The fate of intracoelomic acoustic transmitters in Atlantic Salmon (Salmo salar) post-smolts and wider considerations for casual factors driving tag retention and mortality in fishes

Acoustic telemetry is a widely used method in assaying behavioural dynamics in fishes. Telemetry tags are often surgically implanted in the coelom of the animal and are assumed to have minimal rates of post-release mortality and tag shedding. However, fish are capable expelling tags and mortalities do occur following release, with the mechanism(s) underlying these effects not well understood. The purpose of this research was to address causal factors underlying tagging mortality and tag expulsion in fishes. We conducted an empirical assessment of tag retention and post-surgical mortality rates in post-smolt Atlantic salmon (Salmo salar) fitted with a dummy acoustic tag over a 92 day monitoring period. This was complimented with a meta-analysis of factors affecting tag retention and post-surgical mortality rates in the wider literature. Post-smolt salmon exhibited low rates of mortality following tag implantation ([≤] 5.1%) but had high rates of tag expulsion (54.8%) and impaired growth and a foreign body response was evident. The meta-analysis showed that mortality was generally low across all studies (12.4%) and was largely unaffected by model cofactors. Tag retention rates were high among the studies investigated here (86.7%) and had a weak negative relationship with tag:body mass ratios. Our results suggest that while mortality is often low among tagging studies, including this one, caution must be exercised in assessing stationary tags as they may represent an expelled tag rather than a mortality event. Our results also indicate that tag dimensions are not nearly important as the tag:body mass ratio.