Modeling Atlantic salmon (Salmo salar) and brown trout (S. trutta) population responses and interactions under increased minimum flow in a regulated river

Describing and understanding the relationship between streamflow ecological processes is a classic problem in stream ecology river management. We applied individual-based model inSTREAM to describe dynamic habitat emergent population responses sympatric landlocked Atlantic salmon ( Salmo salar ) lake-migrating brown trout trutta ). This application explicitly describes environmental conditions Gullspång Rapids, residual flow stretch hydropower-regulated River, Sweden (Svenskt elfiskeregister – SERS, 2019) etween September 2008 2018. simulated three static minimum scenarios variable natural regimes, contrasting highly artificial with more dynamics. Our main response was number of large (≥ 12 cm) out-migrants trout, proxy for successful recruitment. The baseline predicted an average production 455 532 per year during 2008–2018 this 11,700 m 2 spawning rearing area. only alternative scenario producing when raised by factor three, as led modest increase out-migrants. Interestingly, none alternatives produced than model, suggesting competitive disadvantage originating from later trout. Density-dependent regulation, well-known phenomenon salmonids, reproduced model. Both intra- interspecific competition evident. While varied regime, sensitivity analyses showed that other input, specifically velocity shelter availability temperature, were just important. Increased shelters (in-stream structures reduces swimming speed drift-feeding fish) increased both silico. conclude system, restoration based on simplistic will have limited effect, unless complemented instream structural complexity. • Individual-based modeling can be used deciding regulations rivers. Predictions about out-migrating fish relevant managers. A stable alters complexity reach could populations.