Management Guide 6: Trout Logic
A good start is essential to ensuring long-term developmental growth and survival for trout. As fish are very susceptible to disease and mishandling,...
Trout Logic Guide:
Water & Nutrition
Part 6
Introduction
A good start is essential to ensuring long-term developmental growth and survival for trout. As fish are very susceptible to disease and mishandling, it is imperative that you utilize the best quality water in combination with optimal nutrition and feed management to have your fish performing to the best of their ability. In the sixth edition of Trout Logic: Good Water and Nutrition, we discuss all things water, feed and nutrition related that can set your hatchery up for success.
This management guide series covers the following topics:
Water Quality
There are many physical and chemical aspects in the water that we must consider for our trout biomass. We will review these aspects in this chapter.
Water quality determines the success or failure of any fish culture operation to a great extent. Physical and chemical characteristics such as suspended solids, temperature, dissolved gases, pH, mineral content, and potential danger of toxic materials should all be considered for managing water quality and running an efficient operation.
Figure 1. Suggested water quality criteria for optimum health of salmonid fishes. Concentrations are in parts per million (PPM)1. Source: Wedemeyer, 1977.
Figure 2. Suggested chemical values for hatchery water supplies, concentration are in parts per million (PPM) 1. Source: Howard n. Larsen, unpublished
1 Source: Fish Hatchery Management
Temperature
No other single factor affects the development and growth of fish as much as water temperature. Metabolic rates of fish increase rapidly as temperature rises, and many biological processes such as spawning and egg development, and hatching are geared to annual temperature changes in the natural environment. Each fish species has a temperature range that it can tolerate, and within that range, it has optimal temperatures for growth and reproduction. For Rainbow Trout, this tolerable range is 0.5 – 25.5°C, with the optimum being 10-16°C.Within a hatchery, temperatures that become too high or low for fish impart stresses that can dramatically affect production and render fish more susceptible to disease. Most chemical substances dissolve more readily as temperature increases; in contrast, and of considerable importance to hatchery operations, gases such as oxygen and carbon dioxide become less soluble as temperatures rise.
Dissolved gases
Nitrogen and oxygen are the two most abundant gases dissolved in water. Although the atmosphere contains almost four times more nitrogen than oxygen in volume, oxygen has twice the solubility of nitrogen in the water. Therefore, freshwater usually contains about twice as much nitrogen as oxygen when in equilibrium with the atmosphere. Carbon dioxide is also present in water, but it usually occurs at much lower concentrations than either nitrogen or oxygen because of its low concentration in the atmosphere. All atmospheric gases dissolve in water, although not in their atmospheric proportions; as mentioned, for example, oxygen is over twice as soluble as nitrogen. Natural waters contain additional dissolved gases that result from erosion of rock and decomposition of organic matter. Several gases have implications for hatchery site selection and management. Oxygen must be above certain minimum concentrations. Other gases must be kept below critical lethal concentrations in hatchery or pond water. As for other aspects of water quality, inappropriate concentrations of dissolved gases in source waters mean added expense for treatment facilities. It should also be noted that the solubility of gasses and the amount of gas dissolved in water vary with the temperature of the water.
Oxygen
Oxygen is the second most abundant gas in water – Nitrogen is the first – and the most important as fish cannot live without it. Concentrations of oxygen, like other gasses, are typically expressed by either parts per million by weight (ppm) milligrams per liter (mg/l) or as percent saturation. In the latter case, saturation refers to the amount of gas dissolved when water and atmospheric phases are in equilibrium. This equilibrium amount (for any gas) decreases-that is less oxygen can be dissolved in water at higher altitudes and, more importantly, at higher temperatures.
For this reason, the relationship between absolute concentrations (parts per million) and relative concentrations (percent saturation) of gases is not straightforward. Special conversion formulae are needed; these can be depicted as nomograms in graphical form. Dissolved oxygen concentrations in hatchery waters are depleted in several ways, but chiefly by respiration of fish and other organisms and by chemical reactions with organic matter (feces, waste feed, decaying plant, and animal remains, etc.).
As temperature increases the metabolic rate of the fish, respiration depletes the oxygen concentration of the water more rapidly, and stress or even death can follow. Good hatchery management must consider fluctuating water temperatures and the resulting change in available oxygen. In ponds, oxygen can be restored during the day by photosynthesis and at any time by wind mixing of the air and water.
In hatchery troughs and raceways, oxygen is supplied by continuously flowing freshwater. However, oxygen deficiencies can arise in ponds and raceways, especially when water is reused or reconditioned. Then, chemical or mechanical aeration techniques must be applied by culturists.
