Female rainbow trout (Oncorhynchus mykiss) from a group that have been selectively bred for resistance to bacterial cold-water disease (CWD). CWD is caused by the bacteria Flavobacterium psychrophilum. It causes tissue erosion and ulceration, and can be fatal. This fish is from a line known as ARS-Fp-R, which has been bred for a heritable resistance to this disease. Selective breeding in this manner is common in farming, where diseases tend to spread rapidly due to crowded conditions.