Lake-habitat use of post-juvenile sea trout over time and space - An acoustic telemetry study in a regulated river

Managing migratory fish species is a major conservational and scientific challenge, due to the diverse habitat requirements of these species and the required connectivity between the habitats. Little is known about the ecology and behavior of wintering trout and as it is believed to be a bottleneck period for salmonids (Salmonidae), new knowledge is needed (Greenberg et al. 2007). The aim of this study was to explore the among-individual spatio-temporal variation in habitat use in post-juvenile sea trout (Salmo trutta morpha trutta) living in the regulated Aurland river-system in Western Norway. This was done using acoustic telemetry technology and triangulation. The lake Vassbygdvatnet is located 6 km upstream from the estuary with Vassbygdelvi River running inn and Aurlandselva running out. The in-lake habitat use and the connectivity in the freshwater phase of sea trout attained the main focus in this study and was determined using vertical, 2D and 3D utilization distribution (UD) methods and the UDs were used to model the behavior of the trout using linear mixed models. The trout was tagged during four periods over the course of 21 months and assigned into three groups: smolt, finnock (i.e., immature individuals that has been to sea) and mature trout according to fish lengths and anadromous traits. Finnocks and mature trout returning from the sea arrived to Vassbygdvatnet during August- September and stayed there for the entire winter, before emigrating in May. A large proportion of the sea trout of all the tagged size groups utilized Vassbygdvatnet during several life stages, where they displayed pelagic behavior mainly utilizing the upper 15 meter depth stratum. Also assigned mature trout stayed within the lake through from September until May and spawning is believed to have occurred within the lake. Adfluvial migrations in the Aurland river and lake system were observed where smolt and finnock migrated downstream from their river-release sites to open waters during spring. Individuals tagged in the river downstream Vassbygdvatnet (Aurlandselva) migrated to the fjord, whereas smolt and finnock released upstream Vassbygdvatnet (Vassbygdelvi) ended up in the lake where just a few migrated further downstream to the fjord. The trout that wintered in the lake was obstructed from downstream migrations during spring by being attracted by the outgoing discharge water leading into Vangen hydropower station. There is a fish ladder between Aurlandselva and Vassbygdvatnet that functions well for migrations in itself but the attraction towards the discharge water of Vangen prevents the trout from finding it. When the water flow was naturalized in Aurlandselva during early summer the trout immediately migrated downriver to the fjord. Detailed analyses of the acoustic telemetry data revealed that 2D and 3D lake habitat use was affected by a complex interplay between fish size, water temperature and the prevailing water maneuvering regime. The trout seemed to prefer the warmest available water temperature during winter. The depth utilization was highly influenced by the temperature, but is also directly influenced by the maneuvering of the hydropower plant and different size groups responded differently towards the different discharge levels. Large variations in the depth use and temperature conditions in Vassbygdvatnet between the two seasons caused the trout to winter at different depths the consecutive winters. The lake was stratified at 4.2°C during the 2012-13 winter where the trout showed little variation in depth use and isothermal at