Movements and population dynamics of brown trout in reservoirs and headwaters

To fully understand movements of fish a multi-faceted, varied approach is required. In this study, a combination of Passive Integrated Transponder (PIT) and acoustic telemetry was used in a complementary approach to understand the movements of brown trout (Salmo trutta) in upland reaches of two reservoir systems in Yorkshire, UK – Grimwith and Langsett Reservoirs. In particular the effects of potential barriers the weirs that are present on both tributaries at Langsett Reservoir on movements were examined. The results were interpreted in relation to fish size and environmental conditions (river flow, water temperature, time of day). Over 1000 fish were PIT-tagged: including fish from both reservoirs and the five tributaries (three at Grimwith, two at Langsett). Twenty fish were acoustically tagged in Grimwith Reservoir.Under the discharge regimes prevailing in this study, weirs appeared to prevent fish from migrating from Langsett Reservoir into the tributaries, as no fish detected immediately downstream of the weirs were subsequently detected upstream. The habitat upstream of the weirs was subjected to analyses that i) had found it to host a significantly less dense population of brown trout than its habitat could carry, and ii) found that it would be able to support spawning trout where the habitat downstream of the weirs either could not at all, or could only do so in a very limited capacity. Therefore, it is possible that the weir was preventing a more complete exploitation, by brown trout, of the habitat.The weirs may also be barriers to downstream migration, as up to 66% of fish detected immediately upstream of the weirs were not subsequently detected downstream. The assumption in this case being that all fish spawned upstream of the barrier should undertake a migration downstream.Acoustically tagged brown trout did not enter Blea Gill Beck, Gate Up Gill or Grimwith Beck, suggesting either that they did not spawn during the study period or spawned elsewhere. Given that Blea Gill Beck, Gate Up Gill and Grimwith Beck are the only tributaries to Grimwith Reservoir that are viable for spawning, the only other possible spawning location is the reservoir itself.The results have been used to make recommendations pertaining to mitigating the effects of the barriers, ultimately that one weir, that on the River Little Don should be selected for fish pass installation