The protein source affects growth and body composition of fish. Fishmeal is an important source of protein in the diets of carnivorous fish species because of its high protein quality and palatability. However, fishmeal is quite expensive and can substantially increase feed costs. Total or partial substitution of fishmeal with less expensive plant and animal protein feedstuffs may help to reduce feed costs, although these sources may be lower in digestibility, palatability and essential amino acids (Robinson and Li 1996).
Fish processing creates a large amount of waste of high nutrient content which, if not properly processed for use in human or animal nutrition, is likely to be deposited in the environment creating pollution problems. By-products from rainbow trout processing for smoking, consisting of heads, bones, tails and intestines, were used as ingredients for gilthead bream diets. The growth and body composition data from this preliminary experiment indicated that trout waste could be used successfully as an ingredient of sea bream diets (Kotzamanis et al. 2001).