begins. Green water and the
tiniest amount of egg yolk (hard boiled) are very suitable for the first couple of days. I
then add vinegar eels to this diet and maintain this feeding for a good week. After the
first week microworms and brine shrimp are substituted and I feed this for another week or
so. If you have no cultures of live food and do not wish to keep them very fine commercial
food ouch as Sera Micron and TetraMin may be substituted, but be careful as the water may
become polluted if these preparations are overfed. At 2 to 3 weeks of age daphnia may be
fed but only a small amount at least 3 times daily. Do not overfeed live food as it can
easily lead to a high mortality rate as I discovered when I overfed my Glossolepis
incisus fry.
Finally you can progress to other foods such to finely ground flake etc. Water changes
should be carried out regularly and when necessary. The pH is not critical, somewhere
around 7 is OK, and it is now time to introduce a small filter such as a corner box type
containing filter carbon, shellgrit and filter wool. Be patient as their growth rate is
rather slow especially when compared to guppies. A word of warning here: do not put them
in with their parents too soon as they will consider them live food and scoff down the
lot. Maintain your fish at around 24 C and feed flake, daphnia, brine shrimp, chopped
earthworms, small ants, cyclops, plenty of vegetables (crushed green peas are good), and
vacuum away what is not eaten. They even enjoy Micro Vit pellets intended for the catfish
that help clean their tank. I also keep duckweed in the tank as a food source!
The future of this fine fish is somewhat debatable as heavy metal pollution from the Ok
Tedi mining project has reached Lake Kutubu, and also many other river systems in New
Guinea. The Government of New Guinea denies that this is happening, but with the dollar
today seeming more important than ever, who knows? Oil has been discovered nearby and
roads now seem imminent (previous access was by helicopter only) and to top it all off the
government wants to establish a settlement of 2000 people on the lake's foreshores. This
will also be disastrous in the long run.
While we have this fine fish let's try and maintain it. Remember 20% water changes
carried out weekly is not overdoing it. Use aged Canberra water for the changes, don't
overfeed, and do supply a heavily planted environment. Lighting is at your discretion and
give them plenty of growing room.
For further reading on New Guinean fishes see Dr Gerald R Allen's book Freshwater
Fishes of New Guinea. Additional information on keeping Rainbowfish may be gleaned
from Australian Native Fishes for Aquariums by Ray Leggett and John R Merrick.
Both are available from our Society's well stocked library. Better still, join the Native
Fish Study Group (see me, Doug Williams or Andrew Boyd for details).
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