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

Acoustic telemetry is a widely used method in assessing behavioural dynamics in fishes. Telemetry transmitters (tags) are often surgically implanted in the coelom of the animal with limited in situ testing and sometimes only assuming that they have minimal rates of post-release tag shedding and mortality. However, fish are capable of 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 tag expulsion and tagging mortality in fishes. We conducted an empirical assessment of tag retention and post-surgical mortality rates in post-smolt Atlantic salmon (Salmo salar) fitted with dummy (non-transmitting) acoustic tags 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 had high rates of tag expulsion (54.8%), impaired growth, and a foreign body response evident but exhibited low rates of mortality following tag implantation (<= 5.1%). 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 tag location data as they may represent an expelled tag rather than a mortality event. Our results also indicate that tag dimensions are not nearly as important as the tag:body mass ratio.