Sperm Competition in Salmon Hatcheries: The Need to Institutionalize Genetically Benign Spawning Protocols

Abstract

Salmon hatcheries in the Pacific Northwest historically spawned adultsby combining eggs from several females and milt from several males in asingle container. This mixed-milt approach leads to significant spermcompetition and highly unequal genetic contributions from male spawners.Sperm competition substantially reduces the genetic effective number ofbreeders (N-b) relative to the actual number of spawners (N-x). Spermcompetition in vitro can also result in undesirable artificial selectionfor life history traits (e.g., age or size at maturity) that arecorrelated 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 usemixed-milt fertilization despite documented genetic effects andpotential risks. The continued use of mixed-milt fertilization may bedue, in part, to insufficient genetic oversight of hatchery operations.As a general rule, salmon hatcheries should discontinue mixed-miltfertilization and institutionalize alternative spawning protocols thatpreclude or minimize sperm competition in vitro. Three alternativeprotocols are described here: pairwise spawning, nested spawning, andfactorial or matrix spawning. The underlying premise of thesealternative protocols is that every adult selected for use as broodstockshould have an equal opportunity-and an equal probability-of producingprogeny. A goal of most hatchery programs should be to minimize geneticchange between the pool of returning adults available for broodstockeach year and the progeny of parents selected as broodstock from thatpool. To achieve this goal, spawning protocols should maximize N-b andminimize artificial selection associated with hatchery propagation. Suchgoals may require increased genetic oversight of hatchery operationscomparable to the level of fish health oversight (pathogen monitoringand disease prevention) currently practiced in salmon hatcheriesthroughout the Pacific Northwest.