One objective in most aquaculture breeding programs is to maintain as much genetic variation as possible from one generation to the next by avoiding mating of closely related individuals. This is especially true for conservation aquaculture, when wild populations are to be enhanced with hatchery reared animals. However, no mating system can actually prevent the accumulation of inbreeding in a closed population over an extended number of generations . As the quantitative geneticists Kimura and Crow stated in their 1963 article On the maximum avoidance of inbreeding, “a system that avoids mating of relatives for as long as possible does so at the expense of a more rapid final approach to homozygosis.” However, some strategies are available to minimize loss of genetic variation over time . W hen they are applied correctly, the result is a mathematical increase in the effective number of reproducing individuals.
Most of the mating systems that have been developed for minimizing inbreeding in small populations involve subdividing the population into a number of groups, and then maintaining distinct breeding groups in subsequent generations . The simplest of these systems is referred to as Circular Mating , wherein individuals within each group are allowed to reproduce, and in the subsequent generation the male offspring from each group are transferred to mate with female offspring from an ‘adjacent’ group . When broodstock numbers are low, inbreeding accumulation rates are somewhat high in earlier generations but they tend to level off over time and actually remain at levels somewhat lower than those attained by other mating systems.
Cyclical mating systems are superior for minimizing inbreeding in earlier generations. They also rely on exchanging males between groups but the exchange patterns vary, cyclically, over generations . In cyclical mating systems, inbreeding levels among groups can bounce up and down from generation to generation, but the long-term trend still results in accumulation .