Origin

River Murray Rainbow Fish is endemic to Australia. (This means that it IS native to Australia and is NOT native to anywhere else.)  It is found naturally in parts of the states of South Australia, Victoria, New South Wales and Queensland.

As its name suggests it is found in the River Murray.  It is also found in many but not all of the Murray’s tributaries and several other river systems and lakes.  It is not always clear whether populations of fish are natural, or have been introduced by Humans.  In Australia, looking at the periodic major floods in this dry continent you can easily see a mechanism for fish to spread from one place to another.  In the River Murray private observations suggest the River Murray Rainbow Fish is more widespread than the official statistics say. 

Size

River Murray Rainbow Fish can reach 10 cm (4 inches long), but most are smaller than this.  The males tend to be bigger than the females.

Temperature

Most of the articles on the internet about the River Murray Rainbow Fish, including the very authoritative Fishbase site, suggest that a temperature of between 22 and 26 degrees C is ideal for this species.  My experience suggests otherwise.  Although this is a very versatile fish which will certainly live healthily in an aquarium, being treated as if it were a tropical fish, the specimens with the best colour tend to be pond fish, subject to the natural rise and fall of temperature with the different seasons as well as the different weather.

Some articles suggest that this fish can take temperatures as low as 10 degrees C, but that there is evidence of high mortality during winter drought with low temperatures.  Some of the web sites containing this information do not acknowledge that it comes for the excellent book “Field guide to Freshwater Fishes of Australia” by G. R. Allen, S.H. Midgley and M. Allen.

It would be possible to take the implication from this observation that the low temperatures are responsible for the deaths.  However, the River Murray Rainbow Fish will live and breed in large ponds even in the Adelaide Hills where I expect that the winter temperatures would drop below 10 degrees C.

At the other end of the scale, the River Murray Rainbow Fish can survive higher temperatures than 26 degrees C but I doubt if extended periods of high temperature are good for them.  Apart from the direct effect of the temperature, there are also indirect effects like the reduced availability of Oxygen in the water.

Water Conditions

The River Murray Rainbow Fish is able to take a wide range of conditions.  Most of the places it occurs naturally have a high pH and the water is quite hard. In an aquarium I suggest that the pH should be between 6.8 and 8. Excessively soft water is not ideal, but most tap waters will be of a suitable hardness.

The River Murray Rainbow Fish can live all right in a normal tropical community tank with a Neutral pH and at about 24 degrees C.  However this is far from the conditions they normally get in the wild and they are unlikely to show their best colours under these conditions.  The best coloured River Murray Rainbow Fish are usually in ponds subject to the naturally varying temperatures as well as having plenty of plants for cover.

Food

The River Murray Rainbow Fish is an omnivore.  Like many of its relatives it eats more plant material than most tropical fish.  They will live on either Tropical or Goldfish food, but this should be supplemented with vegetable matter.  Duckweed (Lemna species) is eaten very readily by River Murray Rainbow Fish.  They also like many types of vegetable.  I cook the harder vegetables enough to soften them.  Zucchini are eaten as are cucumber, green peas, etc.

Like many fish they also relish insect larvae like mosquito larvae, small crustaceans like Daphnia, and small worms.  Our River Murray Rainbow Fish get frozen blood worms once a week as well as frozen brine shrimp once a week on a different day.

Companions

In our shop in Littlehampton in the Adelaide Hills the River Murray Rainbow Fish are kept in a 300 litre unheated aquarium with Goldfish and Paradise fish.  They could just as easily be kept in a tropical tank with most of the types of tropical fish we sell.

Sexing

Apart from being bigger, the male River Murray Rainbow Fish have a strong black edging to several of their fins.  This colour may not be apparent in immature specimens.  The males also tend to have brighter colours than the females.  In breeding condition you can see why this fish was called a rainbow fish.

Breeding

In the wild the River Murray Rainbow Fish breeds in spring when the water temperatures rise.

Similarly, breeding can be stimulated in an aquarium by an increase in temperature. 

One female can produce about 150 eggs.  These are laid a few at a time over a few days.  The fish mainly spawn at the going down of the Sun when it is getting dark and in the early morning, rather than during the middle of the days.

The eggs are colourless and sticky, adhering to fine leaved plants or spawning mops.  The hatching time varies with temperature, but is generally at least 5 days.  At lower temperatures they will take longer to hatch.

Raising the Fry

The newly hatched babies stay in the top centimetre of the surface for their first few days.  They will eat within 24 hours of hatching, but the yolk sacs are not fully adsorbed until they are about 3 days old.

