Frequently asked questions (FAQs)

1. What is a typical, suitable proximate analysis for the diet for rainbow trout?

2. What is a typical feed formulation for rainbow trout diets?

3. What plant protein sources and typical inclusion rates can I consider for rainbow trout feeds?

There are still unknown factors that limit the amount of plant feedstuffs that can be used in diets of carnivorous species like rainbow trout. Plant-derived proteins such as soybean meal and corn gluten meal are used to supply dietary protein in low-fishmeal feeds (refer to question 2 above). In the case of starter feeds, soy protein concentrate or wheat gluten meal is sometimes used, but other plant proteins are generally not included.
Plant sources that have been tested successfully in rainbow trout diets include cottonseed meal (solvent extracted, dehulled) (<31 percent), soybean meal (<31 percent), full fat soybean meal (<73 percent), soy protein concentrate (<31 percent), corn gluten meal (<5 percent), wheat gluten meal (<21 percent), pea meal (<30 percent), rice protein concentrate, barley protein concentrate, canola  (rapeseed) meal (<15 percent) and flaxseed (linseed) meal (<12 percent).
Rice bran, wheat middling, wheat millrun (<35 percent), barley, whole wheat (<35 percent), wheat flour (<35 percent), and whole corn all have protein levels of below 20 percent and are more important as carbohydrate sources.

4. What oils are suitable for inclusion for rainbow trout diets?

Soybean oil (<5 percent) is the oil most commonly added to rainbow trout diets. Soybean oil can replace up to 50 percent of the dietary lipid. Rainbow trout show a general preference for a diet with fish oil over plant oils and with decreasing preference, soya oil, rapeseed oil, sunflower oil and linseed (flaxseed) oil. Vegetable oil mixtures (e.g. [1] 55 percent rapeseed (canola), 30 percent palm oil and 15 percent linseed or [2] 65 percent canola and 35 percent linseed oil) can completely replace fish oil from the beginning of feeding, without major effects on growth, lipid metabolism and the sensory qualities of the flesh. Plant oils affect the fatty acid composition of the fish flesh. Omega-3 polyunsaturated fatty acids add nutritional qualities to the product and this requires fish oil in the diet for a period before harvest.

5. What essential fatty acids are needed for optimal or best growth of rainbow trout?

Linolenic acid is superior to linoleic acid in stimulating growth and improving feed conversion in rainbow trout. The requirement for omega 3 fatty acids for rainbow trout is 1 percent of the diet. The essential fatty acid (EFA) requirement can be met by one percent 18:3w3. Either 20:5w3 or 22:6w3 is superior to 18:3w3 in an EFA value for rainbow trout, and the former two fatty acids in combination are superior to either alone.

6. What antioxidants can I use in my pelleted feed?

Commonly used antioxidants included in aquaculture feeds are BHA (Butylated hydroxyanisole) at 200 ppm, BHT (Butylated hydroxytoluene) at 200 ppm or Ethoxyquin (1,2 dihydro-6-ethoxy-2,2,4- trimethy quinoline) at 150 ppm.

7. What size of feed do I need for the different sizes of fish from first-feeding fry onwards and how frequently should I feed my fish?

Feeding may be done by hand or with a mechanical distribution system. Feed should be distributed throughout the pond as homogeneously as possible. In concrete tanks, trout will feed on pellets that fall to the bottom, but in earthen ponds trout are unlikely to pick up pellets from the bottom.

8. Is my farming intensity considered extensive, semi-intensive or intensive?

Rainbow trout farming is, for the most part, intensive. Extensive production occurs in the re-stocking for commercial fisheries and seeding for sport fisheries of lakes, rivers and reservoirs. Rainbow trout can be grown at densities of over 80 kg/m³ with the aid of high rates of water exchange and aeration/oxygenation.
Intensive commercial farms typically operate at maximum stocking density of 40–50 kg/m³.
Semi-intensive, flow through and reuse systems typically operate with densities 10–20 kg/m³.

9. What yields are possible if I only apply fertilizers and do not use supplemental feeds?

Fertilization and supplemental feeding are not common practises in rainbow trout farming. No natural food is available in flow-through trout farming systems. Therefore all essential nutrients, including vitamins, must be provided by the feed.

10. What is the tolerance of rainbow trout to gossypol found in cottonseed oilcake?

The tolerance limit of juvenile rainbow trout to total gossypol is reported to be 165 mg/kg.