Abstract
Implanting passive integrated transponder (PIT) tags into stream-dwelling fishes generates data on their residency and movements during active tracking surveys and recapture events. However, imperfect detection is an inherent issue due to tagged fish remaining undetected and/or recaptured. The aim was to assess imperfect detection in 1090 PIT-tagged Atlantic salmon Salmo salar (“salmon”) and brown trout Salmo trutta (“trout”) in an upland English stream between September 2022 and September 2024. Tag lengths were 12 and 23 mm and used in both species. Tagging and recapture events were completed in July and September using electric fishing. Active tracking using a mobile antenna was completed between July and September. Eleven percent of recaptured trout had lost their tag, whereas no salmon lost theirs. Nineteen percent of tagged fish were recaptured at least once. The lowest recapture rates were recorded in fish tagged in September and fish were most likely to be recaptured in the site of the smallest channel width. Detection rates during active tracking were significantly higher in fish tagged with 23-mm versus 12-mm tags, with fish detected in multiple tracking surveys always tagged with 23-mm tags. Tag size and site characteristics thus strongly influence imperfect detection during PIT telemetry studies.