In this chapter, we will review management feed strategies.
In this chapter, we will review management feed strategies. We encourage you to check not only these points but also our-in house Feed Trials on our website:
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Various feeding strategies are used on trout farms, including: computerized automatic feeding, self-demand pendulum feeders and/or hand feeding. However, the feeding strategy generally takes into consideration the specific farming conditions (e.g., water temperature, oxygen conditions, water quality). The fish are generally fed a restricted amount according to a table, but it is close to ad libitum to optimise specific growth rate (SGR) and the feed conversion ratio (FCR).
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Assuming that fish are growing exponentially, the specific growth rate (SGR) is defined as:
SGR = (exp ((lnW1 – lnW0)/(T1 – T0)) – 1) * 100
The feed conversion ratio (FCR) is defined as:
FCR = feed administered (kg)/growth gain (kg)
The main difference between restricted feeding and ad libitum feeding is that in restricted feeding the main focus is on the feed utilization and minimum feed loss, whereas the growth potential of the fish is the aim of ad libitum feeding. Restricted feeding is the most widespread strategy applied in Danish freshwater fish farms to improve the utilization of the limited feed allowances and to reduce losses to the environment.
When choosing the feeding strategy, it must be kept in mind that the growth in length of fish occurs in one dimension, whereas the general growth increment (meat, fat, etc.) takes place in three dimensions. For each 1 g of protein growth, 3 g of water is deposited, and fat does not bind water. This means that increased fat deposits are expected with ad libitum feeding. However, the feeding strategy used is not expected to have any influence on the ability of the fish to utilize the feed.
The most common errors in hatcheries are either to overfeed or to underfeed. Overfeeding is wasteful in terms of unconsumed food, but underfeeding is just as wasteful in terms of lost production. To obtain maximum production and feed efficiency during the production cycle, careful attention must be given on a daily basis to the amount of food the fish are receiving and consuming.
The quantity of food required is calculated in terms of percent body weight per day. Because the metabolic rate per unit weight of fish decreases as the fish grows larger, the percent of body weight to be fed per day also decreases. As metabolic rate is typically driven by water temperature, the percent body weight per day also varies with water temperature.
Along with feed rate, the correct feed size is very important. To determine if fish are ready to switch to the next feed size you can subsample the population. The assumption with subsampling, is that the sample you measure is representative of the entire population. One way to sample for fish size/weight is to sample a group of 50 fish for individual fish weight/lengths, and average the weights/lengths, or plot the measurements on a length frequency chart. The other method is to net a sample of fish, put them in to a bucket until the scale reaches a predetermined weight, and then count the number of fish in that sample.
Perform this sampling technique three times and average the counts. If any one of the counts varies by 10% or more from the others, take two more samples (for a total of 5 samples) and average the counts. Sampling using these methods, on a weekly basis, will help hatchery staff keep track of growth rates and allow the culturists to more accurately determine when to change feed size.
One method that is very helpful in determining if the subsample accurately describes the larger population, is sampling to calculate for Coefficient of Variation (CV).
Coefficient of Variation
Used to determine if fish are ready to switch to the next feed size
Used to see how uniform in size your fish are
Used to monitor Growth and Development
Sample at least 100 and not more than 200 fish per sample CV
Same for all Species
A CV of 3-4 is excellent – very tight
A CV of 5-6 is Very Good – fairly tight
A CV of 7-8 is Good –Average tight
A CV of 10 is on the edge – not so tight
A CV of higher than 10 is out of size.
CV can be calculated with the formula below or by using a CV calculator spreadsheet.
CV = standard deviation for the sample divided by mean for the sample
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Swim-up fry (first feeding) are generally fed to satiation which at I0°C is 7-10% of their bodyweight each day. As fish grow, they take proportionately less ration each day. Thus, fingerlings and yearlings are fed anywhere from 6% to 1% of bodyweight per day at l0°C, whereas at 5°C and 15°C they would take 2.5-0.5% and 7.5-1.5% bodyweight/day, respectively. All these levels can be increased, possibly by 20-30%, to improve rates of growth although as satiation is approached, there is generally a reduction in the food conversion ratio or FCR (calculated by dividing the weight of dry food fed by the gain in wet fish weight). There is considerable variation in the FCRs achieved by trout farms although most would expect to average an FCR of at most 1.25. Under ideal conditions FCRs of 1.0 or less are possible. Diets which are low in digestible proteins and/or fats will produce FCRs of 1.5 or more.
