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Aquaculture Feed and Fertilizer Resources Information System
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Rainbow trout fry at first feeding are relatively large with mature digestive systems. They are capable of ingesting small feed particles (<0.5 mm) and digesting pelleted feed, providing very small pellets or crumbles are fed. Hence, live feeds are not used in trout farming. Rainbow trout are fed formulated feeds throughout farm production cycles and to maturation in the case of broodfish (Tables 3 and 4). Tacon and Metian (2008) estimated global production of feed for rainbow trout was 790 000 tonnes in 2006. Most rainbow trout feed is produced by cooking-extrusion to make floating pellets. Extruded pellets are top-dressed with oil to achieve levels of 18–25 percent total lipid (Hardy and Barrows, 2002). Fry and fingerling feeds have lower lipid levels than feeds for grower fish.
Rainbow trout feeds have undergone a shift since the 1970s from being relatively high in protein content and low in total lipid to being lower in protein content and higher in lipid (Hardy, 2002) (Figure 5). Over the same period, the percentage of digestible protein has increased, making modern trout feeds much more efficient and less polluting. An additional change in trout feeds has been a reduction of the percentage of protein supplied by fishmeal and a corresponding increase in the contribution of alternate proteins, mainly plant protein concentrates and rendered animal proteins, such as poultry byproduct meal. Protein sources are blended to produce an amino acid profile in the feed that meets the dietary amino acid requirements of the fish. Generally, lysine and methionine are the first and second limiting amino acids. DL-methionine is supplemented to trout feeds when high levels of soy proteins are used in the formulation.
Another shift in formulation of rainbow trout feeds is replacement of portions of fish oil with plant oils. In 2006, 88 percent of global production of fish oil was consumed in aquafeeds (Tacon and Metian, 2008). Supply and demand forces caused a large increase in the price of fish oil in 2006/07, causing feed producers to replace portions of fish oil with plant oils such as rapeseed oil and soy oil (Table 3). Blended oil is used during portions of the grower phase of production, sometimes with a switch to fish oil near the end of the production cycle to increase omega-3 fatty acids in fillets.  Feed ingredients of plant and animal origin used in the formulation of trout feeds with their general nutritional values and other relevant information are provided in Table 3. The maximum inclusion level of each feedstuff used in trout feed depends on stage of production and the relative prices of ingredients. Protein sources are often compared on the basis of cost per kg protein rather than cost of the ingredient. Plant protein sources suitable for use in rainbow trout feeds were recently reviewed by Gatlin et al. (2007). Feed ingredients used in rainbow trout feed formulations are similar throughout the world (Table 3). Protein sources include fishmeal, poultry byproduct meal, soybean meal, corn gluten meal, blood meal, feather meal and meat and bone meal. In the case of starter feeds, soy protein concentrate or wheat gluten meal is sometimes used, but other plant proteins are generally not included. Krill meal is also sometimes used if it can be found. Lipids used in trout feeds include fish oil, soy oil and canola oil. A source of starch must be included in extruded trout feeds to act as a nutritional binder. Ground, whole wheat, wheat starch and corn starch are examples of ingredients used in feeds. Vitamin and mineral premixes are also added (Tables 6 and 7). If pigmented trout are being produced, astaxanthin is also included.
Sufficient information has been obtained by researchers throughout the world to permit feed formulation to be based in part on available nutrient levels in feed ingredients. Hence, most feed producers formulate trout feeds to contain a minimum digestible protein and energy content. Development of models to predict phosphorus availability in formulated feeds is progressing and likely to become a part of feed formulation in the future (Hua and Bureau, 2005).
As trout feeds transition to lower inclusion levels of fishmeal and higher levels of plant protein concentrates, antinutrients in plant proteins are becoming more important considerations. There are many antinutrients in plant proteins (see Francis, Makkar and Becker, 2001 for a review), but many are inactivated by heat associated with extrusion pelleting. Those that remain active after pelleting include glucosinolates in rapeseed/canola protein concentrate, phytic acid, and unknown factors in soybean meal that cause distal enteritis in the intestine of trout when dietary levels of soybean meal exceed 25–30 percent, depending on the variety of soybean and, perhaps, the strain of rainbow trout.
Examples of rainbow trout feed formulations are shown in Table 3.  Feed additives are added to rainbow trout feeds for a variety of reasons, such as adding microbial phytase to release phosphorus from phytic acid in plant protein concentrates or adding astaxanthin to pigment fillets or eggs in the case of broodfish. Other additives are added to certain feed formulations to enhance feed intake, such as adding betaine to feeds containing rapeseed protein concentrate. Mold inhibitors are generally not added to trout feeds because feeds are typically used soon after production, and rainbow trout farming is generally practiced in areas with relatively cool climates. For feeds produced by compression pelleting, lignin sulfonate is added as a feed binder. Bentonite clay has been shown to reduce levels of aflatoxin, a mold toxin found on grains exposed to high moisture before harvest or during storage (Ellis et al., 2000).
In recent years, feed additives designed to enhance immune function have been developed, including nucleotide products, beta-glucans, prebiotics and probiotics. These products are not yet widely used in rainbow trout farming. Recent reviews can be found in Li and Gatlin (2005) and Gatlin (2002). Feeding schedules for rainbow trout vary with fish size (Table 5). First-feeding fry are fed almost continuously, either by hand or by mechanical feeders. However, care must be taken at this stage to maintain water quality and avoid bacterial gill problems that are aggravated by excess feed particles in suspension in fry rearing water. Trout fry should be transitioned to a small particle as soon as possible and feeding frequency reduced to eight times per day. Once fish reach 1–2 g, feeding frequency can be reduced to four - five times per day, moving to three times per day when fish reach ~5 g. Feeding should be to apparent satiation, and fry should be visually examined for fullness. Any sign of thin fish (pin heads) is an indication of insufficient feeding frequency and/or amount.
As a guideline, trout fingerlings can consume approximately 1 percent of body weight at a feeding. Feeding less at a single feeding tends to increase size variation within a group of trout. Feeding frequency should be adjusted such that the total amount of feed trout fry and fingerlings can consume in a day, expressed as a percent of biomass in a tank, is divided into the appropriate number of feedings to approximate 1 percent of body weight per feeding. As trout grow larger, feed is usually delivered by mechanical or demand feeders. With mechanical feeders, the amount delivered can be established to follow published guidelines for a given size range of fish at a given water temperature. Demand feeders typically deliver an appropriate amount of feed to support rapid and efficient trout growth. Charts providing guidelines for feeding schedules are available from feed producers. Water stability of rainbow trout feeds is a minor issue compared to feeds for other farmed species that feed slowly. Feeds are generally consumed quickly by trout with little loss of nutrients from leaching. |
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