Effective Population Size (Ne)

Effective population size (Ne) is the number of breeding individuals in a population that actually contribute genes to the next generation, always smaller than the census count of broodstock because of unequal family sizes, skewed sex ratios, and variation in reproductive success. In hatchery practice a tank holding 200 broodfish may have an Ne of only 20 to 50, since a few dominant males and highly fecund females leave disproportionate numbers of offspring. The operational rules follow from Ne: inbreeding rises about 1/(2Ne) per generation and genetic diversity is lost at the same rate, so keeping annual inbreeding below 0.5 to 1% requires an Ne of at least 50, and long-term conservation standards call for Ne above 500. Recommended practices include equalizing family contributions, maintaining a 1:1 to 2:1 female-to-male ratio, cryopreserving milt to extend the contribution of valuable males, and using DNA parentage to verify that assumed matings actually occurred. Ne is the central planning number for any selective breeding program: it sets how hard you can select, how many families you can afford to keep, and how quickly inbreeding depression becomes a problem. It connects directly to family selection design, genomic selection intensity, and broodstock management policy.