Feed manufacturers are often the best source of information about the most appropriate daily ration, diet formulation, pellet size and methods of fabrication for particular stocks and sizes of fish and for feeding under differing environmental conditions. They should always be consulted because of their experience with similar farms and conditions. Methods and frequency of feeding are also important in achieving nutritional effectiveness. Generally, first-feeding fry, whether they are trout or salmon, should be fed as soon as they will take food and they should be fed a little and often. In fact, many farms feed early fry stages continuously using automatic feeders. Larger fish need only be fed two or three times a day. In each case the recommended daily ration (% bodyweight per day) is divided by the number of feeds to be employed. Although the different feeds can be of equal amounts, some farms prefer to feed a disproportionately larger meal at the first feed of the day.
There are three methods of feeding: hand, automatic and demand feeding. Smaller farms often administer feed by hand thus saving some capital expenditure. Hand feeding also has certain other advantages over automatic feeding. Firstly, hand feeding enables the feeder to gauge the health of the stock from the strength of the feeding response, i.e. the response during the first moments after feeding and the general appearance and behavior of the fish. In trout farming the absence or reduction of the characteristic frenzied movements or ' boiling of the water' immediately after the pellets hit the water, together with a darkening of some fish and/or abnormal movements (e.g. flashing) are indicators of a water quality or disease problem developing.
Hand feeding also prevents unnecessary wastage of food because feeding can be stopped immediately any pellets remain uneaten by the stock. Floating or slow-sinking pellets can be helpful in assessing the feeding response during hand feeding because they remain on the water surface or water column longer than conventional sinking pellets. Although there is considerable advantage to be gained from hand feeding, for manpower reasons this method is not feasible for larger farms and most of these resort to automatic feeders driven by clockwork, water, compressed air, or electric or battery mechanisms. In general feeders powered by electricity are preferred because of their reliability irrespective of how they are motorised. Automatic feeders generally incorporate a timeclock device. This enables the exact number and timing of the feeds to be specified. Of course, they enable the fish to be fed many more times each day than is possible with hand feeding. Daylight or solar-operated devices have been used effectively to time feeders on cage sites which are too remote or possibly too small to justify manning throughout the day. Whichever type of automatic feeding system is used they are very often linked to, or controlled by, a computer system which will work out feed rates, frequency and amounts, based on stock size, and also keep records for future use.
Automatic feeders are particularly useful for feeding fry. First feeding or swim-up fry benefit greatly from continuous feeding. Used in combination with extended daylength provided by artificial lights in the hatchery, fry may be fed 24 hours each day, although in practice it is better to feed for only 18- 20 hours because all-day feeding may provoke gill problems in the stock . As the stock increase in size, the frequency of feeding can be reduced. Thus, division of the daily ration into eight and five portions are necessary for fish of 0.5 g and 1 g size respectively, whereas for larger fish the ration can be fed in only two or three meals. Large rainbow Trout (>1.5kg) can take their complete daily ration in a single meal. The more meals per day, the more constant are the levels of ammonia, suspended solids, Biological Oxygen Demand (BOD), etc., produced by the stock. By comparison, a single meal will generally produce a peak in effluent production some six to eight hours later. A major disadvantage of early automatic feeders was the small area over which the food was dispensed. Modern feeders can distribute pellets over much of the surface of tanks, ponds, and cages.
Thanks to the advantage of hand feeding in providing information on the general health of the stock, in practice many farms (even some of the largest) feed only 60- 70% of the daily ration automatically, with the remainder being fed by hand. Hand or simple automatic feeding is not sufficient to cope with the largest production units. On these farms' food is distributed over the ponds or raceways by compressed air blowers usually mounted on a vehicle of some type. Compressed air systems are also used to deliver feed through pipes to feeders on cages which are remote from the shore.
