Fish resources in natural water bodies are tending to decrease due to intensified fishing, the extensive construction of hydropower plants on rivers, and the pollution of seas and freshwater basins by indus trial and agricultural wastes. Nowadays only artificial fish rearing can meet man's requirements in fish products. Fish breeding is still very young as compared to plant breeding and animal husbandry. Although fishes have been reared artificially since ancient times in certain Asian countries, this usually included the cultivation of embryos and larvae caught in rivers and lakes. Among the exceptions, only the common carp Cyprinus carpio and the domesticated variety of the crucian carp, the goldfish Carassius auratus, which were cultivated in the East, may be mentioned. Com mon carp breeding began in China about 2000 years ago but was la ter banned by one of the emperors and started again only relatively recently. The goldfish has been cultivated for decorative purposes for about 1000 years. Many remarkable varieties of the goldfish have been developed in China and later in Japan. The first improved breeds (German "races") of the common carp known in Europe appeared after the domestication of the Da nube wild carp in the seventeenth and eighteenth centuries. Local breeds of the carp were probably established somewhat later in Chi na, Japan and Indonesia; even now these breeds have only minor differences as compared to their ancestor, the Asian wild carp.

Fish resources in natural water bodies are tending to decrease due to intensified fishing, the extensive construction of hydropower plants on rivers, and the pollution of seas and freshwater basins by indus trial and agricultural wastes. Nowadays only artificial fish rearing can meet man's requirements in fish products. Fish breeding is still very young as compared to plant breeding and animal husbandry. Although fishes have been reared artificially since ancient times in certain Asian countries, this usually included the cultivation of embryos and larvae caught in rivers and lakes. Among the exceptions, only the common carp Cyprinus carpio and the domesticated variety of the crucian carp, the goldfish Carassius auratus, which were cultivated in the East, may be mentioned. Com mon carp breeding began in China about 2000 years ago but was la ter banned by one of the emperors and started again only relatively recently. The goldfish has been cultivated for decorative purposes for about 1000 years. Many remarkable varieties of the goldfish have been developed in China and later in Japan. The first improved breeds (German "races") of the common carp known in Europe appeared after the domestication of the Da nube wild carp in the seventeenth and eighteenth centuries. Local breeds of the carp were probably established somewhat later in Chi na, Japan and Indonesia; even now these breeds have only minor differences as compared to their ancestor, the Asian wild carp.

The foundation of quantitative genetics theory was developed during the last century and facilitated many successful breeding programs for cultivated plants and t- restrial livestock. The results have been almost universally impressive, and today nearly all agricultural production utilises genetically improved seed and animals. The aquaculture industry can learn a great deal from these experiences, because the basic theory behind selective breeding is the same for all species. The ?rst published selection experiments in aquaculture started in 1920 s to improve disease resistance in ?sh, but it was not before the 1970 s that the ?rst family based breeding program was initiated for Atlantic salmon in Norway by AKVAFORSK. Unfortunately, the subsequent implementation of selective breeding on a wider scale in aquaculture has been slow, and despite the dramatic gains that have been demonstrated in a number of species, less than 10% of world aquaculture production is currently based on improved stocks. For the long-term sustainability of aquaculture production, there is an urgent need to develop and implement e- cient breeding programs for all species under commercial production. The ability for aquaculture to successfully meet the demands of an ever increasing human p- ulation, will rely on genetically improved stocks that utilise feed, water and land resources in an ef?cient way. Technological advances like genome sequences of aquaculture species, and advanced molecular methods means that there are new and exciting prospects for building on these well-established methods into the future.

Fish resources in natural water bodies are tending to decrease due to intensified fishing, the extensive construction of hydropower plants on rivers, and the pollution of seas and freshwater basins by indus trial and agricultural wastes. Nowadays only artificial fish rearing can meet man's requirements in fish products. Fish breeding is still very young as compared to plant breeding and animal husbandry. Although fishes have been reared artificially since ancient times in certain Asian countries, this usually included the cultivation of embryos and larvae caught in rivers and lakes. Among the exceptions, only the common carp Cyprinus carpio and the domesticated variety of the crucian carp, the goldfish Carassius auratus, which were cultivated in the East, may be mentioned. Com mon carp breeding began in China about 2000 years ago but was la ter banned by one of the emperors and started again only relatively recently. The goldfish has been cultivated for decorative purposes for about 1000 years. Many remarkable varieties of the goldfish have been developed in China and later in Japan. The first improved breeds (German "races") of the common carp known in Europe appeared after the domestication of the Da nube wild carp in the seventeenth and eighteenth centuries. Local breeds of the carp were probably established somewhat later in Chi na, Japan and Indonesia; even now these breeds have only minor differences as compared to their ancestor, the Asian wild carp.

Although aquaculture as a biological production system has a long history, systematic and efficient breeding programs to improve economically important traits in the farmed species have rarely been utilized until recently, except for salmonid species. This means that the majority of aquaculture production (more than 90 %) is based on genetically unimproved stocks. In farm animals the situation is vastly different: practically no terrestrial farm production is based on genetically unimproved and undomesticated populations. This difference between aquaculture and livestock production is in spite of the fact that the basic elements of breeding theory are the same for fish and shellfish as for farm animals. One possible reason for the difference is the complexity of reproductive biology in aquatic species, and special consideration needs to be taken in the design of breeding plans for these species. Since 1971 AKVAFORSK, has continuously carried out large scale breeding research projects with salmonid species, and during the latest 15 years also with a number of fresh water and marine species. Results from this work and the results from other institutions around the world have brought forward considerable knowledge, which make the development of efficient breeding programs feasible. The genetic improvement obtained in selection programs for fish and shellfish is remarkable and much higher than what has been achieved in terrestrial farm animals.

This volume, the proceedings of the Fourth International Symposium on Genetics in Aquaculture, builds on the foundations laid down at the first symposium, held in Galway, Ireland in 1982 (Aquaculture, Volume 33), as well as those laid down at the second, held in Davis, California, USA in 1985 (Genetics in Aquaculture II), and the third, held in Trondheim, Norway in 1988 (Genetics in Aquaculture III). It addresses specific problems and developments in this field, demonstrating the tremendous breadth of research activity as well as the complexity of issues in aquaculture genetics. This book will be of great value to aquaculturists, fisheries scientists and marine biologists.(The above-mentioned proceedings of the first three conferences are still available).