8.4 MANAGEMENT PRACTICES FOR FISH

Adding fish to a new aquaponic unit is an important event. It is best to wait until the initial cycling process is totally completed and the biofilter is fully functioning. Ideally, the ammonia and nitrite are at zero and nitrates are beginning to rise. This is the safest time to add fish. If it is decided to add fish before cycling, then a reduced number of fish should be added. This time will be very stressful for the fish, and water changes may be necessary. Cycling the system with fish can actually take longer than fish-less cycling.

The fish must be properly acclimatized to the new water. Be sure to match the temperature and pH, and always acclimatize the fish slowly (as described in Section 7.5). When purchasing fingerlings from a local hatchery, make sure the fish are healthy and check carefully for any signs of disease.

8.4.1 Fish feeding and growth rates

The method of calculating the fish feed using the feed rate ratio applies to mature systems during the grow-out stage of the fish and needs further consideration here. Using the same example from Section 8.1.1, the target biomass for a 1 000 litre tank is 10-20 kg. This would be about 40 harvest-size tilapia. However, during the first 2-3 months, the fish are small and do not eat as much as was calculated (200 g of feed per day) to supply nutrients for the whole grow bed. More specifically, newly stocked fingerling-sized fish weigh about 50 grams. Juvenile fish can be fed about 3 percent of their body weight per day. Therefore, an initial stocking of 40 fingerlings would weigh 2000 g, and together they would eat approximately 60 g of fish feed per day.

A low initial stocking density is a good practice for immature aquaponic systems because it gives the biofilter additional time to develop and allows the plants time to grow and filter more nitrate. The recommendation is to estimate feeding based on body weight, but to carefully monitor feeding behaviour and adjust the ration accordingly. As the fish grow, they begin to eat more food. Moreover, it is recommended to provide a diet comparatively richer in protein to juvenile fish, if different feeds formulations are available and feasible.
After 2-3 months feeding at this rate, the 40 fish will have grown to 80-100 grams each and weigh a total of 3 200-4 000 g. At this point, they should be able to eat 80-100 g of feed per day, which is still only half of that calculated by the feed rate ratio in the earlier example. Continue to feed the fish as much as they will eat, but increase the ration slowly to prevent wasted food. Within a few more months, these same fish will each weigh 500 g with a total biomass of 20 000 grams and eat 200 g of fish feed per day. For tilapia grown in good water quality at 25 °C, it takes 6-8 months to grow from as stocking size of 50 g to a harvest size of 500 g.
Make sure to divide the feeding into morning and afternoon rations. Moreover, juvenile fish benefit from an additional lunch-time feeding. Splitting the ration is healthier for the fish and also healthier for the plants, providing an even distribution of nutrients throughout the day. Spread the feed across the entire surface of the water so all the fish can eat without injuring one another or hitting the side of the tank. Avoid scaring the fish during feeding by refraining from sudden movements. Stand still and observe the fish. Always remove any uneaten fish food after 30 minutes, and adjust the next feeding ration accordingly. If there is no food left after 30 minutes, increase the ration; if there is a lot left, decrease the ration.
A major indicator of healthy fish is a good appetite, so it is important to observe their general feeding behaviour. If their appetite declines, or if they stop feeding altogether, this is a major sign that something is wrong with the unit (most probably poor water quality). Moreover, fish appetite is directly related to water temperature, particularly for tropical fish such as tilapia, so remember to adjust or even stop feeding during colder winter months.

8.4.2 Harvesting and staggered stocking

A constant biomass of fish in the tanks ensures a constant supply of nutrients to the plants. This ensures that the fish eat the amount of feed calculated using the feed rate ratio. The previous example shows how the feeding ration depends on the size of the fish, and small fish are not be able to eat enough feed to supply the full growing area with adequate nutrients. To achieve a constant biomass in the fish tanks, a staggered stocking method should be adopted. This technique involves maintaining three age classes, or cohorts, within the same tank. Approximately every three months, the mature fish (500 g each) are harvested and immediately restocked with new fingerlings (50 g each). This method avoids harvesting all the fish at once, and instead retains a more consistent biomass.
outlines the potential growth rates of tilapia in one tank over a year using the staggered stocking method. The important aspect of this table is that the total weight of the fish varies between 10-25 kg, with an average biomass of 17 kg. This table is a basic guideline depicting optimum conditions for fish growth. In reality factors such as water temperature and stressful environments for fish will distort the figures presented here.

Table 8.2 Potential growth rates of tilapia in one tank over a year using the staggered stocking method Month

Notes:
Fingerling tilapia (1.5 kg 50 g/fish x 30 fish) are stocked every three months. Each fish survives and grows to harvest size (15 kg 500 g/fish x 30 fish) in six months. The asterisk indicates harvest. The range during harvest/stocking months accounts for the range if not all 30 fish are taken at once, ie. the 30 mature fish are harvested throughout the month. This table serves only as a theoretical guide to illustrate staggered harvest and stocking in ideal conditions.
If it is not possible to obtain fingerlings regularly, an aquaponic system can be still managed by stocking a higher number of juvenile fish and by progressively harvesting them during the season to maintain a stable biomass to fertilize the plants. Table 8.3 shows the case of a system stocked every six months with tilapia fingerlings of 50 g. In this case, the first harvest starts from the third month onward. Various combinations

TABLE 8.3
Potential growth rates of tilapia in one tank over a year using a progressive harvest technique

Notes:
Tilapia fingerling are stocked every six months. Staggered harvest starts from the third month to keep the total fish below the maximum stocking biomass of 20 kg/m3. The table shows the theoretical weight of each batch of harvested fish along the year if fish are reared in ideal conditions.
in stocking frequency, fish number and weight can apply, providing that fish biomass stands below the maximum limit of 20 kg/m3. If the fish are mixed-sex, the harvest must firstly target the females to avoid breeding when they reach sexual maturity from the age of five months. Breeding depresses the whole cohort. In the case of mixed-sex tilapia, fish can be initially stocked in a cage and males can then be left free in the tank after sex determination.
Remember that adult tilapia, catfish and trout will predate their smaller siblings if they are stocked together. A technique to keep all of these fish safely in the same fish tank is to isolate the smaller ones in a floating frame. This frame is essentially a floating cage, which can be constructed as a cube with PVC pipe used as frame and covered with plastic mesh. It is important to ensure that larger fish cannot enter the floating cage over the top, so make sure that the sides extend at least 15 cm above the water level. Each of the vulnerable size classes should be kept in separate floating frames in the main fish tank. As the fish grow large enough not to be in danger, they can be moved into the main tank. With this method, it is possible to have up to three different stocking weights in one tank, so it is important that the fish feed pellet size can be eaten by all sizes of fish. Caged fish also have the advantage of being closely monitored to determine the FCR by measuring the weight increment and weight of the feed over a period.

8.4.3 Fish summary

Add fish only after the fish-less cycling process is complete, if applicable.
Feed the fish as much as they eat in 30 minutes, two times per day. Always remove uneaten feed after 30 minutes. Record total feed added. Balance the feeding rate with the number of plants using the feed rate ratio, but avoid over- or under- feeding the fish.
Fish appetite is directly related to water temperature, particularly for topical fish such as tilapia, so remember to adjust feeding during colder winter months. A fingerling tilapia (50 g) will reach harvest size (500 g) in 6-8 weeks under ideal conditions. Staggered stocking is a technique which involves stocking a system with new fingerlings each time some of the mature fish are harvested. It provides a way of maintaining relatively constant biomass, feeding rate and nutrient concentration for the plants.