Rainbowfishes require a highly nutritious diet in order to maintain spawning condition and produce large numbers of eggs. Research studies have shown that during periods of low food availability, a decrease and then cessation of reproduction occurs. During these times rainbowfish must allocate resources to meet basic metabolic functions rather than reproduction. Research has also found that diet strongly affects not only fecundity but also the biochemical make-up of eggs and sperm as well as the growth rate and survival of larvae.
The breeding seasons of rainbowfishes must coincide with the conditions that offer the greatest amount of protection for the eggs, and food and shelter for the newly hatched larvae. The duration and timing of reproductive activity are thus two critical components of the rainbowfishes life-history strategy. Rainbowfishes are often aseasonal spawners, breeding continuously at intervals throughout the year. This strategy increases the chances of at least some eggs surviving during the year. Rainbowfish eggs are attached by adhesive threads or tendrils to aquatic plants and other objects in the water, which hide them from predators.
One study (Badger, 2004) covered a variety of topics on the nutrition of Melanotaenia splendida ranging from identifying what reproductive factors are affected by nutrition to how diet composition affected these factors. This study determined that egg number as opposed to egg diameter was affected by nutritional status, and that it is best to feed Melanotaenia splendida daily to satiation in order to maximise reproduction. The best diet found in this study for maximising reproduction was one with a 43% protein and 12% lipid content, which was supplemented with essential fatty acids. The energy content in the diet effected egg number the most, while the protein content, lipid content, and fatty acid supplementation maximised unfed larval life.
Feeding trials found that rainbowfishes fed every day produced significantly more eggs then fish fed every second day. Average total number of eggs produced by fish fed every 2nd, 3rd, 4th and 5th day showed a clear trend of decline with increased feeding interval. It declined from 152 eggs in those fish fed every day, to 99 eggs in fish fed every 2nd day, 26 eggs for fish fed every 3rd day, 21 eggs for fish fed every 4th day and 19 eggs for fish fed every 5th day. The number of spawns was also affected by feeding, with the highest number of spawns occurring in fish fed daily. Hatching rate also declined from fish fed daily, to fish fed once every five days. All fish in the trials were fed to satiation on feeding days, with excess food siphoned out.
Egg quality also declined during periods of lower food availability. Eggs that had been laid after the onset of reduced feeding frequency or feeding ration had correspondingly lower fertilisation and hatching rates. These two effects may have acted alone or in concert. The eggs that were fertilised also had a decrease in hatching success.
Energy, protein and lipid contents in a diet are important. Whatever is consumed in feed is needed by the fish for a variety of metabolic functions. Any excess that is available after meeting these requirements can then be allocated to other processes such as growth and reproduction. It is therefore necessary to ensure that diets for rainbowfishes contain enough of these nutrients not only to meet essential metabolic functions but also to sustain egg and sperm development.
Both protein and lipid are very important constituents of fish eggs. They are the primary components of vitellogenin, the lipoprotein that is the main energy source in the yolk of eggs. They are also an energy source, therefore it is important to ensure that fish feeds contain enough of each so that female rainbowfish will have enough lipoproteins to allocate to egg production. Many studies have previously linked increased protein level with increased reproductive ability. It has also been shown that while lipid is important to fish reproduction, too much lipid in a diet can be detrimental to egg quality. Too little lipid or poor quality, however, can lead to a decrease in ovary size and lower egg survival to hatching.
Furthermore, too much lipid in a diet can cause deformities in eggs and reduced hatching success. Larval deformity occurred in a study of Melanotaenia splendida as well i.e., in the larvae hatched from eggs obtained from fish fed a 20% lipid content diet. It is surmised that the high lipid content in the diet was the likely cause of larval deformity as it was not seen in the larvae from the 12% or 9% lipid diet trials. When fed a high lipid diet, rainbowfishes may develop lipidosis (fatty degeneration of the internal organs) and eventually die. If too much protein or lipid is supplied in food it may lead to water quality problems that may be detrimental to fish health. A diet containing 43% protein and 12% lipid was deemed to be the more ideal diet. However, in outdoor ponds, natural foods contribute to the total amount of protein and energy available to fish, so results might differ from aquarium studies.
The total dietary protein requirement may be defined as the minimum amount of protein that produces best fish performance (e.g., growth, feed conversion) under a given set of conditions. In addition to total protein, the balance between protein and energy in a diet is critical. When more protein is added to a diet than is needed for growth and other bodily functions, the excess will be metabolised for energy or used to make energy-storage products (e.g., body fat). Excess energy in the diet can also reduce feed consumption and growth.
The major food nutrients (i.e. carbohydrates, proteins and lipids) are required by animals not only as essential materials for the construction of living tissues, but also as sources of stored chemical energy to fuel these processes. The ability of a food to supply energy is therefore of great importance in determining its nutritional value to animals.
Essential Fatty Acids
Essential fatty acids are fatty acids that are needed for growth and maintenance, however, the body is unable to synthesize them. In fish, fatty acids are the main component of egg membranes, maintaining structure and function. Fatty acids have an effect on the condition of female fish as well as a significant impact on spawning performance. Fatty acids are also essential to the development of larvae, and many aspects of development may be affected by the amount and type of fatty acids in a larval diet.
For herbivorous and omnivorous freshwater fish, many of the essential fatty acids are gained from freshwater algae. In most freshwater plants, many of the long chain n-3 and n-6 essential fatty acids are not found in abundant levels especially docosahexaenoic acid (22:6n-3, DHA). Arachidonic acid (20:4n-6, AA) and eicosapentaenoic acid (20:5n-3, EPA) are found in low levels in some freshwater invertebrates such as insect larvae, but are not in abundance, with the result that these fatty acids are still limiting.
The fatty acids in saltwater fish eggs differ from those found in freshwater fish eggs. Although eggs from both saltwater and freshwater fish are rich in DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid), AA (arachidonic acid) is more abundant in eggs of freshwater fish species. Because all three of these fatty acids are found in relatively high amounts in freshwater fish eggs, supplementing dry feed given to rainbowfish with these fatty acids should ensure proper egg formation and larval development.
Melanotaenia splendida fed a diet containing all three fatty acids produced eggs with a 100% survival to eyed embryo and hatching rate. They also had the highest number of eggs, highest number of spawns, and the shortest average spawning interval when compared to results from the other trials in the study (Badger, 2004).
Literature
Badger, A. C. (2004) The effects of nutrition on reproduction in the Eastern Rainbowfish, Melanotaenia splendida splendida. Masters (Research) thesis, James Cook University.
Humphrey, C., Klumpp, D. W. and Pearson, R. G. (2003) Early growth and development of the eastern rainbowfish, Melanotaenia splendida splendida (Peters). I. Morphogenesis and ontogeny. Marine and Freshwater Research 54:17-25.
© Copyright Adrian R. Tappin
Created December, 2008.
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