Tagged for life? Retention rates and effects on growth and condition of tagging - a long-term field study on PIT- and Carlin tagging in European eel

Elin Myrenås, Joacim Näslund, Philip Jacobson, Birgitta Jacobson, Josefin Sundin 

Different types of tags and markers are commonly used for various fish monitoring and tracking purposes. Effects of tags and markers on fish and the retention rates can affect the interpretation of mark-recapture data on both the individual (e.g. growth and body condition) and population level (e.g. survival and pro-duction estimates), making studies of this issue important. In this study, we investigated the effects of tag-ging on the European eel (Anguilla anguilla), using two commonly used tag types, Carlin and Passive Inte-grated Transponder (PIT)-tags. We examined tag retention of Carlin tags and potential effects on growth and body condition of PIT-tags. We used data from several long-term tagging projects conducted in a natu-ral Swedish lake, combining outlet trap monitoring with catch data from a commercial fisher. The longest of these studies spanned over 27 years, covering the majority of the eel’s resident life stage. We found that the retention rate of Carlin tags was 92%, with 64% of the Carlin-tagged silver eels being recaptured. The recapture rate of PIT-tagged eels released as juveniles in the lake was 12%. This recapture rate indicates little or no effect on survival when compared to other studies of both tagged and untagged eels. No major impact of PIT tags on growth and condition in eels was found. We conclude that both Carlin and PIT tags are suitable methods for tagging European eel, each with strengths in its respective area of use.