Wild brown trout (Salmo trutta L.) populations have declined throughout Europe. To counteract this decline, stocking programs re-introduce or supplement existing populations by releasing hatchery-reared smolts. However, these fish suffer high mortality, possibly due to transport and release stress and predators aggregating at the release site. Therefore, this study used acoustic telemetry to test potential benefits of releasing brown trout smolts at night into the Roskilde Fjord, Denmark, to examine whether it would affect survival or migration timing. Despite expectations, no biologically meaningful differences were found between the day and night release smolts on either short-term survival or migration timing. This may suggest that the period of darkness was too short for the released smolt to recover from the stress. Further, brown trout survival was also not affected by fish length or release year. Overall, this study does not support the more logistically complicated nighttime release of hatchery-reared brown trout smolts into the wild. Finally, this study identified improvements for telemetry studies done in this area. This includes suggestions on how to optimize acoustic receiver deployments and directions for future research on brown trout smolt and their predators.
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