Freshwater eels are almost infinitely improbable creatures. They spawn and die in the middle of the ocean, often associated with undersea mountains. Their tra- parent, leaf-like larvae move with ocean currents for months or years until they approach the mouths of freshwater rivers. Then they undergo a dramatic transf- mation in morphology, physiology and behavior. They move from their planktonic oceanic environment, migrate upstream and live for several years as apex fre- water predators. Then, almost impossibly, as they become sexually mature, they reverse their migration downstream to the ocean and back to spawning grounds to complete their life cycle. The dramatic changes in their life cycles are incredible. The efforts to unravel the details of their life history have been truly daunting. Much of the past research was the work of dedicated individuals who devoted their lifetime research to these fishes. Freshwater eels merit a separate chapter in almost any textbook dealing with ichthyology, marine biology or animal migration. We know a great deal about some aspects of the biology of freshwater eels. However, our understanding of their bi- ogy still resembles a work of art as much as a work of science. To some it appears like the sweeping brush strokes of a Japanese Zen landscape, to others it resembles the work of a French impressionist, and to still others it appears as magic realism.
The European eel Anguilla anguilla is a common fish species in West European countries supporting several thousands of small-scale fisheries. In June 2007, it has been listed in Annex B of CITES after the 99% decline in recruitment observed over the last decades. A European Council Regulation was issued on September 2007 to arrest the collapse of the eel stock. In this book, we discuss the factors that may contribute to the collapse of the European eel population. With respect to conservation measures two important aspects have to be considered: quality and quantity of escaping silver eels. Clearly, a poor condition of the silver eels can never be compensated by large numbers. Qualitative parameters were thus far hardly available for management applications. The main goal of this book is to provide useful quality parameters for migrating silver eels, indicating their contribution to recruitment. The process of migration and maturation also provides information about the natural conditions for eel reproduction. Therefore, this book will also be very useful for eel aquaculture; it may provide tools for reproduction as well as for suppression of precocious maturation. An integrative approach regarding eel maturation requires a combination of ecology and physiology. In this book we included both aspects, and we hope that this will contribute to the restoration of the European eel population.
