Abstract
The conservation status of freshwater fish is of concern throughout the world, a third of all known species having become extinct or being endangered. Many of them have fragmented populations of small size whose future is in doubt. Conservation biology is the discipline that is used to study such populations, so that they can be managed in a way that will ensure their long-term survival. The history, development and future of conservation biology are described. Following repeated restocking of brown trout (Salmo trutta) dating from 1906, the marble trout (Salmo marmoratus) seems to have disappeared as a result of genetic pollution (hybridisation) from the lower reaches of the Soca River in Slovenia. A project for rehabilitating the marble trout has been undertaken and will serve as an example to illustrate an application of conservation biology to freshwater fish. A preliminary exploratory stage (1993-1995) that resulted in the publication in 1996 of an Action Plan, written in English and Slovenian, suggested that the cause of the disappearance of the marble trout was hybridisation, established the validity of the project in the region in question and provided us with essential information (occurrence of genetically pure populations) that could be used to define our future strategy. We chose genetic rehabilitation, i.e., the replacement of a population of introduced and introgressed fish by a population of genetically pure fish of the native species, rather than a programme of eradicating undesirable fish by chemical methods, which would have been incompatible with the region's context. Our overall strategy therefore had two main aims: to ensure the long-term survival of populations of pure marble trout (species conservation) and to rehabilitate the genes of marble trout in the hybridisation zone until foreign genes have almost been eliminated.