Ponds that are drained, dried, and then filled with well water are much safer for culturing fry than are ponds filled with surface water.
Starting with water that does not contain zooplankton makes it much easier to predict when the right size zooplankton for the fry will appear in the ponds.
It also helps to ensure that zooplankton, fish predators, parasites, diseases, and a variety of fry-eating insects are not abundant in the ponds when fish are stocked.
If surface water must be used, it should be filtered through a filter fine enough to prevent even small copepods from passing through (125-micron mesh).
Ponds should be properly fertilized as they are being filled. If fry will depend on zooplankton for food, a combination of organic and inorganic fertilizers is best. Organic fertilizers are the basis of the food chain that nourishes bacteria, protozoans, zooplankton, and eventually the fish fry.
(Some organic fertilizers, such as rice bran, are fine enough to be directly consumed by zooplankton). As organic fertilizers decompose, their nutrients are used by phytoplankton, which is consumed by some types of fry. Or, the phytoplankton is eaten by protozoans or zooplankton before they are eaten by fry.
Nutrients from organic fertilizers are released over time, so they produce less drastic changes in plankton populations than do inorganic fertilizers. Inorganic fertilizers add nutrients to the pond instantly. A phytoplankton- based food chain can develop very rapidly without the need for bacterial action.
However, the nutrients are often used up very rapidly by the tiny plants, and the risk of a bloom “crash” is greater than it is with organic fertilizers. Using a combination of organic and inorganic fertilizers results in a greater diversity of plankton than if either fertilizer type is used alone, and reduces the potential for a dangerous bloom crash.
Fertilizer nutrients are used quickly in the pond environment. Some nutrients are trapped in the bottom mud or otherwise lost from water. Therefore, nutrients should be replenished often. Frequent applications of small amounts are more effective than a single large application for maintaining a supply of fry food organisms.
Timing of fry stocking:
The proper timing of fry stocking, in relation to filling and fertilizing the ponds, can make the difference between having an abundant harvest or a complete crop loss. Proper timing is also important for optimum growth of the fry.
Ponds must contain the appropriate type and size of food when fry are stocked. Large fry stocked into ponds with very tiny zooplankton may grow slowly because the fry must expend so much energy to catch an adequate amount of food. Likewise, if the zooplankton are mostly too large for the fry to eat they may starve, or become prey of cyclopoids or insects.
When ponds are filled and fertilized, the plant and animal populations that invade or hatch from within the bottom mud pass through somewhat predictable changes in sizes and species. This process is called succession.
At first there are usually a few small species in large concentrations. Later there will be many species in an array of sizes, but each in moderate concentrations. The average size of organisms also gets larger with time. The early community is unstable and great changes can occur quickly; later, the greater diversity of organisms makes the community more stable.
Knowing how succession happens in fry culture ponds will help a producer be more successful. Figure 2 illustrates the successional process in ponds in Arkansas during the spring when temperatures are 21 to 24o C (70 to 75o F) (sunshine bass fry were also present in these ponds). When ponds are first filled with well water, there are few living organisms and few nutrients.
The water rapidly gains nutrients from the bottom, particularly when soluble inorganic fertilizers are added. It also gains nutrients, but more slowly, as organic fertilizers are decomposed by bacteria. Phytoplankton and other bacteria rapidly use released nutrients. Within a few days, growing populations of phytoplankton may provide a green tinge or “bloom” to the water.
This indicates that there is a growing food base for single celled protozoans and other zooplankton. In many ponds the water first appears brownish. This happens when the bacterial food levels are large enough to cause huge protozoan or rotifer blooms without much phytoplankton being present.
Although some protozoans may be large enough for tiny fry to eat, it is the next stages in succession that are of greatest importance to fry. Rotifers usually appear first. Rotifers feed on bacteria and phytoplankton, and then reproduce to form huge populations.
When water temperature is 21 to 27o C (70 to 80o F), rotifers can go from nearly nonexistent to concentrations in the thousands per liter by the second week after a pond is filled. As rotifers eat their own food supply the population drops drastically.
