Comparative studies on body composition of rainbow trout (Salmo gairdneri R.) in relation to type of diet and growth rate

Abstract

Three populations of rainbow trout (mean initial live weight 40 g) were fed experimentally under almost the same conditions for 49 wk. Changes in the relative proportions of the major whole body constituents (water, protein, fat and ash) were examined in relation to type and amount of diet and growth rate. Two pelleted dry diets (A and B) and one mixed diet (C) were used. The amount of food given daily to the fish in the case of diets A and B was continuously based on fish body weight and water temperature. The population which received diet C (raw material and pelleted dry food) was fed on maximum ratio. Sampling was carried out approximately every 2 mo. After 25 wk, and besides the regular sampling, fish were sampled from each population and analyses were made on their fillets. The same analyses were carried out on fillets and whole body material on a sample of wild fish, of almost the same age, fed on a variety of food organisms in a stream near the experimental tanks. When results were expressed in terms of dry weight, the analyses of whole body material showed that ash content remained fairly constant in all populations throughout the experimental time. With increasing body weight and age the percentages of water and protein decreased and the percentage fat content increased in all populations and especially in fish fed on diet C which appeared to have the maximum growth rate and final mean body weight. Fish fed diet B showed the lowest growth rate and final mean body weight and had low changes in the major body constituents throughout the experimental period. The lowest and the most regular rate in the changes of these parameters appeared in fish fed on diet A which had a sufficient growth rate and mean final body weight. Analyses of the fillets for the 3 different types of food used yielded different results. The analyzed wild fish had the highest percentage of water content and, expressing the results in terms of dry weight, the highest percentage of ash and protein content, while they appeared to have the lowest percentage of fat content, on both whole body material and fillets. Since the 3 types of diets used gave 3 different growth rates, and 3 different body compositions of the reared trout, relative proportions and changes of the 4 body constituents and effects on the growth rate of trout may be strongly affected by the type of food.