Conservation Of Genetic Variation In Harvested Salmon Populations

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Management of the sub-structured salmonid species may benefit by considering the genetic structure of the species. In the Atlantic salmon (Salmo salar), genetic studies show that populations are genetically distinct, but also that they do not exist in isolation. In this paper, we look at genetic management in a group of populations that are harvested together in the ocean, and separately in fresh water. The scientific problem can be formulated as to combine goals of optimal harvesting and effective population size in a group of populations interconnected by migration. By developing a model that maximises harvesting yield of a group of populations, subject to constraints set by maintaining the total effective size, Tufto & Hindar (2003, J. Theor. Biol. 222:273-81) showed that: (1) Considerable gain can be made in total effective size in a group of populations through harvesting based on knowledge about genetic structure, (2) in source-sink population systems, the total effective size can be increased without reducing total yield by first reducing the harvest in the smallest (sink) populations, (3) when populations differ in their degree of isolation, it pays to harvest relatively less in isolated populations, and (4) in cases with directionality in the migration pattern, the total effective size can become less than the sum of the subpopulation sizes. We apply the model to a set of ten salmon populations in western Norway, which is dominated numerically by the River Lærdalselva population. We show that the total effective size of this population system is to a large extent dependent on the effective size of the Lærdalselva population. On the other hand, the smaller populations contribute more to the total per fish, than what the Lærdalselva population does. We discuss the results in light of conservation genetic theory and empirical results on the fitness consequences of loss of genetic variation in salmonids, and conclude that the genetic consequences of harvesting need to be assessed both at the level of local (sub)populations and at the level of the total population.