The theoretical causes of inbreeding depression are reviewed. Studies to measure depression at a series of inbreeding levels in rainbow trout populations using the method of inbred-outbred half-sib families are discussed. While the actual depression estimates varied widely between populations and inbreeding levels, significant levels of depression were found in many hatchery, field, and brood stock performance traits after only one generation of brother-sister mating. Especially susceptible to inbreeding depression, were measures of growth based upon attained fish weight at a given age in each test situation examined: 147 day weight (0.0 to 19.1%), 364 day weight (6.2 to 62.8%), weight after 6 months in fishing pond (−4.6 to 25.4%), weight after 12 months in fishing pond (13.0 to 29.1%), 2-year male weight (11.3 to 55.3%), and 2-year female weight (10.2 to 57.0%). Other hatchery performance traits that showed inbreeding depression were: egg hatch-ability (−0.2 to 53.1%), fry survival (0.4 to 8.2%), feed conversion efficiency (5.0 to 9.0%), fish length at 2-years of age (4.7 to 36.9%), and egg mass produced at 2-years (12.1 to 57.0%). Field performance traits that yielded inbreeding depression were percent recovery of fish stocked (−2.5 to 41.1%) and biomass index (16.2 to 47.7%).

Literature on the effects of inbreeding in fish populations is reviewed and the breeding approaches for controlling the rate of inbreeding accumulation in brood stock populations are outlined and discussed.