Abstract

Multiple attempts to introduce coho salmon in South America were carried out during the early twentieth century, with results that were initially largely unsuccessful. However, the species is now widely distributed along the Pacific Ocean coast of southern South America, where artificial propagation and aquaculture escapes have facilitated the emergence of naturalized populations, including occurrences in the southernmost regions of the continent. Here we present (i) a synthesis of the coho salmon distribution in South America based on records collected between 1981 and 2024 from aquatic researchers from Argentina, Brazil, Chile, and Uruguay, and (ii) a map of ocean thermal suitability of non-native coho salmon in the Pacific Ocean and Atlantic Ocean of South America, based on thermal preferences in their native range, to understand oceanic invasion pathways. Our results showed a 25.5% expansion in the distribution range of spawners and juveniles over the last decade, extending from 38 degrees 47 ' S (Imperial River, Chile) to 54 degrees 58 ' S (Moat River, Argentina), encompassing 19 basins draining into the S Pacific Ocean and S Atlantic Ocean. When considering records of non-reproductive adults and marine occurrences, the contemporary distribution of coho salmon extends from 27 degrees 8 ' S (Bombinhas, Brazil) to 54 degrees 58 ' S (Moat River, Argentina), covering 22 basins. Coho salmon showed lower colonization success than related Pacific salmon species such as Chinook salmon or rainbow trout, yet it maintains a continuous distribution in the S Pacific Ocean and spatially isolated occurrences in the S Atlantic Ocean. The Santa Cruz River in Argentina may represent the northern limit of a continuous S Atlantic Ocean distribution of coho salmon, whereas the more northerly records may correspond to vagrant individuals or long-distance dispersal events potentially facilitated by suitable thermal habitats along the Falklands/Malvinas Current. Taken together, our findings provide evidence of an ongoing process of invasion and naturalization of coho salmon in South America. The use of different marking approaches may help distinguish between escaped and naturalized individuals, aiding in the evaluation of expansion patterns. The information generated here is essential to support management strategies for a species that provides socioeconomic value while also exerting significant ecological impacts.