Do invasive Brown Trout threaten Atlantic Salmon in North America?

ABSTRACT Objective Brown Trout Salmo trutta have been introduced from Europe into waters in eastern North America and may be contributing to the current population declines in native Atlantic Salmon Salmo salar where the two congeneric species overlap. Despite living in sympatry in European systems, gaps in knowledge about Brown Trout as an invasive species remain with respect to their role as competitors and predators of native Atlantic Salmon in North American rivers. Here, we combined information about the potential sources of antagonism by invasive Brown Trout to assess whether they present a threat to native Atlantic Salmon. Methods We reviewed the relevant literature and consulted with data sources on the distribution of Brown Trout in eastern Canada. We identified the main ways in which Brown Trout may affect Atlantic Salmon in North American waters. Where relevant literature was lacking, we sought comparative literature and suggested future research. Results First, Brown Trout and Atlantic Salmon exhibit some spatiotemporal segregation during spawning timing in North American rivers, but Brown Trout could increase interspecific competition for habitat and alter spawning dynamics and, ultimately, populations of Atlantic Salmon. Although there have been concerns about hybridization, this risk is mitigated by the low viability of Atlantic Salmon × Brown Trout hybrids; nevertheless, hybridization could impact salmon production. Third, juvenile competition between Atlantic Salmon and Brown Trout may arise from shared food and habitat resources, with Brown Trout potentially outcompeting Atlantic Salmon due to their aggressive behavior and faster growth rates. Brown Trout are also opportunistic predators of Atlantic Salmon, potentially contributing to further declines and hindering recovery efforts for Atlantic Salmon in rivers of North America, where small salmon populations are increasingly vulnerable to predation pressure. Finally, Brown Trout can affect their new food webs directly by decreasing the survival of Atlantic Salmon, ultimately reducing nutrient availability, and indirectly by altering energy pathways (i.e., new connections between previously unlinked habitats), which can reshape trophic interactions. Conclusions The expansion of invasive Brown Trout in North America may be limited by genetic diversity, but they could overcome evolutionary constraints over time, similar to other invasive salmonids as they adapt to new environments. The potential sources of antagonism that are considered here provide a foundational understanding of the interactions between Brown Trout and Atlantic Salmon, but additional research is needed to fully assess their ecological impacts and the long-term consequences of Brown Trout invasions in rivers of North America where Atlantic Salmon conservation is increasingly a priority for river managers.