How much hatchery-reared brown trout move in a large, deep subalpine lake? An acoustic telemetry study

Abstract Fish movement into large, deep lakes has been rarely investigated due to the complexity and extent of such ecosystems. Among the different monitoring methods available, acoustic telemetry enables to study spatial ecology of aquatic organisms and to investigate their behavior in lentic environments. In this study, movement of 69 hatchery-reared adult brown trout (size 43–61 cm) marked with acoustic transmitters was monitored in the large and deep subalpine Lake Lugano (Switzerland and Italy). Trout were tracked for six consecutive months by 7 hydrophones (April-August 2022), positioned in a non-overlapping array. Trout movement was reconstructed using R packages specific for acoustic telemetry (Actel and RSP), which also allowed to translate tracking information into utilization distribution (UD) areas for each fish. The effects of different environmental variables (rainfall, water discharge of the two main tributaries of Lake Lugano, atmospheric pressure, cloud coverage and moon phases) on trout movement was tested, but none of these variables seemed to correlate with fish movement. Although trout marked with acoustic transmitters were hatchery-reared fish, the extent of their total movement throughout the study period was unexpected, with many trout moving for more than 100 km in few months. The extent of such movement patterns is potentially related to an explorative behavior, which is more pronounced in hatchery-reared fish due to their more aggressive and bolder attitude. The association of these behavioral traits, shaped by domestication, could expose hatchery-reared fish to higher risks and post-release mortality in the wild.