More specifically, the amount of food that a young fish can consume during the preceding summer. Because the young trout and salmon do little feeding during winter, but they live off their fat reserves that they accumulated during earlier, good feeding periods. The fat reserves provide fuel to run the body engine through the lean winter period. Those with bigger tanks can go farther and last longer. Activities are reduced because the water temperature is low.
Territoriality is broken down and schooling is more common.
Metabolism is slower, but the fish still need energy to maintain their basic needs for living such as swimming and breathing. The food supply is also diminished and there is little feeding activity. They need to feed as heavily as possible when the food is available. The fish that live in the best summer habitat will be able to get the most food with the least effort and grow fat to fill their tank. Summer feeding and, in particular, summer fat deposition is one of the most important factors that will insure survival through winter.