Feeding and Nutrition
At present there is little information available concerning the quantitative dietary nutrient requirements of rainbowfishes under aquarium conditions, or regarding the nutrient requirements of the first-feeding larvae and broodstock of rainbowfishes.
Data from aquaculture research is commonly extrapolated and applied to aquarium species, which often proves to be unsatisfactory because of the differences in fish species and variation in diet formulations which is aimed at maximum growth in a short time period. This might be of value for the commercial farming of ornamental species, but would be unsuitable for rainbowfishes kept in home aquaria. In addition, the majority of research has focused on feeding fish to satiation, measuring the food intake, and linking this to growth performance and utilisation.
Under aquarium conditions, rainbowfishes characteristically display a considerable range of growth rates. Despite this, and on the basis of ‘laboratory based’ nutritional feeding trials conducted to date, some generalised conclusions may be drawn regarding the recommended nutrient levels. Nevertheless, there are still many unanswered questions when it comes to the nutritional requirements of rainbowfish species.
The formulation of adequate diets for organisms in an aquatic environment gives a greater challenge to those involved in their care, compared to terrestrial species. In general the maintenance energy requirement of fish is less than 10% of the maintenance energy required by birds or mammals. The low maintenance energy requirement is partly due to the poikilothermic nature of fish. Fish also exert less energy on posture and have an energetic advantage over mammals in their nitrogenous waste management as they excrete mainly ammonia instead of urea or uric acid, thus losing less energy in protein catabolism and excretion of nitrogenous waste.
The provision of energy and other nutrients to poikilothermic animals can take many forms and is generally chosen by reference to their natural ecology. However, it is pertinent to note that feeding diets high in protein to herbivorous and omnivorous fish may result in increased tank pollution as excess protein is for the most part excreted as waste.
Feeding strategies and anatomical differences between fish species make formulation of one diet for a community of species quite difficult. Fish are unique in that only a small proportion of species use plant material as their primary food source. Those species consume nearly their body weight in vegetation daily.
Most fish eat a diet rich in protein and fat and digestion of plant materials by these species is poor. Therefore, most commercial fish diets contain protein levels above 30%, and diets for very young fish may contain nearly 60% protein. Commercial diets are partially cooked during production, which increases the digestibility of plant products. Lipids are also an important energy source for fish, and a source of essential fatty acids. The most desirable fats are unsaturated.
Some research on the nutritional requirements of freshwater ornamental species in a commercial production environment has been conducted, mainly in Singapore. Protein requirements varied from around 30% dietary protein for growing omnivorous goldfish (Carassius auratus) to 50% for the carnivorous discus (Symphysodon aequifasciata). Whereas, mineral (phosphorus, iron, magnesium, zinc) requirements have received some attention in feeding Poecilia reticulata, few researches have concentrated on vitamin requirements of ornamental species.
Maintenance energy levels of ornamental fish varied from 0.068 kJ per day for Paracheirodon innesi to 0.51 kJ per day for Trichogaster microlepis, kept at a water temperature of 26°C. However, large commercial producers of aquarium fish in Singapore emphasise the importance of regular supplementation of formulated feeds with live feed, as the inclusion of live feed improves growth.
A study to determine the effect of increasing levels of dietary protein on the common swordtail (Xiphophorus helleri) was carried out by Chong et al., 2004. Five semipurified diets having similar caloric values, containing 20%, 30%, 40%, 50% and 60% dietary protein were used. Results showed that while the 20% and 30% protein produced the lowest specific growth rate values, there was no significant difference between 40% and 60% dietary treatments. The 20% dietary treatment also displayed lowest protein content in both ovaries and muscle of female fish. Fry production was highest from females fed with 50% and 60% protein, followed by the 30% and 40% protein while the diet containing 20% protein produced lowest number of fry.
A significant correlation was also obtained between number of fry produced and the weight of female fish, indicating that size is a major factor influencing production. Relative fecundity was lowest for the 20% protein diet followed by the 30–40% and 40–60% protein diets. There were no significant differences in both weight and length of fry produced among the dietary treatments. Based on these results, they suggested that a minimum of 30% protein be included in the diet of female swordtail broodstock.
In another study of Xiphophorus helleri (Kruger et al., 2001), three protein levels (30%, 38% and 45%) at three different dietary lipid concentrations (6%, 8% and 12%) were used to formulate nine different diets that were fed for 60 days to 6–8 weeks old juvenile Xiphophorus helleri. From that study it was suggested that a diet of at least 45% protein and 6% lipid concentration is needed for the best specific growth rate and feed conversion ratio.
Larval goldfish were found to grow best on prepared diets containing about 50% protein (Sales & Janssens 2003). Juvenile goldfish grew best on prepared diets containing about 40% protein. As they grow to adulthood, their nutritional requirements change. Adult goldfish require only 29% protein, and they will continue to grow when receiving only 1% body weight of a diet containing 36% protein.