56 DECEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG the course of 15,000 years, directional selection resulted in the between-population genetic diversity of Pacific Salmon populations increasing as they gradually adapted to their local freshwater habitats (Figure 3). Body shape, age of maturation, the response of swim-up fry to current, and pathogen resistance all illustrate local adaptation in Pacific Salmon. The morphology of adjacent populations of Sockeye Salmon spawners in Alaska has evolved to fit their spawning habitats. River spawning males have shallow shapes suited to the reduced water depth of their spawning areas, while beach spawning males have deeper shapes which can be accommodated by the greater water depth of their spawning habitat (Oke et al. 2019). The energetically expensive humps and jaws male Chinook salmon (O. tshawytscha) utilize in male-male competition for mates are reduced in populations migrating further inland than in those with lesser migration distances to spawning grounds (Kinnison et al. 2003). These inherited local adaptations evolved over the course of 26 generations. Sockeye salmon spawning in streams above nursery lakes produce fry that swim with the current that carries them downstream into the lake, while those spawning in streams below nursery lakes produce fry that swim against the current so they can move upstream into the lake (Brannon 1972). Fraser River inland populations of Chinook with low outmigration survival to the ocean display greater rates of yearling precocity than coastal populations with higher outmigration survival to the ocean (Taylor 1989). Pacific Northwest steelhead (O. mykiss) from watersheds lacking the pathogen Ceratomyxa shasta have less resistance to the pathogen than those from basins where the pathogen is endemic (Buchanan et al. 1983). Allowing for some within population variation to accommodate year-to-year environmental variability, the evolution of spawn timing and egg development rates in chum (O. gorbuscha) salmon appears to be locally adapted (Beacham and Murray 1987). This local adaptation tracks spring food availability to ensure first feeding fry emerge from their redds (nests) during peak periods of prey availability. Many other adaptations to local environments may exist within Pacific Salmon populations. High altitude populations, with lower dissolved oxygen saturations, probably evolved respiratory mechanisms with better oxygen extraction capability than low elevation populations, as occurs in the Glyptosternoid catfish of Tibet (Ma et al. 2015). Similarly, the more rigorous climb to high altitudes, especially for far inland populations, favors the evolution of physiological and morphological adaptations providing greater migratory endurance than populations in low elevation coastal habitats. Natural selection is expected to favor traits within populations that produce smolt downstream migration timing coinciding with the best mix of migration corridor temperatures, flows, predation pressure and ocean feeding conditions with the greatest survival advantage. With the exception of food availability, local adult return times are expected to evolve similarly. As a population’s risks associated with ocean migration increase, natural selection should favor traits associated with increased precocity, iteroparity, and full life cycle freshwater resident life histories. As mentioned earlier, year-to-year environmental variation results in polymorphic traits within a population being balanced. An example of balanced polymorphism may be displayed by inland populations of Spring Chinook Salmon where some individuals overwinter in cold shallow headwater habitat, while others undergo fall migrations to the deeper and warmer waters of the main channel to avoid winter anchor ice (Hillman et al. 1987). Another potential example is found in coastal Fall Chinook Salmon populations which can have up to five smolt outmigration strategies ranging from 0-age fry to yearling smolts (Reimers 1991). This combination promotes population survival during years when freshwater or ocean mortality is seasonally catastrophic. An extreme example of balanced polymorphism is found FIGURE 2. Hypothetical illustration of how directional selection favoring one morph over another might gradually shift an expressed trait’s average. Climate change driven warm water years select against late freshwater reentry (a) and shift average reentry timing earlier (b). Fisheries favoring harvest of earlier portion of return (c) shift average reentry timing later (d). FIGURE 3. Illustration of how evolution of traits favoring survival in local habitats resulted in genetic divergence between Columbia River populations following the initial colonization event 12,000 years ago. The increase in between-population genetic diversity and decreased within-population diversity produced by local adaptation is also illustrated. The letter codes (E, e, K, k, F, f) represent biallelic genes (those with two forms) for hypothetical traits under natural or sexual selection pressure.
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