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Aquaculture Feed and Fertilizer Resources Information System
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Atlantic salmon are cultured in freshwater tanks and raceways in flow-through systems prior to transfer to sea. In the marine environment, they are farmed in sea cages where they depend solely on formulated feeds. The use of fertilizers to increase the natural productivity of aquatic organisms is therefore not necessary either in hatcheries or sea cages. Supplemental feeds and feeding is not practiced for the hatchery and cage culture of Atlantic salmon. Atlantic salmon feeds formulated for various stages of development and production cycle in freshwater and seawater are broadly classified as freshwater (starter, grower, smolt transfer), seawater grower and broodstock feeds (Tables 3, 6 and 7). Freshwater feeds contain 45–54 percent protein and 16–24 percent lipid. The protein content is decreased after salmon fry reach fingerling size. Feed manufacturers use seawater transfer feeds for salmon going through parr-smolt transformation. These diets contain salt, betaine, amino acids, nucleotides and other supplements to improve the osmotic adaptation of smolts to seawater and for better survival. Smolts are fed marine grower feeds after the seawater acclimation is complete. The protein content is reduced from 45–48 percent to 36–42 percent and lipid content increased from 24 to 30–40 percent during their seawater grow-out phase to market-size salmon (~ 4 kg). Most feeds used are highly digestible and the fines are negligible, which allows minimum impact of aquaculture feeds on the environment.
Starter feeds are made by crumbling extruded pellets, but agglomeration technology is now widely used to produce modern starter feeds for salmon and marine fish. Most salmon feeds are manufactured by extrusion technology to produce slow-sinking pellets. A vacuum infusion coating process allows fat to penetrate the pellet, and this has led to a new generation of salmon feeds containing high amounts of lipid ranging from 18 to 40 percent. Certain heat unstable nutrients such as ascorbic acid, astaxanthin, feed attractants and other additives are also externally coated. The diameter of pellets can vary from 1 to 11 mm or larger. Feeds are delivered to marine cage sites packed in large bags (500 kg or larger) by boat. Salmon hatchery feeds are packed in 25 kg bags (Figure 6). Under feed regulations, each bag must have a label that provides information on proximate composition, certain nutrients (e.g. phosphorus) and feed additives, as well as the feed ingredients used for feed formulation (Figure 7). A wide range of ingredients is used in the formulation of Atlantic salmon feeds (Tables 4 and 5). They are selected on the basis of available energy content and nutrient composition as determined through chemical analyses. They are broadly classified as to the source of protein (amino acid), energy, essential fatty acids, vitamins and minerals. Several protein supplements such as high-quality fishmeal, plant protein products (soybean meal, corn gluten meal, canola meal, pea meal), animal by-product meal (poultry by-product meal, meat meal, blood meal, hydrolyzed feather meal) and crustacean meal (krill, shrimp, crab) are used in salmon feed formulation depending upon economics and availability. The nutritional value of common feed ingredients used in feed formulation is provided in Tables 4, 5 and 8. One of the major shifts in the selection of salmon feeds has been from the use of fishmeal and fish oils to terrestrial vegetable oils and plant proteins due to the high demand for marine by-products by the global aquaculture industry (Tacon and Metian, 2008). Several feed additives are added to salmon feeds to enhance growth, flesh pigmentation, physical properties, digestibility, osmoregulation, palatability and preservation of the feed. Several carotenoids, including synthetic astaxanthin and canthaxanthin, and certain natural supplements such as yeast (e.g. Phaffia rhodozyma), algae (e.g. Hematococcus pluvialis) and crustacean products (e.g. krill and shrimp) are used to impart an attractive pink-red colour to the salmon flesh. Enzymes, particularly microbial phytase, increase the bioavailability of phytic acid in oilseed proteins. Certain amino acids, peptides and betaine are added to increase feed intake. Other feed additives include immunostimulants (e.g. b-glucans and nucleotides), prebiotics and probiotics (Gatlin, 2002). Selenium and vitamins C and E in conjunction with other immunostimulants have been also used for disease prevention. Antioxidants such as ethoxyquin are added to fishmeal and fish oil to increase their stability; however, farmed salmon feeds are used within a short period after their manufacture, thus reducing the use of synthetic antioxidants. Natural tocopherols have antioxidant activity, however a-tocoherol acetate supplement in the diet has limited antioxidant activity until hydrolyzed in the digestive tract. Although several binders are available to improve the stability of feed pellets, salmon feed manufacturers rely on gelatinization of starch during extrusion to improve feed stability. After hatching from eggs, swim-up fry (alevins) depend on endogenous nutrients from the yolk sac. The most appropriate time to start feeding small-size particles in the form of granules or crumbles is when this reserve is completely absorbed. At this stage, they are switched to starter feeds and fed frequently by automatic feeders (Figure 8) or belt type clockwork feeders throughout daylight hours or during a set photoperiod regime. Swim-up fry are fed in slight excess so that slow-sinking feed is visible for capture. However, an excessive amount of feed particles suspended in the water column may affect gill respiration and predispose fry to bacterial infections. The frequency of feeding is dependent on the water temperature and body size. Generally, feeding is reduced from 8–12 times or more daily to 3–4 times a day for fingerlings and parr (Table 9). Feed consumption is readily reduced by poor water quality, low water exchange, high density of fry and physiological state of fish.
In sea cages, feeding practice includes hand feeding (Figures 9a and b) and the use of automatic feeders equipped with video monitoring systems (Figure 11). It has been observed that peak feeding time during summer months is in the early morning, followed by another peak 12 hours later (Kadri et al., 1991). In addition to temperature, appetite also depends upon gut fullness and the evacuation rate of food from the gut. Modern demands feeders and feeding schedules developed by feed manufacturers based on fish size, water temperature and energy content of feed provide appropriate directions to achieve maximum growth and feed utilization under diverse environmental and culture conditions (Table 10). |
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