Invasive Brook trout (Salvelinus fontinalis) presence may reduce dominance-linked growth and shift diel activity in brown trout (Salmo trutta)

Salmonid fishes optimize their habitat use through temporal and spatial activity patterns to balance foraging success, competitive ability, and predator avoidance. Under stable social hierarchies, dominant individuals typically occupy profitable stream positions and achieve higher growth rates. However, the introduction of non-native salmonid species can disrupt these established social hierarchies and alter optimal habitat use patterns. Moreover, the removal of invasive species is often difficult to fully accomplish and may disproportionately target certain behavioural phenotypes, potentially exacerbating the impact of invasive species. Using seminatural stream flumes, this study compared dominance-growth relationship, habitat use, and diel activity patterns between allopatric brown trout groups and sympatric groups of brown trout and invasive brook trout, under varying flow conditions and brook trout removal scenarios. Our results revealed that brook trout invasion disrupted the fundamental dominance-growth relationship observed in allopatric brown trout, indicating that the costs of maintaining dominance in the presence of brook trout outweigh the benefits. Additionally, brook trout invasion significantly altered brown trout diel activity patterns and spatial habitat distribution, even after partial removal of brook trout. These findings demonstrate that invasive species can disrupt established link between dominance, growth and habitat use in native fish communities.