Marine ranching offers an interesting solution to obtaining more animal protein from the aquatic environment. This involves the use of hatchery management to support, augment, or create new fisheries. Although subtly different in meaning, the terms "marine ranching", "hatchery enhancement","culture-based fisheries" are synonymous for the purpose of this paper.

Three main criticisms have been levelled at the use of ranched fish to replenish natural populations: lack of evidence that such enhancement benefitted the fishery; the high cost of maintaining the hatcheries; and the fear of losing natural genetic diversity through interaction of hatchery fish with native populations. Moreover, ranching has not had a particularly bright history. The continuing decline of cod stocks despite the release of billions of Atlantic cod into the Atlantic from both North America and Norway provides an example. Reasons for this lack of success include the following: the decline of the fishery was inadequately addressed and a rather simplistic approach to enhancement was adopted - dumping millions of larvae into appropriate water bodies and hoping; most programmes were not monitored and hatchery fish were not tagged, thereby making it difficult to differentiate between hatchery and native fish.

Nevertheless, despite fears of loss of genetic diversity and other harmful effects through interaction of hatchery and native fish, the goal of many enhancement programmes is for the wild fish to mix with hatchery fish and form a new and larger mixed population. Therefore, an important option for certain enhancement programmes and for certain aquatic species will be to try and create fish in the hatchery that are compatible with wild stocks and the environment. To achieve this, the hatchery enhancement programme must have clearly defined objectives and goals. A major objective should be to conserve genetic resources in wild and cultured stocks. Given that most of these resources reside in wild populations and that fisheries make up the largest single source of animal protein, every effort should be made to protect the wild resources.

This paper aims at outlining general principles and procedures that should be incorporated into a hatchery enhancement or ranching programme to conserve and utilize genetic resources (biodiversity). Items addressed include the understanding of natural levels and organization of genetic diversity, the objective of ranching programmes, the understanding of the reasons for decline in original fishery, special considerations for the establishment of a new species, assessment of hatchery technology and husbandry, assessment of ecological requirements of ranched species, and the monitoring of both the hatchery and wild populations.

The paper concludes that aquatic ranching is a complicated area involving technology, biology, ecology, socio-economics and politics.

A recent meeting of experts from around the world established a "responsible approach" to hatchery enhancement that involves multi- disciplinary assessment, monitoring and action. Although genetic considerations alone will not ensure a successful ranching programme, the long-term success of production and conservation efforts will be impossible without applying the principles of genetic resource management.