Useful tips from Novatek: How much is enough when feeding your fish?

When feeding your fish, one of the most important factors to consider is your feeding strategy. Feeding costs can be as much as 30 to 80% of the total production cost of an aquaculture enterprise, depending on the type of production system and intensity of production. Choosing the appropriate feeding strategy is therefore important to help minimise feeding costs while maintaining optimal growth.

The selection of a feeding strategy may be influenced by the following:

Water temperature

Water temperatures below or above the recommended optimal range for a species will decrease the feeding rate.

Photo period

The photo period or the time exposed to light can affect the growth, feeding efficiency, locomotor activity and sexual maturation of aquaculture species. Generally, longer photo periods lead to increased somatic growth due to increased feed intake.

Water quality

Nutrients and organic matter from excess feeds and faeces can cause rapid deterioration of water quality. Poor water quality, can adversely affect the fish. unit will decrease as the mass per fish increases. Juveniles require higher feeding rates compared to older fish.

Fish stocking density

The higher the biomass of a system, the higher the absolute feeding requirements.

Natural water productivity

In a pond system, the natural water productivity refers to the quantity of live or natural feed available to the fish. The higher the productivity, the lower the feeding requirements of the fish.

Feed moisture content

The higher the moisture content of the diet, the lower the concentration of the nutrients, and consequently the higher the feeding rate should be.

Feed energy content

This has the most significant influence on the feeding rate. Fish generally eat to satisfy their energy requirements. The energy concentration of the diet will therefore directly determine the quantity of “other nutrients” the fish consumes. Thus, the importance of a balanced (energy to protein) diet is clear.

Aside from the factors that can directly affect the fish, the following should also be considered when selecting your feeding strategy:

• Market value of the species being produced, • Availability of labour, training requirements and cost, • Financial resources (for example cash flow, access to finance), • Cost of production inputs (fertiliser, feed, electricity, water, fuel, et cetera), • Type of production system (cage system, raceways, dam, et cetera) and • Water availability and replacement rate.

In terms of selecting your feeding strategy, there are several strategies you can employ. Two common strategies that can be used are ad-libitum feeding or feeding at a predetermined feeding level. It is important to note that, depending on your situation, you will often find a balance between these two strategies that suits your needs.

Ad-libitum feeding: Ad-lib feeding entails the feeding of fish until all the fish are satisfied. The ability of the person feeding the fish to judge when the fish have had enough is very important. Experience, a high standard of management and good supervision are important for optimal production. Although ad-lib feeding is a well-established strategy to feed fish, there are some risks associated with it. Bad management of ad-lib feeding can lead to overfeeding. This reduces the resulting percentage meat weight of the

carcass and can lead to feed wastage which has a negative economic implication and can decrease water quality. On the other hand, you may end up underfeeding your fish which leads to a lowered growth performance. With good management, ad-lib feeding ensures that the growth potential of the fish is pushed to the maximum. It will also help to prevent competition as enough feed reduces competition between fish, leading to less variations in size.

Predetermined feeding level:

Prescriptive feeding tables are compiled to assist the farmer in attaining optimum growth rates and feed conversion efficiencies, while keeping excretion levels low. These tables contain recommended feeding levels to be administered to the fish at certain stages of growth, and at specific water temperatures. In such tables, a predetermined quantity of feed is suggested for the fish. This is usually calculated as a percentage of the body mass of the fish. The extent to which values in these tables should be followed depends on the situation. In certain cases, like bad water quality and/or low oxygen levels, these recommendations are not strictly applicable. Therefore, it remains important to monitor the reaction of the fish with changing production circumstances to make sensible decisions concerning feeding management. Another aid in determining feeding levels is computer programmes or feeding management platforms in which variables like water temperature and fish size are used to determine feeding rates. Here, advanced feeding projections can be done that will not only indicate the quantity of feed per group of fish, but is also a handy aid for economic planning. Using a prescribed quantity of feed based on the biomass of the fish in a specific pond will be most effective if the biomass of each pond is known and an accurate estimate of the feed conversion can be made. Regardless of the type of feeding strategy you plan to employ, it is important to understand the feeding rate and feeding frequency. The feeding rate refers to the quantity of feed given to a production unit per day and is calculated using the relative feed intake (RFI) of the fish. The RFI is the quantity of feed given to the fish as a percentage of biomass. The feeding frequency refers to the number of feedings per day. As tilapia feed continuously, they respond better to frequent feeding than other fish. In general, the younger the fish, the higher the feed frequency. This is due to the underdeveloped and smaller gut when compared to older fish. As the fish grow, the feeding frequency can be decreased.

More frequent feeding is recommended when:

• fish are small, • natural food is inadequate, • the feed used has a low water stability, and • when the nutrient density of the feed is low.

Biomass is calculated as:

BM = W * A n

Where: W = Average individual body weight of fish in a production unit n = Number of fish stocked in a production unit A = Area of production unit (m2)

Some practical implications of feeding frequency:

Fish need oxygen for normal life processes, and requirements will rise drastically after feed intake. This is because the nutrients that are absorbed by the digestive system are being metabolised, which unlocks the energy part of the feed. The higher the metabolic rate, and consequently energy release, the more oxygen is required. If a fish receives more of a certain type of feed, or the same quantity of feed with a higher energy content, more oxygen is required for digestion and metabolism of the feed. Since the oxygen content of the water is influenced by external factors, such as high temperatures or algae blooms, oxygen deficiencies are sometimes experienced. This will affect feed consumption, which necessitates adjusting the feeding level to the oxygen concentration available in the water. When using feeds with a low energy content, it is possible (under normal circumstances) to feed the fish until they are satisfied without any problems. Maximum oxygen consumption will seldom exceed the oxygen availability. This value could be exceeded, however, if the fish is fed to satisfaction with a high-energy feed. Therefore, feeding practices should be adjusted when high-energy feeds are used. Instead of distributing a large quantity of feed once off, feed should be presented in more frequent, smaller portions. Feed intake is thus spread evenly through the day, and the oxygen requirements are likewise less drastic and stretched over a longer period of time.

Some useful calculations to calculate feeding rate or daily feeding allowance are as follows:

Relative feed intake is calculated as:

RFI = Fl * 100 Wt + Wo 2 * t

Where: RFI = Relative feed intake (% bodyweight per day) t = Time (days) Fl = Feed consumption after t days (g) Wo = Initial weight of the fish (g) Wt = Final weight after t days (g)

Feeding rate / daily feeding allowance is calculated as:

RFI = BM * S * RFI

Where: FR = Feeding rate (kg/day) BM = Biomass of fish per unit area (kg/ha) S = Percentage survival rate (%) RFI = Relative feed intake (%)

Example: Fish in 1 ha pond = 5 000 Average individual weight after 30 days = 30 g Estimated survival = 90% RFI = 8%

DFA = (5 x 50 000) x 0,90 x 0,08 = 18

In conclusion, not only does the type and quality of feed have great importance, but so too does the choice of feeding strategy when it comes to production costs and overall farm performance.

For more information on feeding strategy, please contact Robert Kanyembo at fish.ta@novatek.co.zm