Whole Lake Behaviour of Sea-Run Brown Trout During Pre-Spawning Months

Sea-run brown trout use lake habitat extensively when it is available both before and after spawning. However, the ways in which brown trout actually use lakes remain poorly resolved. In Norway, some of these lakes are used as reservoirs for the effluent of high-head power stations that tunnel mountain water through turbines, discharging the water into the lakes that support migratory species like trout. In this study, we calculated the positions of tagged trout in Lake Vassbygdivatnet, western Norway from July–November 2021 using acoustic telemetry and YAPS software in the R programming environment. We fitted kernel density estimators to the positioning data to estimate core areas where the trout were found within the lake and identified hotspots close to shore near locations in the lake where cascades discharged from the surrounding mountains. However, a resource selection function type analysis identified no affinity of the trout for the area immediately adjacent to the high-head power station outlet. Finally, a hidden Markov model determined that the nearshore lake margin was mostly used by trout for more passive states (low directionality, short steps), whereas the middle of the lake was more frequently used for active states (high directionality, long steps). The findings demonstrate for the first time that mountain cascades discharging into lakes provide habitat that attracts trout and that there is no clear attraction towards the power station outlet during the pre-spawn period. Risk of entrainment in the power station tunnel therefore does not seem to be a major risk factor for migrating trout. However, fisheries managers may wish to use these findings when regulating fishing activities in lakes like Vassbygdivatnet where trout stage in preparation for spawning.