Feeding Rainbowfishes
Rainbowfishes are frequently described as meiophagous omnivores 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 data on the diets of most species of rainbowfishes 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 and their larvae are the most important food resource for most rainbowfish species, with microcrustaceans, 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 contribute almost 35% of the total mean diet. Other important food items were microcrustaceans (14.4%), algae (8.5%), pollen, seeds and terrestrial insects (8%).
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. Planktonic invertebrates (mostly zooplankton) are importance in their early life history stages. The availability of appropriate zooplankton is an important determinant of mortality levels endured by larval rainbowfish populations and thus is an important determinant of recruitment into the adult population.
Studies in the Alligator Rivers of the Northern Territory (Bishop et al., 1981), bearing in mind that this is probably by far the most thoroughly surveyed aquatic environment in Australia, with years of accumulated research available, found rainbowfishes were omnivorous, feeding opportunistically across substrates and in surface waters, with possibly less emphasis on mid-water areas. Traces of hydrophytes, oligochaetes, gastropods, arachnids, crustaceans, fish (larvae or juvenile), terrestrial plants, detritus and inorganic material were found in the stomach of rainbowfishes. Their diet was found to vary in relation to the habitat they occupy.
In the mainchannel waterbodies they eat mainly aquatic insects, with small amounts of terrestrial insects, plant material and algae. In perennial streams, algae and terrestrial plant material are less important, while aquatic insects and, to a lesser extent, oligochaetes and microcrustaceans, are consumed. The diet in the lowland sandy creekbeds had much larger algal and terrestrial insect components. Specimens examined from the floodplains feed mainly on aquatic arachnids and aquatic insects, and a small amount of algae. In the pools and riffles that enter the floodplain in the wet season they feed mainly on non-aquatic insect forms such as winged diptera and ants. A variety of aquatic insects are eaten; the main identifiable species being chironomid larvae and pupae, and coleopterans. The main terrestrial insects were formicids (ants), while the main microcrustaceans were cladocerans, ostracods and conchostracans. The algal component consists mainly of green filamentous species.
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 (Balcombe et al., 2005) 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%), and aquatic insects (5.6%). 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. The natural diet of Rhadinocentrus ornatus was reported to consist mainly of terrestrial insects. They feed on a variety of terrestrial and aquatic insects, insect larvae, and small aquatic crustaceans. Algae and pollen (mostly pollen) also appear to be a major alternative food.
The natural diet of Pseudomugil tenellus includes algae, microcrustaceans and aquatic insects. The identifiable algae were green filamentous and blue-green algae and dinoflagellates. The microcrustaceans were mainly cladocerans, ostracods and copepods. Chironomid larvae were the main aquatic insects eaten. Other food items found in the stomachs were terrestrial insects and miscellaneous organic matter. One study found that the diet in the late-dry season was mainly based on detritus (with associated unidentified organic material) and small quantities of chironomid larvae and pupae, and algae; no microcrustaceans were eaten. In the early-wet season microcrustaceans appeared in the diet and detritus decreased in importance; aquatic insects also appeared in the diet during this season. In the mid-wet season Pseudomugil tenellus ate mainly microcrustaceans (particularly cladocerans) with smaller amounts of terrestrial and aquatic insects. By the late-wet— early-dry season algae were the main component of the diet.
Melanotaenia fluviatilis consume high numbers of cladocerans throughout the year and copepods were consumed in high numbers in all seasons except winter. Cladoceran eggs were recorded in high numbers in spring and summer, and were likely to have been detached from adult cladocerans in the digestive tract of the fish, rather than having been consumed as individual food items. The highest numbers of ostracods and dipteran larvae were consumed during spring, while algae were only consumed in summer and autumn. Fish eggs were recorded in the stomach contents of some fish during spring and summer.
Other diet categories were either recorded in low numbers or abundance counts did not apply (such as detritus and plant material). In another study large numbers of copepods were consumed in spring and summer. Algae were recorded in high numbers in summer only. Dipteran larvae were recorded in stomach contents in spring and autumn, whilst ostracods were recorded in summer and winter. Zooplankton was recorded in the stomach of fish from summer and autumn. Unidentified material (organic and inorganic detritus) constituted a major proportion of the diet in both studies.
Nevertheless, dietary analysis can reveal contradictory information on the food resources available to rainbowfishes, particularly across the seasons. While some foods may represent the most energetically rich resource they may not always be accessible. For example higher quality prey may not be available all year round due to natural seasonal declines in these resources and in such instances fishes may switch to poorer quality food.
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 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 (filterfeeders) material from the water, or collect deposited material on the 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 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—earlydry 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 fish 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 nonexistent and fish 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 rainbowfishes. Few people, however, will be able to duplicate the natural combination of the many foods consumed by rainbowfishes in their natural environment. Therefore, it is necessary to provide favourable aquarium conditions and careful feeding with specially formulated diets in an attempt to satisfy all their nutritional requirements.