The report reviews arguments for the preservation of the genetic resources of fish and various strategies for accomplishing this in both marine and continental waters. It reviews the significance of genetic diversity, especially within populations and subpopulations, to the viability of aquatic ecosystems and to the fitness of fish populations, and discuss the various means by which genetic impoverishment comes about. The report emphasizes the importance of maintaining breeding populations of an effective size of at least 50 for short-term fitness and of at least 500 for long-term survival; and of avoiding "genetic bottlenecks" created by reduction of breeding populations to small size for one or more generations.