In recent years, progress in fish biology has advanced at an unprecedented rate and has led to many breakthroughs in the field. This book provides a wealth of information on the strategies that fish adopt with respect to waters with markedly different physical and chemical characteristics. It shows how their physiology, behaviour and lifestyles are adapted to exploit particular niches and gives comprehensive insight into fish life under extreme conditions. The readers are introduced to the ways in which fish exemplify many phenomena of general biological interest - the existence of competitors, chaos, and predator-prey interaction. Fish pathology as well as the components of the immune system are addressed. In this book, original and at times controversial views are presented, areas which have so far received inadequate attention are highlighted and avenues for further research are suggested.