This chapter focuses on the respiratory systems and metabolic rates in fishes. Fish living in the cold waters found at the poles are exposed to extreme temperatures that have important biochemical and physiological consequences. Because of the effects of temperature on the rate of biochemical reactions, fish exposed to cold water have reduced the metabolic rates. It is well known that the metabolic rate of fish decreases with temperature. Based on early experiments on goldfish acutely exposed to cold, the concept of metabolic cold adaptation (MCA) was born. This chapter also describes that metabolic rate is influenced by a wide range of factors (temperature, body mass, activity level, feeding state, etc.); therefore, there has been quite a lot of discussion on the best way to determine the standard metabolic rate correctly.