Title: Detection of juvenile coho salmon (Oncorhynchus kisutch) indicates widespread successful reproduction and different life-history strategies in the extreme south of Patagonia

Abstract

The native fish biodiversity of Patagonia has been highly modified by a long history of salmonid introductions from the Northern Hemisphere for recreational fishing and aquaculture purposes. Coho salmon, Oncorhynchus kisutch, is the most recent salmonid established in Patagonia, first recorded in Chile in 2014 and 2019. In this work, we report for the first time the occurrence of juvenile coho salmon in rivers of Tierra del Fuego Island, in the extreme south of Patagonia (Argentina, 52.5º S–54.9° S). Fifteen rivers were surveyed by electrofishing between 21st January and 14th February (austral summer) 2020, yielding juveniles in eight rivers flowing into the south coast of the island. The fork lengths of the fish captured indicated age 0+ and 1+ fish in three rivers, whereas only 0+ juveniles were captured in the other five rivers. Body size differences among coho salmon individuals caught in two rivers are discussed in the context of their different thermal regimes, providing the first step in describing the early life history of these populations. Understanding the ecology and expanding distribution of coho salmon in Patagonia is crucial to predicting future changes in their life histories due to local adaptations and phenotypic plasticity, and their potential effects on native species and other salmonids from predation, competition for food, or habitat displacement.