Sperm competition in salmon hatcheries: The need to institutionalize genetically benign spawning protocols

Salmon hatcheries in the Pacific Northwest historically spawned adults by combining eggs from several females and milt from several males in a single container. This mixed-milt approach leads to significant sperm competition and highly unequal genetic contributions from male spawners. Sperm competition substantially reduces the genetic effective number of breeders (N-b) relative to the actual number of spawners (N-x). Sperm competition in vitro can also result in undesirable artificial selection for life history traits (e.g., age or size at maturity) that are correlated phenotypically with sperm potency and fertilization success. A large number of salmon hatcheries in the Pacific Northwest, particularly those within some state agencies, continue to use mixed-milt fertilization despite documented genetic effects and potential risks. The continued use of mixed-milt fertilization may be due, in part, to insufficient genetic oversight of hatchery operations. As a general rule, salmon hatcheries should discontinue mixed-milt fertilization and institutionalize alternative spawning protocols that preclude or minimize sperm competition in vitro. Three alternative protocols are described here: pairwise spawning, nested spawning, and factorial or matrix spawning. The underlying premise of these alternative protocols is that every adult selected for use as broodstock should have an equal opportunity-and an equal probability-of producing progeny. A goal of most hatchery programs should be to minimize genetic change between the pool of returning adults available for broodstock each year and the progeny of parents selected as broodstock from that pool. To achieve this goal, spawning protocols should maximize N-b and minimize artificial selection associated with hatchery propagation. Such goals may require increased genetic oversight of hatchery operations comparable to the level of fish health oversight (pathogen monitoring and disease prevention) currently practiced in salmon hatcheries throughout the Pacific Northwest.