Body Composition of Brown Trout (Salmo trutta L.) in Relation to Temperature and Ration Size

Abstract

Analyses were made of the body composition of brown trout (live wt 9-314 g) fed on various ration sizes ranging from zero to maximum rations at 9 different water temperatures (range 3.8-21.7.degree. C). Ash content remained fairly constant in the feeding experiments and can be estimated from the wet weight of the trout. The per cent water content decreased and the per cent fat, per cent protein and energy values all increased with increasing ration size and usually with increasing body weight. Temperature affected the rates of change in body constituents in relation to ration size and time. The rates were very low below 6.degree. C, increased progressively above 6.degree. C with maximum values at about 13.degree. C, remained high for trout on zero rations in the range 13-20.degree. C but decreased to very low values for trout on maximum rations. They were also high above 20.degree. C but changes associated with zero rations also occurred in trout on maximum rations. Equations were developed to describe the relationship between water content and the other body constituents, and the relationship between body constituents and both the weight and condition of the trout. These equations were applicable to both wild and hatchery-reared trout feeding on a variety of food organisms in both the stream and the laboratory. If only water content is determined, the equations provide satisfactory estimates (with 95% confidence limits) of fat content, protein content and energy values. If it is impossible to determine the water content, then the body constituents can be estimated from the weight and condition of the trout, but only with wide confidence limits.