Size selection from fishways and potential evolutionary responses in a threatened Atlantic salmon population

The evolutionary effects of harvest on wild fish populations have been documented around the world; however, sublethal selective pressures can also cause evolutionary changes in phenotypes. For migratory fishes, passage facilities may represent instances of nonlethal selective pressure. Our analysis of 6 years of passage data suggests that certain fish passage facilities on the Penobscot River have been exerting selective pressure against large‐bodied, anadromous Atlantic salmon (Salmo salar). At the second and third dams in the river, a 91‐cm salmon was 21%–27% and 12%–16% less likely to pass than a 45‐cm salmon, respectively. Fish size positively influences egg survival and number and is a heritable trait. Therefore, in a wild‐reproducing population, exclusion of large fish from spawning areas may have population‐level impacts. In the Penobscot River, most returning adults derive from a hatchery program that collects its broodstock after passing the first dam in the river. Analysis of fork lengths of salmon returning to the Penobscot River from 1978 to 2012 provided mixed support for evolution of size at maturity in different age classes in a pattern that may be expected from interactions with conservation hatchery operations. Additionally, slow‐maturing and iteroparous individuals that represent the largest salmon size classes were essentially lost from the population during that time, and Penobscot River fish have shorter fork lengths at maturity than Atlantic salmon in undammed systems.