Stress can be described as the physiological response to a stressor.
In other words, stress is an internal physiological state that is caused by external conditions.
Stress can also be described as an internal hormonal response of a living organism caused by environmental or other external factors that moves that organism out of its normal physiological resting state, or homeostasis (Selye, 1973).
Stress can disturb the normal physiological equilibrium or homeostasis of fish by forcing a reallocation of energy within its system.