In general, water flowing into hatcheries should be at or near 100% oxygen saturation and a level of > 7.0 mg/l. In raceway systems, where large numbers of fish are cultured intensively, oxygen contents of the water should not drop below 80% saturation. In ponds, where fish densities are lower (extensive culture) than in raceways, lower concentrations can be tolerated for short periods. But in either type of holding unit, if the fish are subjected to more extended periods at concentrations below 5.0 mg/l growth, survival will be severely compromised. A continual saturation of 80% or more for Trout provides a desirable oxygen supply.
Nitrogen
Some aquatic bacteria and algae may fix molecular nitrogen (N2), but it is biologically inert as far as fish are concerned. Dissolved nitrogen may be ignored in a fish culture if it remains at 100% saturation or below. However, at supersaturation levels as low as 102%, it can induce gas bubble disease in fish.
Theoretically, gas bubble disease can be caused by any supersaturated gas, but the problem is almost always due to excess nitrogen in practice. When water is supersaturated with gas, fish blood tends to become so as well. Because oxygen is used for respiration, and carbon dioxide enters the physiology of blood and cells, excess amounts of these gases in the water are taken out of the solution in the fish body. However, nitrogen, being inert, stays supersaturated in the blood. Any reduction in pressure on the gas, or localized increase in body temperature, can bring such nitrogen out of solution to form bubbles; the process is analogous to "bends" in human divers. Such bubbles (emboli) can lodge in blood vessels and restrict respiratory circulation, leading to death by asphyxiation. In some cases, fish may develop apparent bubbles in the gills, between fin rays, or under the skin, and the pressure of nitrogen bubbles may cause eyes to bulge from their sockets.
Gas supersaturation can occur when air is introduced into water under high pressure, subsequently lowered, or when water is heated. Water that has plunged over waterfalls or dams, water drawn from deep wells, or water-heated from snowmelt is potentially supersaturated. Air sucked in by a water pump can supersaturate a water system.
Carbon Dioxide
All waters contain some dissolved carbon dioxide. Generally, waters supporting good fish populations have less than 5.0 parts per million carbon dioxide. Spring and well water, which frequently are deficient in oxygen, often have a high carbon dioxide content. Both conditions can be corrected easily with efficient aerating devices. Carbon dioxide over 20 parts per million may be harmful to fish. Lower carbon dioxide concentrations may be detrimental if the dissolved oxygen content drops to 3- 5 parts per million. It is doubtful that freshwater fishes can live throughout the year with an average carbon dioxide content as high as 12 parts per million.
Toxic gases
In very low concentrations, hydrogen sulfide (H2S) and hydrogen cyanide (HCN) can kill fish. Hydrogen sulfide derives mainly from the anaerobic decomposition of sulfur compounds in sediments; a few parts per billion are lethal. Hydrogen cyanide is a contaminant from several industrial processes and is toxic at concentrations of 0.1 part per million or less.
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Suspended and Dissolved Solids
Suspended solids make the water cloudy or opaque; they include chemical precipitates, flocculated organic matter, living and dead planktonic organisms, etc.
Suspended and Dissolved Solids
"Solids" in water leave tangible residues when the water is filtered (suspended solids) or evaporated to dryness (dissolved solids). Suspended solids make the water cloudy or opaque; they include chemical precipitates, flocculated organic matter, living and dead planktonic organisms, and sediment stirred up from the bottom on a pond, stream, or raceway. Dissolved solids may color the water but remain clear and transparent; they include anything in proper solution.
Suspended solids
"Turbidity" is the term associated with the presence of suspended solids. Analytically, turbidity refers to the penetration of light through water (the lesser the penetration, the greater the turbidity). Still, the word is used less formally to imply concentration (weight of solids per weight of water).
Turbidity over 100,000 parts per million does not affect fish directly, and most natural waters have far lower concentrations than this. However, abundant suspended particles can make it more difficult for fish to find food or avoid predation. To the extent they settle out, such solids can smother fish eggs and the bottom organisms that fish may need for food. Turbid waters can clog hatchery pumps, filters, and pipelines.
In general, turbidity less than 2,000 parts per million is acceptable for fish culture.
Acidity
Acidity refers to the ability of dissolved chemicals to "donate" hydrogen ions (H+). The standard measure of acidity is pH, the negative logarithm of hydrogen-ion activity. The pH scale ranges from 1 to 14; the lower the number, the greater the acidity. A pH value of 7 is neutral; that is, there are as many donors of hydrogen ions as acceptors in solution.
Ninety percent of natural waters have pH values in the range 6.7- 8.2, and fish should not be cultured outside the range of 6.5- 9.0. Many fish can live in waters of more extreme pH, even for extended periods, but at the cost of reduced growth and reproduction. Fish have less tolerance of pH extremes at higher temperatures. Ammonia toxicity becomes an important consideration at high pH.