The natural first food is infusoria.  This can be supplemented with commercial fry foods.  As they get bigger they are able to eat larger live foods.

Frogs and Rainbow Fish

Some authorities have listed tadpoles as one of the food items of the River Murray Rainbow Fish.  Despite this, the River Murray Rainbow Fish is one of the fish that can co-exist with breeding frogs even in a moderate sized pond.

People are becoming more conscious of the environment and some are attempting to have a frog pond in their garden.  If you just put a pond in, the frogs may breed, but so will mosquitoes.  The obvious answer to this pest is to add some mosquito larvae eating fish.  Although Goldfish are excellent at eating the wrigglers, they also eat the young tadpoles and frogs will usually fail to breed in ponds populated with Goldfish.

The River Murray Rainbow Fish is one of the fish used to control mosquitoes under these conditions.  In smaller ponds people often go for the White Cloud Mountain Minnow which is also safe with tadpoles.

Availability

In the United States of America, Australian Rainbow Fish are fairly popular.  However, most of the ones sold in that country are tropical Australian Rainbow Fish, and Melanotaenia fluviatilis is not very often sold.

Even in Australia it is not one of the more popular species in several states.  In fact it seems that only in South Australia is Melanotaenia fluviatilis sold in reasonable quantities.

Pest Fish

You should never introduce an animal into an ecosystem it is not native to.  Although the River Murray Rainbow Fish is listed as being native to four of the five mainland states of Australia it is not native to all parts of any of these states.  Normal precautions need to be taken to avoid the accidental introduction of this fish into natural waterways it is not native to.

Hybrids

The River Murray Rainbow Fish, Melanotaenia fluviatilis, is a recognised “species” but it can interbreed readily with Melanotaenia duboulayi.  There are many other hybrids which can be produced within the Australian rainbow fish group.  This adds to the difficulties of distinguishing between a separate species and a geographical variation within the species.  As a conservationist I am against the production of hybrids when they can threaten the purity of the original population.

This is another reason for preventing the River Murray Rainbow Fish from getting into waterways that are naturally populated by other species of rainbow fish.

Conservation Status

The IUCN Red list has not evaluated the risk of the River Murray Rainbow Fish becoming extinct in the near future.  Commercial and anecdotal evidence suggest that this species is not in immediate danger of extinction, but there are a number of things that are cause for concern.

The main natural habitat of the River Murray Rainbow Fish is the River Murray and its tributaries.  This is a river system that has been extensively modified and in many resects degraded by human activity. The damaging changes have generally been ordered, encouraged or approved of, by our governments of the day.  These things include the introduction of exotic species like European Carp, Mosquito Fish and Red Finned Perch.  These are displacing the native fish in many places.  One of the reasons these foreign fish are taking over is that they are better able to live in habitats degraded by Humans.

Another, related, cause for concern stems from an observation made by David Dunn.  Mr. Dunn has observed the baby fish of the river after a high speed boat has just past.  These included dead rainbow fish babies, and live carp babies, perhaps implying that the baby Rainbow fish are not well able to take the turbulence caused by the boats.

While this observation was only by one person, and his interpretation is not bound to be correct, it is still worrying.

Common Names

In English the “River Murray Rainbow Fish” also called the “Crimson Spotted Rainbow Fish”, the “Crimson Spotted Jewel Fish”, “Murray River Rainbowfish”, “Murray River Sunfish”, “Murray Darling Sunfish”, “Pink Eared Rainbowfish” and “Pink Ear”.

In German it is called “Australischer Perlmutterregenbogenfisch” (Pearl Australian Native Rainbow Fish).  In Denmark it is called “Australsk regnbuefisk”, in Finnish it is called “Jokisateenkaarikala”.  In Mandarin Chinese it is called “河虹銀漢魚”, or “河虹银汉鱼” or “緋紅點鰭魚”, or “绯红点鳍鱼”. 

It is interesting to note that all the English word specifically related to this species, rather than to all Australian Rainbow fish, are in Australian English.  The lack of names in US or UK English is an indication of the lack of popularity of this fish in these countries.  Some of the names in other languages are also general names for all the Australian rainbow fish.

Scientific Names

River Murray Rainbow Fish’s accepted scientific name is “Melanotaenia fluviatilis”   (Castelnau, 1878).  Other names that have been given to this fish are “Aristeus fluviatilis”   (Castelnau, 1878) and   “Melanotaenia splendida fluviatilis” (Castelnau, 1878).

Sources