Effect of 12 mm PIT tags on the survival, growth, and ecologically significant behaviours of juvenile critically endangered endemic

Understanding the movement ecology and habitat use of freshwater fish is crucial for their conservation, with telemetry being an essential tool. Current tagging methods are restricted by the minimum body size of fish, as tags should not exceed 2% of the fish's body mass. This limitation hinders the study of juvenile small-bodied endangered species, as the reduced fitness of tagged individuals can negatively impact their populations. This study examines the feasibility of using 12-mm PIT tags on juvenile and subadult individuals (63–90 mm fork length) of the critically endangered Rhône streber, Zingel asper. Results indicate that tagging does not significantly affect survival, growth or behavioural traits, despite the tag weight being between 2.2% and 5.4% of the fish's weight. Tag retention was 100%, with 90% of individuals having the PIT tag fully encapsulated in the body cavity wall after 55 days. The only possible negative effect observed was a tendency, albeit statistically non-significant, for slower escape velocity in the smallest tagged individuals. These findings reduce the known minimum body size limit for 12-mm PIT tags for this species. However, the smallest individuals may be more sensitive to tagging, warranting careful consideration to avoid potential negative impacts on wild populations.