Some farms use a third method of feeding, known as 'demand' feeding. Demand feeders are devices in which the appetite of the fish determines the amounts of food dispersed. Usually, demand feeders consist of a food hopper with an aperture whose opening is controlled by a movable gate. Attached to this gate is a pendulum whose tip extends down into the water where it can be nudged by the fish. Lateral movements of the pendulum cause the gate mechanism to open, allowing food to flow out of the food hopper into the water. The fish are thus able to demand food. Disadvantages of demand feeders include the gate mechanism sticking open and/or inadvertent or unnecessary operation by fish, both resulting in food waste. Often farms using demand feeders, like those primarily using automatic feeders, combine these methods with a proportion of the feed being administered by hand.
Whichever feeding method, feeding frequency and daily ration size is employed, the effectiveness of these regimes should be checked every one or two weeks for every batch of fish on a farm. These checks are carried out by weighing a representative sample of fish in a particular tank or pond. Knowing the total number of fish in the tank and hence their total weight, and the total amounts of food fed, enables the growth and FCR to be calculated and any deficiencies in performance from the expected norm remedied accordingly. Careful recording of weights, growth, FCR etc., enables most farms to predict what performance is to be expected from their stocks under a variety of environmental conditions.
The amount of feed trout require depends on water temperature and fish size. Smaller fish have faster metabolic rates and need more feed relative to their body weight than do larger fish. Because fish are poikilo- thermic (cold-blooded), their body temperatures and metabolic rates vary with water temperature. Fish in warmer water need more feed than fish in cooler water. The minimum temperature for growth in trout is about 38°F (3.3°C). At this temperature and below, appetites are suppressed, digestive systems operate very slowly, and trout require only a maintenance diet (0.5 to 1.8 percent of body weight per day, depending upon fish size). Feeding more than this wastes feed.
In warm water -above 68°F (20°C), a trout’s digestive system does not use nutrients well and more of the consumed feed is only partially digested before being eliminated. This nutrient loading of the water, coupled with the generally lower oxygen levels in warm water, can easily lead to respiratory distress. In warm water, feeding rates should be reduced enough to maintain good water quality and avoid wasting feed. The optimum temperatures for growing trout are 55°F to 65°F (12.8°C to 18.3°C). At this temperature range feeding rates should be at maximum levels (1.5% to >6.0% of body weight per day).
The best way to determine the correct amount and size of feed for trout production is to use a published feeding chart, usually provided by the feed manufacturer. These charts are useful guides, but you may need to make adjustments to fit specific conditions on your farm. Under most circumstances, fish need to be fed less than they will eat. Overfeeding will cause the fish to use the feed less efficiently and will not increase growth rates significantly. To determine the appropriate amount of feed, know the number and size of the fish on your farm. At water temperatures above 55°F (12.8°C), make a sample count of the fish at least monthly and adjust feeding percentages accordingly. In cooler waters, a sample count every 1 to 2 months usually is adequate. Good growth records for trout on your farm will help you predict seasonal growth rates. Do not overfeed. Once feed settles to the bottom of the tank, small trout will ignore it. Excess feed reduces water quality and promotes disease. Remove any excess feed promptly.
Southern Regional Aquculture Center . Publication number 223, May 1990
Figure 1. Examples of feeding rates for rainbow trout. All values are in percent of body weight to be fed each day. A grower should obtain a feeding chart from the feed supplier that is tailored to that feed formulation.
Figure 2. Sample feed sizes and number of daily feedings for rainbow trout.
Once a high-quality feed has been selected and the correct amount of feed determined, the next consideration is how to feed the fish.
The best method depends on the size of the fish. Trout will begin to consume prepared diets within 7 to 10 days after hatching. At first, fry should be fed a small amount by hand eight to ten times per day until all the fish are actively feeding. A large kitchen strainer makes an excellent tool for distributing the finely ground starter feeds used for trout. After the initial feed training, an automatic feeder is most practical, with two or three hand feedings daily so that you can observe the fish.
As the fry grow, the frequency of feeding can be gradually decreased to about five times per day. When fed nearly to satiation, trout will consume roughly 1 to 2 percent of their body weight in dry feed at each feeding. The feeding frequency should be adjusted to obtain the desired feeding percentage. Fry gain weight rapidly and should be sample counted weekly for the first 4 to 6 weeks. The daily feed ration should be adjusted according to their weight. Feed should be distributed over at least two- thirds of the water surface when fry are less than 2 inches long. This gives them easy access to the feed and helps to keep a uniform size within the population.