Even within the relatively narrow range of pH 6.5-9.0, fish species vary in their optimum pH for growth. Generally, those species that live naturally in cold or cool waters of low primary productivity (low algal photosynthesis) do better at pH 6.5- 9. Trout are an example; excessive mortality can occur at pH above 9.0.
The affected fish rapidly spin near the surface of the water and attempt to leave the water. Whitening of the eyes and complete blindness also occur, as well as fraying of the fins and gills with the frayed portions turning white. Death usually follows in a few hours. Fish of warmer climates, where intense summer photosynthesis can raise pH to nearly 10 each day, do better at pH 7.5- 9. Striped bass and catfish are typical of this group.
Alkalinity and hardness
Alkalinity and hardness imply similar things about water quality, but they represent different types of measurements.
Alkalinity refers to an ability to accept hydrogen ions (or to neutralize acid) and is a direct counterpart of acidity. The anion (negatively charged) bases involved are mainly carbonate (CO:;) and bicarbonate (H C O 3) ions; alkalinity refers to these alone (or these plus O H- ) and is expressed in terms of equivalent concentrations of calcium carbonate (C a C 0 3).,
Hardness represents the concentration of calcium (C a++) and magnesium (M g++) cations, also expressed as the CaC0 3- equivalent concentration. The same carbonate rocks that ultimately are responsible for most of the alkalinity in water are the main sources of calcium and magnesium as well, so values of alkalinity and hardness often are quite similar when all are expressed as CaC0 3 equivalents.
Fish grow well over a wide range of alkalinity and hardness, but values of 120- 400 parts per million are optimum. At very low alkalinity, water loses its ability to buffer against changes in acidity, and pH may fluctuate quickly and widely to the detriment of fish. Fish are also more sensitive to some toxic pollutants at low alkalinity.
Toxic materials
Various substances toxic to fish occur widely in water supplies as a result of industrial and agricultural pollution. Chief among these are heavy metals and pesticides.
Heavy Metals: There is a wide range of reported values for the toxicity of heavy metals to fish. Concentrations that will kill 50% of various species of fish in 96 hours range from 90 to 40,900 parts per billion (ppb) for zinc, 46 to 10,000 ppb for copper, and 470 to 9,000 ppb for cadmium. Generally, trout and salmon are more susceptible to heavy metals than most other fishes; minute amounts of zinc leached from galvanized hatchery pipe can cause heavy losses among trout fry, for example. Heavy metals such as copper, lead, zinc, cadmium and mercury should be avoided in fish hatchery water supplies, as should galvanized steel, copper, and brass fittings in water pipe, especially in hatcheries served by poorly buffered water.
Ultra Violet (UV) filters
Feed Quality and Storage
Feed quality and storage are related to the success of the operation. Feed is expensive in trout production. In addition, good storage will help to maintain the quality of the feed-in time.
Feed Quality and Storage
In this chapter, we will review management feed quality and storage. We encourage you to review not only these points but also our in-house Feed Trials on our website:
Feed quality
Trout feeds have significantly been improved in the past decade. Fish meal is still the primary source of protein, but protein digestibility has been improved, and ash content has been reduced by using fish meal processed at lower temperatures (“low-temp” fish meal). Also, diets now have higher energy levels that help fish use protein more efficiently.
Increasing the energy level in the diet limits the fish’s use of protein as an energy source. Trout are grown efficiently with dietary fat levels (mainly from fish oils) as high as 18 to 28 percent, provided the ratio of digestible protein to energy remains in the correct range. This ratio is expressed as grams digestible protein per megajoule of digestible energy.
Ask your feed manufacturer to tell you the ratio of digestible protein to energy in your fish feed, especially if you plan to use high-energy diets. For typical high-energy diets the ratio should be about 20:1. Feeds with ratios significantly higher than 20 may contain excess protein or large amounts of protein that trout cannot digest easily.
Feeds with lower ratios may contain excess fat and could affect flesh quality and dress-out percentages. However, specific diet formulations may vary considerably from this ratio and still be highly efficient if adequately formulated.
Feed storage and inventory control
Since feed is the highest cost component of a trout farming operation, consideration needs to be given to proper feed storage and handling. Freshness of feed is another consideration, although feed manufacturers use ingredients that are much more stable than they used to be. Some manufacturers have extended shelf life guarantees to 12 months.
A good example of a feed storage room. The space is clean, with no outside light, and the walls are insulated.
Purchase feed that has been recently manufactured and stored adequately by the feed supplier. Feeds that have the manufacturer’s date stamped on the bags will prevent the purchasing of old feed. Where possible, feed should be stored in an air-conditioned building for temperature and humidity control. Otherwise, feed should be stored in a cool, dry area off the ground on pallets and at least one foot away from any walls to avoid condensation. Make sure to rotate inventory; first in, first out. If silos are being used, try to move feed through silos regularly.
Feed that will be used quickly, can be stored in a barrel with a lid, as long as the room does not experience significant temperature changes.
