Abstract
BackgroundPassive 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.ResultsTag 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.ConclusionThe 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.
Highlights
Passive integrated transponder (PIT) tags are widely used in studies of various aspects of fish survival, movement and behaviour
PIT tag retention rates have been shown to vary among species, and even within species depending on life stage, sex or tag placement [2, 3]
Fish were measured for fork length (LF) and individuals without detected PIT tags were removed from the experiment and euthanized using an overdose of anaesthetics
Summary
Passive integrated transponder (PIT) tags are widely used in studies of various aspects of fish survival, movement and behaviour. Despite the wide use of PIT tags in both laboratory experiments, field studies and population monitoring of Atlantic salmon, we are not aware of any published information on long-term retention of PIT tags spanning juvenile to adult life stages. Anadromous salmon from a single cohort can have variation of several years in both time spent in the river as juveniles and as adult in the ocean [9]. This means that for salmon tagged as juveniles, correction of survival estimates requires knowledge on temporal patterns in tag retention rates. We present data on temporal changes in retention rates over a period relevant for studies of wild one-sea winter Atlantic salmon
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