In laboratory research, the rainbow trout has become a counterpart to the white rat, because that fish is an adaptable species available in much of the developed world and stocks from egg through adult are available throughout the year. Moreover, many strains are recognized, and their propagation and laboratory maintenance are not particularly demanding. Also, knowledge of rainbow trout nutrition, husbandry, diseases, immune responses, toxicology, and carcinogenesis exceeds that of any other salmonid or coldwater teleost. The rainbow trout is the logical surrogate species in many studies of other salmonids.