When feeds are stored for long periods or under poor conditions, fish health problems may arise from molds and fungi and from vitamin degradation and rancidity of oils in the feed. Controlling rodents and insects is also important in maintaining nutrient quality and aflatoxin-free feeds.
Poor feed storage- The feed is stored in the incubation room, which will lead to excess moisture entering the feed bags. This can lead to mold colonizing the feed.
Molds and Fungi
Feeds stored for a long time and probably contaminated with molds appear stale, are discolored, lump together and smell musty. Moldy foods are often saturated with moisture and appear to ‘sweat.’ Any containers used to store food (bins, automatic feeders) should be cleaned thoroughly on a bi-monthly basis to prevent mold growth on their surfaces (which may be hidden by newly placed fresh feed).
Mold in fish feed becomes a problem when toxic species of molds exist in sufficient quantities to produce aflatoxins. Aflatoxins are chemicals produced by some species of naturally occurring fungi(Aspergillus flavus and Aspergillus parasiticus), commonly known as molds. Aflatoxins are common contaminants of oilseed crops such as cottonseed, peanut meal, and corn. Wheat, sunflower, soybean, fish meal, and complete feeds can also be contaminated with aflatoxins. These molds can grow in grains and prepared feeds intended for fish production when storage conditions are suboptimal: temperatures of 27°C (80°F) or warmer and moisture at levels greater than 14%.
Consumption of aflatoxins can result in a disease known as aflatoxicosis. Fry are more susceptible to aflatoxicosis than adults. Initial findings associated with aflatoxicosis include pale gills, impaired blood clotting, anemia, poor growth rates, or lack of weight gain. Prolonged feeding of low concentrations of aflatoxins causes liver tumors, which appear as pale-yellow lesions and which can spread to the kidney. Increases in fish mortality may also be observed. Aflatoxins can cause disease indirectly through their effects on essential nutrients in the diet. These toxins can destroy fat-soluble antioxidants, such as vitamin A, and water-soluble antioxidants and vitamins such as vitamin C (necessary for immune function) and thiamin (essential for metabolic and nervous function). Aflatoxins depress the immune system, making fish more susceptible to bacterial, viral, or parasitic diseases. These subtle effects often go unnoticed, and profits are lost due to decreased production efficiency, such as slow growth, reduced harvest weight, higher FCR, and increased medical costs.
Aflatoxicosis is now rare in the rainbow trout industry due to strict regulations in most countries for aflatoxin screening in oilseeds, corn, and other feed ingredients.
Vitamin degradation
Vitamin degradation in feed can’t be seen or detected by smell. Vitamin degradation is accelerated by:
- Heat
- Oxygen
- Moisture
- Ultraviolet Light
- Age – Use feed within recommended shelf life.
Improvements over the years include:
- Use of more stable forms of Vitamin C.
- Feeds formulated with higher levels of vitamins to compensate for degradation.
Rancidity
Lipids (fish oils and poultry oils), lipid-containing ingredients, and complete feeds primarily degrade through oxidation reactions resulting in rancidity. Antioxidants are added to the feed to counteract this process, but they are gradually depleted over time. Rancidity is typically associated with loss of nutritional value and the development of off-odors. Rancid feed smells like paint, turpentine, or other similar chemical smells. The oxidation of lipids can also result in lower feed conversion, decreased weight gain, and increased disease or mortality in cultured fish. Lower energy value, protein quality, and vitamin retention in feed can be accompanied by the formation of toxic oxidation metabolites. Higher moisture feeds are more susceptible to rancidity. Temperature swings can accelerate lipid oxidation by creating pockets of high moisture. Proper storage and handling of feed can significantly increase the lifespan of the feed. The rancid feed should be disposed of and not used. If the feed has passed its expiration date, you can choose to still feed as long as there is no mold or rancidity. Mix with newer feed to be safe. Ideally, feed is used before the expiration date, but we sometimes recognize that you may have feed left over from the year before.
Feeding Strategy
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:
Read our Feed Trials
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
- W0 = biomass at the beginning of the period
- W1 = biomass at the end of the period
- T1 – T0 = feeding days in the period.
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.
Trout feeding information and guidelines
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
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Used to determine if fish are ready to switch to the next feed size
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Used to see how uniform in size your fish are
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Used to monitor Growth and Development
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Sample at least 100 and not more than 200 fish per sample CV
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Same for all Species
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A CV of 3-4 is excellent – very tight
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A CV of 5-6 is Very Good – fairly tight
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A CV of 7-8 is Good –Average tight
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A CV of 10 is on the edge – not so tight
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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
CV=stdev(range)/average(range)*100
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Feeding methods, frequency, and rates
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.
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Feeding practices
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.
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How to feed 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.
Special purpose feeding
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.