Then copepod nauplii, adult copepods and cladocerans make their appearance. Cyclopoid copepods prey on small rotifers. Calanoid copepods and herbivorous cladocerans out-compete them for phytoplankton. Together, copepods and cladocerans prevent a re-bloom of the smallest rotifers.
However, modest populations of larger rotifers may appear after several weeks, particularly when fish fry prey on the rotifers’ competitors and predators—cladocerans, copepods and insects. Because of the ephemeral nature of high density rotifer populations, timing is critical if the fry being stocked are so small that they can eat only rotifer-sized prey.
Most fry 6 mm long or less fall into that category. Fry must be stocked just before the rotifer population begins its rapid growth. If the fry are stocked when rotifer populations are rapidly rising there will be plenty of food and the fry should grow rapidly and be large enough to eat copepod nauplii and larger zooplankton when those organisms appear.
The fry will also have a much better chance of being large enough to avoid being eaten by cyclopoid copepods. Larger fry (more than 6 mm) should be stocked into ponds as populations of copepod nauplii, copepods and cladocerans begin to climb.
That usually happens 2 to 3 weeks after ponds are filled when water temperature is 21 to 27o C (70 to 80o F). The fry will then have the right size food for rapid growth and can better escape predation from aquatic insects that soon begin to populate the pond.
In general, fry must have zooplankton to survive, or at least to be healthy and grow rapidly. Most fry are not particular about the types of zooplankton they eat, but the organisms must be small enough to fit into their mouths. To maximize survival, stock any fry just as populations of zooplankton small enough for the fry to eat are rapidly increasing and before invading predators become numerous.
Stocking even large fry into a pond that has been filled for more than 3 to 4 weeks during warm weather can result in high mortality. By that time, a variety of fry predators have invaded the pond and begun to reproduce. These include insects such as back-swimmers, diving beetles and whirlygig beetles. Later, even larger insects such as water scorpions, giant water beetles and the larval stages of dragonflies will appear.
Insects begin to colonize as soon as ponds are filled during warm weather. However, it usually takes several weeks for their populations to reach levels threatening to small fish. Predaceous cyclopoid copepods are often a much greater threat to fry than insects. Many of these tiny zooplankton will prey upon fry unless, or until, the fry are large enough to prey upon them.
Cyclopoid copepods often are abundant in ponds after about 10 days when water temperature is 20 to 25o C (68 to 77o F). Because there are no legal means of controlling undesirable zooplankton or insects, it is important that fry, particularly small fry, be stocked into ponds as early as appropriate after ponds are filled.
The effects of weather must also be considered when stocking fry. Temperature has a profound effect on the successional process. Figure 3 shows that the colder the water the more time is required for rotifers to reach their initial peak population.
Researchers are developing methods of predicting when some zooplankton events will occur under different temperatures. Figure 3 shows that the time it takes to reach an initial rotifer peak is related to the mean morning water temperture in the following way: Days to rotifer peak = 29.7 – 0.95 (average morning water temperature in o C) = 46.57 – 0.53 (average morning water temperature in o F).
If you know the average water temperature on a farm for selected dates you can predict how long it will take to reach a peak in the rotifer population. If the fry being rasied require rotifers, they should be stocked several days before the peak.
There is also a relationship between the mean average daily air temperature on days between filling the pond and reaching the peak and the time it takes rotifers to reach their initial peak density (Fig. 4).
The relationship is Days to reach rotifer peak = 27.4 – 0.89 (mean average daily air temperature in o C) = 43.22 – 0.49 (mean average daily air temperature in o F) where average daily air temperature is defined as high daily temperature plus low daily temperature divided by 2.
Fish farmers can use this relationship to approximate the time required for rotifer populations to reach an initial peak starting any spring day for any location. To do that, normal, daily average air temperatures for the location are substituted into the equation and a curve is drawn. Figure 4 illustrates this procedure for Stuttgart, Arkansas.
Author:
Gerald M. Ludwig
