Inbreeding Depression

Inbreeding depression is the loss of fitness, growth, survival, and reproductive performance that occurs when related fish are mated and offspring become homozygous for deleterious recessive alleles. In aquaculture it accumulates quietly because a handful of broodstock pairs can produce thousands of offspring that seed entire farms, and in closed hatchery populations the rate of inbreeding rises roughly as 1/(2Ne) per generation, where Ne is the effective population size. Measurable effects include 1 to 2% per 10% inbreeding coefficient (F) in body weight, elevated larval malformation rates, reduced sperm quality, and sharply lower disease resistance; salmonid studies show survival penalties of 5 to 15% at F values above 0.10. Practical defenses are deliberate: maintain at least 50 to 100 unrelated broodstock per generation, rotate mating pairs with pedigree or DNA parentage verification, cryopreserve milt from founding males, and monitor realized inbreeding with genomic markers rather than assuming pedigrees are correct. Inbreeding depression is the main reason small, isolated hatchery lines decline after a few generations, and it undermines the gains from any selective breeding program. It is the direct counterweight to family selection intensity and must be managed together with effective population size and heterosis exploitation in hybrid programs.