Tagger effects in aquatic telemetry: Short‐term and delayed effects of surgery in Atlantic Salmon smolts

Abstract

AbstractObjectiveAn assumption of biotelemetry is that animal performance is unaffected by the tagging process and tag burden, yet this assumption is often untested or not thoroughly explored. Our objective was to explore how transmitter implantation procedures influenced Atlantic Salmon Salmo salar smolt survival and migratory performance.MethodsWe monitored radio‐tagged smolts, first in the hatchery and then in a river with a receiver array. We assessed survival and in‐river performance in relation to surgeon, surgery duration, processing order, and fish size.ResultMortality was 13.3% during an 8‐day hatchery‐observation period but was higher for fish that were processed by one of two experienced surgeons (25% vs. 2%). Mortality peaked 3 days postsurgery and was higher for smaller fish and fish that were tagged during morning tagging sessions (versus afternoons). The size effect changed over time, being greatest during the first 2 days postsurgery and continuing thereafter at a diminished level. Fish performance once released into a river also differed between surgeons (migration initiation 66% vs. 82%; to‐lake migration success 22% vs. 43%) and, consistent with hatchery observations fish that were tagged in the morning by one surgeon, performed poorly once released.ConclusionWe highlight the immediate and lingering effects of surgical procedures on smolt survival that, if not accounted for, could bias inferences about the study population. Researchers should anticipate tagger effects during study design to ensure potential tagger effects (i.e., surgeon, order tagged, conditions during tagging) are balanced across study groups of interest. Testing for a fixed tagger effect in analyses may not always be adequate because a tagger effect may covary with processing order and fish size and may change over time.