High mortality of rainbow trout ( Salmo gairdneri) in Yugoslavian fish farms caused by inadequate feed quality

In 1984, 70–80% of the fry died on many Yugoslavian rainbow trout farms. Significant mortality was observed in the second or third week after first feeding. Virological, bacteriological and parasitological examinations of the fry revealed no infectious disease. Physiochemical analyses showed that the water quality could not have caused the fry mortality. Samples of the imported dry diet were analysed for proximate composition, vitamin content, amino acid composition, peroxide value, acid value and histamine content. Proximate composition and vitamin content were normal while the amino acid composition indicated that the isoleucine: leucine ratio was higher than normal and that the valine content was high. Peroxide values were acceptable, acid value was very high, while very high histamine values indicated that the protein used in the diet had been partly decomposed. Microscopic analyses showed that the feed contained 10% wheat, 15–20% fish meal, 10% blood meal and 60–65% meat and bone meal. When the feed was replaced by a dry diet obtained from a different manufacturer some fish recovered (15–20%), but not without lasting consequences for later growth.