Effect of hydrolytically changed and oxidized fat in dry pellets on the health of rainbow trout, Oncorhynchus mykiss (Richardson)

Abstract. (1+) rainbow trout, Oncorhynchus mykiss (Richardson), were reared by two methods (flow-through tanks at an average water temperature of 9°C; floating cages at a water temperature of 17°C) under different hydrological conditions for 120 days to study the effect of the presence of oxidized and hydrolytically altered fats in dry pellets on the red blood count and on the liver somatic index (LSI) and on the spleen somatic index (SSI). The unfavourable effect of hydrolytical decomposition and oxidization of fat on the blood indices was demonstrated by correlation and regression analyses. A high linear (r = -0.852) and parabolic (I = 0.890) dependence of the peroxide value (PV), the 2-thiobarbituric acid number (TBAn) and acid value (AV) on the red blood cell count (RBCc), haematocrit (PCV) and haemoglobin (Hb) was found on the basis of these analyses. This effect was most marked at a low water temperature when it was diagnostically important to respect seriously the decrease in RBCc to 0.78T/l, in PCV to 0.291/l and in Hb to 52g/l. In the cages the changes in the haemogram manifested themselves only in the fish given heavily oxidized fats in the feed (above 0.50% iodine, i.e. 43 mEq/kg oil): their Hb decreased from 83 to 69g/l and PCV from 0.44 to 0.381/l. Rainbow trout reacted to the presence of oxidized fats in feeds by increasing their LSI (2.31 vs 1.73%) and SSI (0.30 vs 0.23%). It is important for lipoid liver degeneration (LLD) prevention to determine the ALT and AST aminotransferase activitics which increased to an almost 4-fold level in the diseased fish (11.7 and 0.96 μcat/l).