Fish are not completely efficient (FEs of 100 %, FCRs of 1.0). When fed 5 pounds of feed, fish cannot exhibit 5 pounds of growth because they must use some of the energy in feed for metabolic heat, digestive processing, respiration, nerve impulses, salt balance, swimming, and other living activities. Feed conversion ratios will vary among species, sizes and activity levels of fish, environmental parameters and the culture system used.