The importance of identifying spatial population structure in restocking and stock enhancement programmes

Most animal species show detectable genetic differentiation between populations, but the extent and pattern of this differentiation varies considerably between species. Some show gradual differentiation due to isolation by distance, some show chaotic patchiness, and some show relative uniformity over large distances with striking discontinuities over short distances. These varying patterns reflect both the dispersal powers of the organism and its population history. The evolution of locally adapted genotypes is facilitated in populations with restricted gene flow, and such co-adapted genotypes may then vary from population to population depending on local selective forces. Restocking and stock enhancement programmes need to be aware of the stock structure of the target species, as the introduction of genotypes unrepresentative of the augmented population can have negative effects. Swamping the native population with large numbers of genotypes from a few matings, even if derived from the native population, can also be detrimental. It follows that, wherever possible, restocking and stock enhancement programmes should use broodstock taken directly from the population to be enhanced, and that large numbers of broodstock should be used. If broodstock cannot be taken from the population to be enhanced, they should be taken from the genetically most similar population available. Restocking and stock enhancement programmes should be genetically monitored to determine their impacts and outcomes.