Enter Salmon: Trophic risk mediates riverine barrier-crossing behaviours of parr

Freshwater biota are highly threatened by global stressors, among which fragmentation by artificial barriers has gained increasing attention in recent years. Barriers limit the movements of native species, notably the migrations of keystone fish, such as Atlantic salmon (Salmo salar), with ecological and socio-economic consequences. The severity of barrier impacts on salmon smolt and adult migratory stages has widely overshadowed the effects on juvenile river-resident stages, despite the importance of habitat availability during these years. We present the first study of crossing behaviours displayed by salmon parr traversing obstacles while considering trophic context-dependencies. Using semi-natural flume tanks and a multifactorial design, we examined two of the most common weir designs (vertical and v-notch) and simulated risk-reward conditions (model predator and chemosensory food resource cues), with approaching, assessing, attempting and crossing behaviours analysed. We found that fish movements throughout the tank were significantly impeded by both barrier designs compared to barrier-free controls, of which the v-notch presented a moderately greater challenge. These effects were further mediated by perceived predatory threat, with the presence of a potential predator significantly reducing the probability and latency of fish approaching, reducing the probability of assessing the barrier, as well as shortening the average duration of barrier crossings. Resource presence had minimal effects on the behaviours investigated, except for a significant interaction with predator presence influencing assessment duration. Our study demonstrates that barriers, regardless of their design, place significant limitations on habitats available to parr by reducing overall movement in the system. However, the behaviour of salmon parr around artificial barriers is context-dependent, highlighting the complexity of fish-barrier interactions and raising important questions about the potential benefits and unknown consequences of barrier removals within existing ecological networks.