Where does the rainbow end? A case study of self-sustaining rainbow trout Oncorhynchus mykiss populations in tropical rivers

The great physiological, behavioral and morphological plasticity of salmonids contributed to the success of their introduction into a wide range of habitats. However, rainbow trout ( Oncorhynchus mykiss ) is the only salmonid species to live in the tropics. Like for any cold-water species, water temperature is assumed to be a major constraint to the presence and expansion of rainbow trout. In this study, we investigated key drivers of the establishment of self-sustaining populations of rainbow trout in the tropical rivers of Reunion Island (West Indian Ocean). We collected detailed records of the introduction history of rainbow trout at 38 sites across the island over 80 years to determine whether the presence/absence of trout could be related to the introduction effort. Surprisingly, the presence of trout was limited to 10 known sites and seemed independent of the introduction effort. There was no evidence of large-scale expansion of trout away from introduction sites, and the risk of expansion from current self-sustaining populations appeared low. We then quantified how seasonal water temperatures influenced the presence of self-sustaining populations of rainbow trout. As expected, low temperature is a prerequisite for the presence of trout, which is favored at high elevations in this tropical region. We provide evidence that the presence of trout is limited by temperature in summer and winter, which makes it vulnerable to global warming throughout its life cycle. By translating threshold temperatures into elevation, we estimated the probability of the presence of trout, spatially, after introduction along all perennial rivers of the island. Upscaling results from a few study sites to a larger spatial scale is a useful tool for managers in this and other data-poor areas. ### Competing Interest Statement The authors have declared no competing interest.