Rainbow trout - Nutritional deficiencies

Modern commercial trout feeds are unlikely to be outright deficient in essential nutrients. Nevertheless, deficiencies can conceivably occur when mistakes in feed manufacture occur, although this is rare (See Table 8, Table 9 & Table 10). More commonly, when feed formulations are altered by adding or removing feed ingredients conditioned nutritional deficiencies, defined as deficiencies that occur despite seemingly adequate levels of essential nutrients in feeds, can sometimes result. Examples include so-called “screamer disease” in Atlantic salmon (Roberts, Hardy and Sugiura, 2001) and nutritional cataracts reported in Atlantic salmon (Waagbo et al., 2003). An earlier example of this was the widespread incidence of lens cataracts occurring in Pacific salmon hatcheries in the early 1980s when a combination of high-ash fishmeal and plant protein (cottonseed meal providing phytic acid) caused a deficiency in zinc, even though the zinc content of the feed exceeded the dietary requirement. The most common pathologies associated with nutritional deficiencies are cataracts due to zinc deficiency or other nutrient imbalances; skeletal deformities associated with ascorbic acid deficiency, mineral imbalances or inadequate levels of available phosphorus; steatitis (inflammation of fatty tissue) associated with insufficient antioxidant levels in feeds containing oxidizing lipids; and enlarged, abnormal liver associated with excessive available carbohydrate. Other common feed problems are associated with poor growth, reduced feed intake and high feed conversion ratios. All are non-specific signs of problems that are not limited to nutrition. Such problems can have many causes, making identification very difficult.