Most rely on the water to regulate their body temperature. Water enables them to move and migrate. Water is also very important for spawning and successful reproduction. Let's consider each of these points. Fish need water to breathe.
Salmonids are very active fish and they require lots of oxygen.
They need water that is nearly saturated with oxygen to survive. Oxygen levels also vary from place to place and time to time. The effects of dissolved oxygen levels on fish are related to the effects of temperature. This is because cold water can hold more oxygen than warm water. Fish have a lower metabolic rate in very cold water and, in general, need less oxygen than in warmer water.