After fingerlings are moved out to tanks or earthen ponds, there are several feeding alternatives. Hand feeding each day until the fishes’ appetites are suppressed usually produces the best combination of feed conversion efficiency and growth rate. However, hand feeding is labor intensive and may not be practical on a large commercial farm. Hand feeding is the best way to train fish to use demand feeders or to administer medicated feed to sick fish.
Several types of automatic and mechanical feeders are available for trout farming, including electric, water powered, and solar powered feeders with variable timers. There are feeders that use compressed air to blow feed out over the water surface at pre-set intervals, and truck or trailer mounted units that have hydraulically operated blower feeders. The type of feeder commonly used on commercial trout farms is the demand feeder (Fig. 1). It consists of a hopper for holding the feed pellets and, below the hopper opening, a movable disc attached to a pendulum extending into the water. Trout longer than 5 inches can easily be trained to feed themselves.
With careful adjustment of demand feeders, rapid weight gain and efficient feed utilization can be attained. The use of demand feeders can eliminate the sharp oxygen decline that occurs when fish are fed by hand or machine a few times each day.
Demand feeders also reduce the labor cost associated with daily hand feeding. Disadvantages include the tendency to overfeed because of improper feeder adjustment, and food release only in a small section of the pond or tank. Overfeeding with demand feeders can be a problem with larger trout.
Demand feeders should be located at intervals of about 25 to 30 feet along the tank walls. Several days’ feed can be loaded, but for best feeding efficiency it should not be replaced until the feeding period has passed. Adjust the feeder so that the feed is removed over the entire time for which the feeder is loaded. Even if demand feeders are used, feeding according to a feed chart is recommended for best performance.
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Figure 3. Demand feeder used in trout production
Whether feeding by hand or with a mechanical distribution system, feed should be distributed throughout the pond and should not accumulate on the bottom. In concrete tanks, trout will feed on some pellets that fall to the bottom, but trout will rarely pick up pellets from the bottom of earthen ponds.
A good way to ensure that all the trout in a tank have access to the feed when hand feeding or using automatic feeders is to distribute twice as many feed pellets as fish throughout the tank in a 5- to 10- minute period. Repeat this process at 10-minute intervals until all the ration for that feeding has been distributed or until the feeding activity declines.
Feeding should be restricted when water temperatures drops below 40°F (4.5°C) or rises above 68°F (20°C). Feeding rates also should be reduced or feed withheld entirely when fish are sick. Fish should always be kept off feed for a while before handling or transporting. For routine handling, such as grading or vaccinating, 24 hours without food is sufficient. If fish are to be transported off the farm or are to be processed, they should be kept off feed for at least 3 to 4 days, or longer if the water temperature is low. Trout producers do not usually use finishing diets before processing, but feed may be withheld for several weeks if the fat content of fillets needs to be reduced.
There are specialty trout feeds for specific production goals. Phosphorus levels in some feeds have been reduced to 0.7 to 0.9 percent by weight in order to reduce the amount of phosphorus released to the environment from trout culture. Highly digestible or “nutrient-dense” diets are available for use where reducing solid waste is a concern. Nutrient-dense diets are typically high in fish meal protein and lipids and low in carbohydrates, especially uncooked starches, and fibrous materials.
There are also specialty feeds containing antibiotics (tetracycline hydrochloride or potentiated sufadimethoxine), immune stimulants (beta-glucans and other yeast derivatives or other compounds), or carotenoid pigments (canthaxanthin or astaxanthin). They are more expensive than regular diets and should be used only when appropriate. Feeds containing antibiotics should be used only after the diagnosis of a bacterial condition susceptible to treatment. Immune stimulants have only recently become available and are not yet in wide-spread use. Feeds with carotenoid pigments impart a pink or red color to the flesh and do not affect fish health or growth rate. Pigmentation can be achieved in about 3 months when fish are actively growing, and in about 6 months in cold water. Other specialty diets include an enriched diet for broodfish and a high-fat diet (16 to 24 percent fat) for producing an oilier fish used for smoking or for specialty markets.