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. Very little is known about the exact nutritional requirements of rainbowfishes. When rainbowfishes are kept in aquaria and fed the best of the currently available formulated diets, the growth rates are considerably less than for rainbowfishes grown in outdoor earthen ponds. This is because in a pond situation the rainbowfishes have access to the full range of naturally occurring foods. Rainbowfishes grown in outdoor ponds are also a lot more colourful.
Aquatic life in ponds is very important for rainbowfishes. This is particularly so for developing fish whose regular diets are comprise of zooplankton and phytoplankton. In the commercial culture of larval fish of various species, management of the zooplankton forage base is critical to successful transition of larvae to the juvenile stage. In addition, information regarding the relative status of plankton (zooplankton and phytoplankton) communities gives insight into water quality parameters and the possible success or failure of the pond.
The food that is available in ponds can be divided into three broad categories these being plant material, animal material and detritus (decomposing fragments of organic material derived from both plants and animals). Plant material can come from many sources including aquatic plants, reeds and grasses growing on the pond edge. With all of these fresh plant materials, especially ones containing low protein, the actual nutrient value to rainbowfishes is relatively low. In many cases when they are eating plant material they are only acquiring a few vitamins and minerals. Plant materials become far more nutritious after they have been in the pond for a couple of weeks after they begin to decompose. At this stage they are colonised by tiny aquatic animals, bacteria and fungi and begin to break down into detritus.
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| Natural Zooplankton photo© Dave Wilson
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The most important form of animal material in a rainbowfishes diet comes from the ponds aquatic insects and zooplankton (tiny aquatic animals). There are a number of different species of animals that inhabit the pond. These animals are very high in protein which is necessary for growth. Zooplankton is an important part of the diet of all freshwater fish. Rainbowfishes will seek out and eat aquatic worms, snails, insect larvae and when they can catch them, the free swimming insects. The fast free swimming insects more frequently enter the diet when they die and settle to the bottom. Juvenile rainbowfishes are more predatory than the adults and require a higher proportion of animal protein in their diets. They are active predators and are well adapted to catching the smaller free swimming forms such as daphnia and copepods. All forms of zooplankton should be encouraged. This can best be done by promoting phytoplankton (microscopic free floating algae) and suitable water quality. Large amounts of commercial artificial feeds should not be used to feed ponds as it is bad for water quality and is an ineffective method for feeding rainbowfishes.
As plants decompose they become broken down into tiny fragments. The fragments become colonised by bacteria and fungi which feed off the decomposing material. These tiny fragments and the microscopic plants, animals, bacteria and fungi associated with them are known as 'detritus'. Detritus is a major component of the rainbowfishes diet at all stages of their life cycle. The tiny plant fragments themselves are not very nutritious but the micro-organisms associated with them are a readily digestible, nutritious, protein rich food source. The naturally occurring detrital food available can be supplemented by adding small amounts of plant materials such as hay and lucerne. Phytoplankton cells in the water eventually die and settle to the bottom to form detritus. Organic materials will break down most rapidly and effectively to form healthy detrital communities in slightly alkaline waters (pH 7.4 to 8.5).
All factors occurring in the pond, whether physical, chemical or biological, influence the pond ecosystem. The pond ecosystem is only just beginning to be understood and is of course extremely complicated and intricate. As aquariculturalists, we try to manipulate the ecosystem so as to produce an optimal environment for fish growth. Food is just one component of this complex system.
Although little live phytoplankton is eaten directly by rainbowfishes, it is one of the most important components of the pond food chain. Some ponds will support adequate plankton communities without any assistance. However most ponds require a form of fertilising in order to promote productivity. There is no point fertilising ponds that have very low pH values (< 5) or very low total hardness (< 20ppm). In these situations liming is first required.
Likewise ponds with very high clay turbidity will not respond to fertilisation. However suspended clay particles can provide suitable sites for active bacterial colonisation and these ponds often have very good natural zooplankton populations.
The dynamic characteristics of zooplankton populations have led researchers to use particular fertilisation techniques and species-specific zooplankton inoculations in culture. The intent of these management techniques was to maintain high densities of desirable zooplankton species in culture ponds.
Commercial aquaculturists often use fertilisation to improve their food base. Fertilisers may be either inorganic or organic based. Inorganic fertilisers are those that take the form of granular or liquid fertilisers having high phosphorus content and, to a smaller degree, nitrogen (phosphorus is often the limiting nutrient in freshwater). The premise behind using inorganic fertilisers is that by applying needed nutrients, phytoplankton populations' increase. These increased populations of phytoplankton, often called a 'bloom', will then increase the number of zooplankton in the pond, which then eat the phytoplankton. However, it has been shown that large phytoplankton populations alone do not necessarily increase zooplankton populations; zooplankton will eat more fungi and bacteria associated with decaying organic substances than phytoplankton directly. In fact, these large populations of phytoplankton often lead to lower water quality through increased pH and low morning dissolved oxygen levels.
Some researchers have had considerable success in managing zooplankton populations through phytoplankton management. The most important diet component of these animals has been shown to be small algae (1-25 µm). Algae larger than 50 µm or algae with spines or in colonies were usually rejected.
Organic fertilisers are often used to promote desirable zooplankton species. Organic fertilisers may be animal manures, hay and lucerne (ground or meal), or soybean meal. Organic fertilisers should have low carbon: nitrogen ratios and have fine particle sizes to allow rapid decomposition. As previously indicated, zooplankton will consume fungi and bacteria associated with decaying organic material. However, the use of organic fertilisers may cause dissolved oxygen and ammonia problems during the initial decomposition.
