ABSTRACT
This chapter presents the physiological responses of fish to hypoxia or anoxia. Hypoxia refers to any condition in which the amount of oxygen is measurably below air saturation levels. The chapter provides the process of respiratory gas exchange and transport during hypoxia to the biochemical changes involved in anaerobic metabolism during anoxia. There are several potential causes of environmental hypoxia, some of which are not due to human activities. Some species of freshwater fish are extremely resistant to low levels of oxygen, or even anoxia. Within the gill, if transport of oxygen by the blood is compromised by some chemicals such as nitrate, there is a reduced ability to tolerate hypoxia. The main reason for adjusting the various processes involved in taking up oxygen from the water is to increase respiratory independence (or regulation). Acclimation to hypoxia would seem to be a very important ability for those fish that experience chronic conditions of low oxygen.
