Simulated Demographic and Evolutionary Impacts of Size-Selective Passage at Dams on Atlantic Salmon Body Size

Atlantic Salmon have a complex life history that involves a multi-year freshwater juvenile phase, several years maturing at sea, and an eventual return to their natal rivers to spawn. Range-wide, their migrations have been impacted by the construction of dams. The relationships between body size, fecundity, and juvenile survival are well documented in Atlantic Salmon. Large-bodied individuals are important contributors to spawning populations, producing higher-quality, more viable eggs. Additionally, body-size is a heritable trait in salmonids. Recent research has characterized size selective passage at several dams favoring smaller salmonids, with potential ramifications for spawning demographics and long-term population structure. We used an individual based model to explore the possible influence of dam-mediated size selection on a migrating population of Atlantic Salmon. We assessed the sensitivity of population demographics to hypothetical hydrosystem configurations including dams in series, complete removal of all dams, removal of a subset of dams, and passage upgrades at some dams. Spawning escapement, egg production, and population structure (including size at age and average age at maturity) were assessed on an annual timestep to evaluate demographic changes over time. Heritability estimates for size-at-age in each individual also allowed for evaluation of evolutionary impacts of size-selection at dams.