Spatial ecology and population dynamics of brown trout Salmo trutta L. in reservoirs and headwater tributaries

This investigation compared the spatial ecology and population dynamics of brown trout Salmo trutta L . between reservoirs with (impact; Langsett Reservoir) and without (control; Grimwith Reservoir) barriers to fish movements into afferent headwater tributaries, including the effectiveness of a fish pass to remediate connectivity. Passive Integrated Transponder (PIT) telemetry revealed fish that emigrated from Langsett and Grimwith reservoirs were 1-3 and 0-2 years old, respectively, and predominantly did so in March-May and October-December in both. Weirs at Langsett Reservoir (emigration rate = 26%) appeared to thwart emigration relative to Grimwith Reservoir (emigration rate = 85%). Acoustic telemetry (2D positions) in the impact reservoir revealed the largest home range was in October – December (monthly K95 ± S.D. up to 26.9 ± 6.69 ha in November), activity was influenced by both month and time of day, and fish occupied shallow water depths (relative to reservoir depth), especially at night. Large proportions of brown trout tagged in Grimwith and Langsett reservoirs (42.9% and 64.1%, respectively) and fish that emigrated (37.2% and 27.7%, respectively) were detected moving upstream into tributaries. At both reservoirs, peak immigration for 3- and ≥4-year-old fish occurred in October-December, although upstream movements occurred throughout the year and by all age classes. Three brown trout passed upstream of each of the weirs on River Little Don (prior to fish pass construction; 3% of those that approached from downstream) and Thickwoods Brook (throughout the study; 2%). Overall fish pass solution passage efficiency was 14% but was higher for 2- and 3-year-old fish (32%), which was comparable to fish translocated from upstream (33%). Passage predominantly occurred at lower river levels than fish pass entrance / attraction, which was also lower than during approaches to the weir. A Before-After Control-Impact (BACI) design found that although juvenile (0+, but not >0+) brown trout densities were lower after fish pass construction, the reduction was significantly less than at control sites, i.e., the fish pass had a positive effect. Overall, this investigation significantly furthers our understanding of brown trout spatial ecology and population dynamics in reservoirs and headwater tributaries. ### Competing Interest Statement The authors have declared no competing interest.