First record of a synergistic interaction between invasive salmonids in South America

Ecological interactions among multiple invaders can aggravate their effects on invaded systems. Many salmonids including rainbow trout Oncorhynchus mykiss and Chinook salmon O. tshawytscha have established naturalized populations and co-occur in rivers worldwide. Predation of eggs and juveniles has been well documented among salmonids in their native range in the Northern Hemisphere. It has been unclear to what extent these interactions exist in invaded rivers of the Patagonia. Here, we report and quantify rainbow trout predation of Chinook salmon eggs in the Cisnes River Basin, southern Chile. We used hook and line angling to sample rainbow trout (n = 80) around salmon spawning areas before, during, and after the spawning season of Chinook salmon. Stomach contents obtained during two consecutive years show consistent evidence that rainbow trout opportunistically prey on Chinook salmon eggs. Salmon eggs occurred in 75 and 50% of the trout stomachs analyzed in 2019 and 2020, respectively. The number of salmon eggs (mean ± SD) per trout stomach ranged between 17.95 ± 24.29 in 2019 and 7.45 ± 12.90 in 2020. We assumed that consumed eggs were primarily non-viable or would have had low probabilities of survival suggesting a synergistic interaction between rainbow trout and Chinook salmon. Our findings provide baseline information that contribute to demographic studies and the management of invasive salmonids in rivers of this region and elsewhere.