Long-term PIT tag retention rates in Atlantic salmon (Salmo salar)

Background: Passive integrated transponder (PIT) tags are widely used in studies of various aspects of fish survival, movement and behaviour. Quality of such studies depends on the fish retaining the tags over the study period and that the tagging procedure or the tag does not influence behaviour or survival. Here we document PIT tag retention rates in hatchery-reared Atlantic salmon over a 533-day period from the late juvenile freshwater stage (pre-smolts) to young adults after 1 year in seawater. Fish were marked with 12-mm PIT tags, injected into their body cavity, as age 1+ pre-smolt and scanned for presence of PIT tags five times during the study. Results: Tag retention for the entire period was 91% and varied between periods (96.09–99.89%). For individual time steps, daily retention rate was lowest in the first period following tagging (days 0–49). After this period retention rate increased substantially, before again dropping close to initial levels at the two last periods (days 173–533). Length of fish was only significantly related to retention during the first period. No difference in retention rates was found between males and females. A subset of fish without detected tags was X-rayed to verify that lack of PIT detection was due to tag loss. Conclusion: The retention rates observed in this study clearly show that tag loss needs to be accounted for when analysing PIT tag data on Atlantic salmon. Further, the temporal changes in retention rate clearly challenge previous assertions that tag loss is something that primarily occurs shortly after tagging or during spawning.