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Rainbowfishes are frequently described as omnivorous or opportunistic feeders, meaning that they will eat almost anything that is available. However, just because they will consume something does not necessarily mean that they will grow well on it. Rainbowfishes are found in a diverse range of habitats, therefore it is not surprising that this diversity is reflected in their feeding habits. The size and variety of their diet can differ widely from one species to another and from one habitat to another. In fact, quantitative descriptions of the diets of many rainbowfish species are limited.
In their natural habitat rainbowfishes consume nearly 100% live foods. They can be seen around sub-surface aquatic vegetation feeding on a variety of terrestrial insects, small aquatic crustaceans, insect larvae, worms, phytoplankton, zooplankton, aquatic plant material, detritus (inanimate suspended matter principally of organic origin), and where the opportunity occurs, smaller fish will also be consumed.
Aquatic insects are the most important food resource for many rainbowfish species, with microcrustaceans (mostly zooplankton), algae and food from terrestrial origin (mainly terrestrial insects) also providing important food sources. A review of published dietary information for rainbowfishes shows that aquatic insects were the most important dietary component, contributing almost 35% of the total mean diet. Other important food categories were small aquatic crustaceans (14.4%), pollen, seeds and other terrestrial items (8%), algae (8.5%) and miscellaneous organic matter (4.2%).
Food sources of allochthonous origin (e.g., terrestrial insects) form a significant component of the diet of rainbowfishes. Rainbowfish consume such items once they have entered the aquatic environment. Terrestrially derived food form an important alternative food source during times of low aquatic food availability. For example, aquatic food items may not be available all year round due to natural seasonal declines in these resources and in such instances rainbowfishes may switch to other food sources. In large floodplain rivers, terrestrial food items increase in importance as water floods on to the floodplains. Rainbowfishes in floodplain rivers consumed a variety of food types, such as dipterans, crustaceans, fish (larvae or juvenile) and other aquatic fauna. Of these food types, chironomid larvae, and a range of zooplankton, particularly ostracods, cladocerans and conchostracans are prominent in their diet.
One study (Rayner, 2006) on the diet of Pseudomugil signifer and Melanotaenia splendida in the Mulgrave River in north Queensland found these species specialised on a combination of aerial and surface insects (28-92%), particularly emerging chironomid nymphs, and terrestrial invertebrates (up to 43%), principally green ants (Oecophylla smaragdina), which had presumably fallen on to the water surface from overhanging vegetation. In another study it was found that terrestrial fauna was a major food group consumed by Melanotaenia splendida tatei. Their diet consisted of terrestrial insects (67.4%), other terrestrial invertebrates (10.2%), algae (16.8%), aquatic insects (5.6%) and fish (1.1%). In addition to the consumption of some aquatic insects and algae, this species fed chiefly upon terrestrial arthropods, many of which were flying insects (e.g., ants, wasps and dipterans). Other items included aquatic dipterans, coleopteran larvae and zooplankton. The aquatic dipterans were mostly chironomid larvae, while the zooplankton prey consisted chiefly of conchostracans and cladocerans. Terrestrial foods included isopods, scolopendridid centipedes, or a variety of alighting insects such as dipterans, hymenopterans and coleopterans. Up to 100% terrestrial insects can be consumed during the dry season.
Other studies have shown that rainbowfishes have a preference for foods such as cladocerans, ostracods, copepods, rotifers and other invertebrates. The stomach contents of specimens collected from the Noosa River in Queensland consisted of copepods, cladocerans and shrimp (Caridina sp.) and a variety of aquatic insects (especially chironomid larvae). A small percentage of algal material and other plant tissues plus a few terrestrial insects were also found. However, terrestrial sources of food were relatively unimportant in their diet in this study. Diet analysis can reveal contradictory information on the food resources available to rainbowfishes, particularly across the seasons and habitats.
Nutrients in freshwater rivers are mostly derived from sources outside the stream itself, mainly in the form of organic material derived from the riparian vegetation. Some primary production occurs from algae and in-stream vegetation, although this is comparatively small, with upland streams too shaded or too cold, and lowland streams too turbid for effective photosynthesis. Leaves falling into the stream are eaten directly by invertebrates known as "shredders", such as some stonefly and caddisfly larvae, which shred or chew the softer parts of the plant material. Material not consumed by the shredders is colonised by microorganisms such as aquatic fungi and bacteria and broken down to progressively smaller sizes. Freshwater algae also colonise the leaves and twigs. Invertebrates known as "scrapers", such as some mayflies and snails feed directly on the fungi and algae. As the organic material is broken down, the resultant finer material then serves as a food source for other invertebrates, which filter (filter-feeders) material from the water, or collect, deposited material on stream bed (detritus feeders). These animals, in turn are preyed upon by other invertebrates and animals such as rainbowfishes. Rainbowfishes are then a food source for water birds and other fish-eating animals. Without the initial inputs of organic material or invertebrates, the entire food chain of the stream system is compromised. Hence, the importance of riparian vegetation to stream health cannot be underestimated.
Available evidence suggests that feeding rates for rainbowfish species are higher in spring and summer than in autumn and winter, possibly because of higher metabolic demands during the warmer months. This time also represents an important period for larval development and growth of juveniles. The season of greatest feeding activity in tropical rivers is the wet season. Feeding activity increases most dramatically between the late-dry season and the early-wet season. By the mid-wet season feeding activity has peaked, and then decreases slightly by the late-wet to early-dry season. An examination of variations in body condition indicated that most species obtained their best condition from the mid-wet to the mid-dry season, with a peak in the late-wet early-dry season. During the dry season, when the water volume of their habitat is greatly reduced, the only food they get is whatever happens to fall into the water.
Adult rainbowfishes can survive for weeks or even months on very little food, but without a plentiful food supply they will not flourish. If food is in short supply, growth will be reduced or non-existent and they may lose weight and become more susceptible to disease. During spawning egg numbers will be fewer. Juveniles and larvae in contrast are much more dependent upon a regular food supply and may starve within days or even hours.
Scientific data from stomach content analyses in wild populations can be helpful in selecting the right type of diet for your fish. Few people, however, will be able to duplicate the natural combination of the many foods consumed by rainbowfishes in their natural environment.
Literature
Bishop, K.A., Allen, S.A., Pollard, D.A. and Cook, M.G. (1981) Ecological studies on the freshwater fishes of the Alligator Rivers region, Northern Territory. Research Report 4, Volume 111, Autecological Studies. Canberra: Office of the Supervising Scientist for the Alligator Rivers Region.
Pusey, B.J., Arthington, A.H., Bird, J.R., and Close, P.G. (2001) Reproduction in three species of rainbowfish (Melanotaeniidae) from rainforest streams in northern Queensland, Australia. Ecology of Freshwater Fish 10:75-87.
Pusey, B.J., Kennard, M.J. and Arthington, A.H. (2004) Freshwater Fishes of North-Eastern Australia. CSIRO Publishing, Victoria.
Rayner, T. S. (2006) The trophic ecology of the freshwater fishes of an Australian rainforest river. Thesis submitted for the degree of Doctor of Philosophy in the School of Marine and Tropical Biology James Cook University.
© Copyright Adrian R. Tappin
Updated January, 2009.
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