World Aquaculture - December 2023

WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2023 55 coupled with the strong homing ability of Pacific Salmon, restricted gene flow between microhabitat populations setting the stage for local adaptation (Waples et al. 2008). Local adaptation results from natural selection acting on a population’s genetic composition to increase the frequency of traits providing the greatest survival and reproductive advantage in a given habitat. A review and meta-analysis of salmonids found on average local populations had 1.20 times higher fitness than foreign populations (Fraser et al. 2011). Most behavioral, physiological and morphological traits within a population are polymorphic (multiple forms of a trait) that fall along a bell-shaped curve. As Pacific Salmon spawning, swim-up and outmigration timing form a bell-shaped curve they are likely polymorphic traits. Most polymorphic traits are balanced with those traits on the left end of the curve having the best survival in some years, while those traits on the right end of the curve having higher survival in other years. Figure 1 illustrates this potential balanced polymorphism for ocean returning Sockeye Salmon (O. nerka) reentering Columbia River freshwater during May-June-July. In cold water summers natural selection favors late freshwater reentry providing additional ocean feeding time which increases nutritional reserves for upstream migration and reproduction. In hot water summers natural selection favors early freshwater reentry that allows fish to reach their cool headwater spawning grounds prior to the development of lethal temperatures in the freshwater migratory corridor. In other cases, directional selection leads to steady erosion of traits on one end or the other of the curve improving the population’s fitness in a specific local habitat. Over the course of millennia this leads to slow shifting of the bell-shaped curve to the left or right of the original average (Figure 2). This directional selection results in the traits of populations living in different habitats slowly diverging. In extreme cases, anatomical, physiological and behavioral traits providing the greatest survival and reproductive advantage in local habitats may become fixed (monomorphic). Trait fixation decreases within-population genetic diversity and most often occurs in small breeding populations living near the margins of their species range, where directional selection is greatest (Eckert et al. 2008). Over Captive rearing is useful for mitigating habitat loss, enhancing fisheries, rebuilding depleted natural runs and reintroducing fish to restored habitat (Maynard and Trial 2014). Unlike full life cycle commercial fish farming, where domestic selection may be applied to favor traits producing maximum quality and yield at the lowest cost, ranching (mitigation and enhancement) and conservation (recovery and restoration) need to retain traits within the cultured population favoring survival in the natural environment. Programs allowing cultured fish to breed with the wild population (Integrated Hatcheries) or seeking to rebuild or restore natural populations must retain the genetic traits and epigenetic effects (environmental induced effects that can be passed on to following generations) required for successful full life cycle reproduction in the population’s natural environment. Achieving this goal requires avoiding domestication selection as much as possible. Although it is easy to avoid intentional domestication selection by just not doing it, reducing unintentional domestication selection is challenging. It requires an understanding of evolutionary principals, the population’s natural habitat and careful planning to maintain the source population’s adaptive traits during culture. Evolutionary Principles The key evolutionary principals needing consideration will be illustrated with Pacific Salmon (Oncorhynchus sp.) of the Pacific Northwest Coast of the United States of America. Pacific Salmon are anadromous fish, spawning and rearing for a portion of their life cycle in freshwater before migrating to sea where they put on over 95% of their final body weight. This anadromous life history and resulting straying allowed these species to recolonize Pacific Northwest streams, rivers and lakes following glacial retreat beginning around 17,000 years ago. Over the ensuing 2,000 years of glacial retreat new salmon habitat opened up from the mouth of the Columbia River and Puget Sound to the Canadian border. Differences in elevation, distance from the ocean, prevailing weather, soil types, vegetation, competitors, predators, pathogens and other factors created a diverse array of salmon microhabitats within these basins. The island-like nature of these freshwater habitats, Balancing Natural and Artificial Selection in Captive Rearing Programs Desmond J. Maynard (CONTINUED ON PAGE 56) FIGURE 1. Illustration of balanced polymorphism associated with year-toyear environmental variation around a long-term average. In hot water years natural selection favors maturing adults that reenter freshwater prior to the development of lethal migratory corridor temperatures. In cold water years, latter freshwater reentry timing is favored as it provides the opportunity for increased ocean feeding and nutritional reserves for increased migratory stamina and egg production.