WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2023 57 FIGURE 4. Natural inland freshwater spawning and rearing habitat of Pacific Salmon. in sockeye salmon and steelhead trout populations having both anadromous and residual life history types. The anadromous go to sea for half or more of their life, while the residuals remain in freshwater for their full life cycle. The opposing strategies are kept in balance with residual life histories being favored in years of poor migratory conditions and the anadromous strategy favored in years of good migratory conditions. Retention of existing polymorphic traits within local populations is crucial to enable populations to adapt to environmental change. Beyond local adaptations, natural selection shapes many reproductive traits of Pacific Salmon. Natural and sexual selection favors female nest site location, substrate preference, nest construction, nest defense, egg size and fecundity traits providing the greatest offspring survival. Male traits favored by natural and sexual selection include intrasexual competitive strategies, secondary sexual characteristics, and milt production resulting in the greatest contribution to the next generation. Natural and Artificial Considerations during Captive Rearing Populations taken into aquaculture are traditionally reared in very different environmental conditions than they were sourced from (Figures 4 and 5). Density, metabolic waste concentrations, water chemistry, temperature profiles, substrate types, light levels, feeding conditions (food shape, size, motion, chemistry, hardness, presentation manner and feeding schedule), current velocity, water depth, structural complexity and numerous other factors typically differ from the population’s natural environment. Although aquaculture provides higher survival than nature, selection still acts to drive fitness by eliminating traits unsuited to artificial rearing conditions (jumping against screens or out of rearing vessels) and allowing for retention of traits naturally selected against in nature. Selection also alters within-population trait frequency through post-release effects on survival and reproductive capability. As an example, fish with traits selected against in nature that permit faster growth during culture will be larger at release and less prone to starvation and predation. As a result, more fish with this faster growth trait will return to the hatchery and increase the trait’s proportion in the next generation (Blouin et al. 2021). Artificial spawning has the potential to alter the natural frequency of primary-to-secondary sexual traits within a population. Artificially spawned females genetically programed to use more of their energetic reserves for egg production will contribute more offspring to the next generation than those females with traits favoring greater use of their energetic reserves for nest defense. This is confirmed by the observation that populations artificially spawned for multiple generations have less distinct secondary sexual characteristics and greater egg biomass than their source natural populations (Fleming and Gross 1989, Reich 2023). In years of high returns, the unrepresentative filling of egg takes with fish from the earlier portion of the spawning population has resulted in shifting spawning times a month or more earlier than those of the wild source populations (Figure 6) (Quinn et al. 2002, Tilloston et al. 2019). Techniques for retaining the natural balance of polymorphic traits in artificial production programs are available and often put into practice. Salmon mitigation and enhancement operations provide some natural selection by sending fish to sea to compete for food, avoid predators and successfully navigate to, through, and back from their natural ocean habitat. An inland New York State Atlantic salmon (Salmon salar) program at Adirondack Fish Hatchery used a modification of this approach to provide some natural selection by releasing juveniles into an enclosed lake and then recapturing them as maturing adults for use as broodstock. The on-site incubation and emerging fry release utilized by chum salmon programs maintains local natural temperature profiles and provides many of the selective rigors of natural living. The spawning channels (West and Mason 1987) used for Canadian sockeye salmon production go further by allowing both natural and sexual selection pressure to operate in the maintenance of locally adapted traits. Integrated hatchery programs refresh naturally selected traits in the captive population by incorporating wild broodstock into the hatchery’s spawning population. Integrated programs also try to maintain naturally and sexually selected traits within the source (CONTINUED ON PAGE 58) FIGURE 5. Typical mitigation and enhancement hatchery rearing environment for Pacific Salmon during the freshwater swimup fry to smolt (seaward migrating) stage.
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