Revealing a rapid shift in the phenology of the adult spawning migration of an introduced Chinook salmon population in Patagonia

Chinook salmon (Oncorhynchus tshawytscha) is invading South America. Both the high plasticity and genetic diversity of introduced propagules have been hypothesized to be responsible for the success of this species’ invasion. Yet, the influence of environmental variability on the expressed phenology of the adult spawning migration has been overlooked in this region. Here, we examined the consistency in timing, duration, and relative abundance of adult salmon migrants and their associations with environmental river conditions and surrounding ocean in a regulated river system in Patagonia. We conducted monthly long-term snorkeling fish surveys (2010–2019) and collected associated environmental information from the river and ocean. We observed a recent increase in duration of the spawning migration and a decline in the relative abundance of adult migrants. A warming phase of the Southern Pacific Ocean (during the two previous years) was associated to an extended migration season, whereas a colder river in fall was associated to a lower number of adult migrants. Collectively, our findings suggest that rapid phenological shifts could occur in a recently established salmon population (circa 1980). This process could be explained by novel selective pressures and expression of life history traits in response to novel environmental regimes. Further long-term surveys of introduced salmon can aid in parsing the relationships between environmental regimes and the biology and persistence of these self-sustained populations.