While culturing larval fish, the aquarist needs to periodically check zooplankton populations in culture ponds. The main problem is obtaining good representative samples when the ponds are heavily infested with filamentous algae or vascular plants. Irrespective to sampling technique, zooplankton samples should be obtained in a variety of locations in the pond and at the same time of the day. The reasons for this are that zooplankton are often in clumped numbers throughout the pond and do migrate vertically during the day.
Consistency in sampling is paramount to obtaining good quantitative samples. The number of zooplankton needed for successful culturing of larval fish is affected by the number, age and species stocked. In general terms, zooplankton populations should be approximately 100 to 500 animals per litre. Specific constituents of the zooplankton samples, e.g., size and species, are best determined by the species and the life stage of fish being cultured.
Zooplankton are classified as rotifers, cladocerans or copepods. The ability of rotifers and cladocerans to reproduce parthenogenetically (asexually) enables them to react quickly to unfavourable and favourable environmental conditions.
Rotifers have the shortest life span (12 days) and can reach their peak reproductive level in about 3-5 days. At 20°C, the egg-to-egg span is 2-3 days with the total young per adult lifespan being 15-25 days. Cladocerans and copepods have similar life spans of approximately 50 days, but with different peak reproductive periods. To reach their peak reproductive capacity, cladocerans require 14-15 days while copepods require 24 days. Copepods, which have only sexual reproduction, require longer periods to increase their population levels.
Cladocerans are desirable fish prey since they have high caloric value and are readily consumed by most fry. However, cladoceran populations usually decline rapidly when subjected to predation in culture ponds. On the other hand, copepods, because they are swift swimmers are better able to maintain their populations during the later stages of a culture season. Egg-to-egg generation times are slower than for cladocerans (13-15 days for copepods compared to 7-8 days for cladocerans at 20°C), but life spans are similar (approximately 50 days at 20°C). The total young per adult lifespan is 400-600 for cladocerans compared to 250-500 for copepods at this temperature.
Although rotifers are the first zooplankton to reach large numbers in newly filled culture ponds, they are soon out competed by both cladocerans and copepods for the available food resources. There is also a difference in filtering rates for these animals. Cladocerans have the highest filtering rates, followed by copepods and then by rotifers. The high filtering rates and total young per adult lifespan give cladocerans a definite ecological advantage over rotifers and copepods. However, increased predation by fish upon cladocerans does decrease these ecological advantages.
Models of zooplankton succession patterns and species composition in large reservoirs and lakes may not be applicable to intensively fertilised culture ponds. In a study of fertilised culture ponds without fish, it was found that copepod adults and nauplii, and Daphnia spp. populations reached maximum mean densities in an average of 23.5 days. Rapid population declines of copepod adults and nauplii occurred in 5.3 days, respectively, while Daphnia spp. and Bosmina spp. populations decreased significantly within 7.3 days after reaching maximum densities.
Researchers have differed in their recommendations concerning the time between filling the ponds and fry stocking. Some recommend that culture ponds be filled 2-3 weeks prior to fry stocking to allow time for maturation of zooplankton populations. However, not all fish species require the same size of prey at the onset of feeding. For instance, some species have very small mouths that require them to consume small prey, such as rotifers and early instars of cladocerans. Improved production may be achieved by stocking these fry into culture ponds filled only 2-3 days before stocking.
Zooplankton, namely cladocerans, which are coloured a deep red are often indicators of low dissolved oxygen conditions, and quickly become clear when placed into well-oxygenated waters.
This coloration is based on the increased amount of haemoglobin that these animals have to compensate for low oxygen levels in the environment; however, this increased amount of haemoglobin comes at a cost. The increased number of diapause eggs in cladocerans also indicates another indication of poor environmental conditions. These diapause eggs are often quite large and dark and are produced when these animals are forced to undergo sexual reproduction in preparation of unfavourable environmental conditions.
When a cladoceran is food-limited, it matures at a smaller size and produces smaller offspring. The main response of Daphnia pulex to low food levels was a reduction in size specific food intake and egg size. However, food concentration did not affect length/weight relationships; instar duration and weight-specific investment of energy in reproduction.
Cladoceran populations also consist of smaller individuals in water bodies with large populations of vertebrate predators. Large-bodied species, e.g., Daphnia pulex, tend to be fewer in ponds with large predator bases. In these situations, smaller species or smaller individuals within a given species have improved chances of escaping predation than larger individuals (based on prey visibility). However, smaller animals can also be selected when predators are other invertebrates, such as midge larvae, or backswimmers.
Chemical characteristics refer to the water quality parameters that are measured within a pond. Water quality in ponds changes continuously and is affected by physical and biological characteristics. With this in mind water quality should be monitored regularly. This can be achieved by recording simple visible water characteristics such as water colour, clarity, plant and animal life. Relatively inexpensive testing kits and recording probes (more expensive) can be purchased from analytical supply stores.
Whether your pond is a old truck tyre or a backyard masterpiece with waterfalls and hidden lights, good water quality must be maintained. If not, the pond declines in beauty and the fish become stressed and susceptible to diseases. Once the basics of water quality are understood and practiced, maintenance will become second nature and require only a few hours per week.
© Copyright Adrian R. Tappin
Created July, 2005.
Updated February, 2007.
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