Abstract
BackgroundThe experimental effects of surgically implanting fish with acoustic transmitters are likely to have negative effects on survival and behaviour. Measuring the extent of these negative effects is important if we wish to extrapolate inferences from tagged animals to un-manipulated animals. In this study, we examine the effect of surgery and post-tagging recovery time on the survival and migration rate of acoustically tagged wild Atlantic Salmon (Salmo salar) smolts through freshwater, estuarine and ocean phases of migration. Four treatment groups were used: pre-smolt captured in the fall that overwintered in a hatchery and were tagged either 75 days prior to release (winter hatchery) or within 24 h prior to release (spring hatchery) and smolt captured during the spring smolt run, tagged 24 h prior to release and released during the day (day-released) or night (night-released).ResultsThe spring hatchery treatment group served as a reference treatment group such that recovery time (comparison to winter hatchery treatment) and hatchery effects (comparison to day-released and night-released treatments) could both be discerned. The hatchery effect increased migration rate, whereas short recovery times and captivity in a hatchery negatively affected survival. These effects were most pronounced within the first 5 days and/or 48 km downstream post-release, however, the residual recovery time effects appeared to persist during the transition from the estuary into salt water.ConclusionsEven with smolts originating from the wild and spending relatively little time within the hatchery environment, post-release survival was still negatively affected. Migration speed was faster for hatchery smolts, but is likely only due to their larger size. Recovery time effects were most prominent during the initial migration period in freshwater and again in the transition from the estuary to saltwater which may be due to added stress during these transitional zones. As surgery-related bias will likely never be completely removed from telemetry studies, it is important to quantify and account for these effects in situ when making inferences on the un-manipulated component of the population.
Highlights
Acoustic telemetry has become a broadly applied technology for studying the migration dynamics and for estimating survival rates of many fish species, including juvenile stages of wild Atlantic salmon in both fresh and salt water [1,2,3,4,5]
With the increasing application of acoustic telemetry in the study of survival and behaviour of fish, it is important to quantify the impact of the experimental manipulation associated with capture and surgical implantation of acoustic transmitters in fish on the biological characteristics of interest
Post hoc treatment comparisons showed only spring hatchery smolts to be significantly longer than all other treatments (Table 1; Fig. 2)
Summary
Acoustic telemetry has become a broadly applied technology for studying the migration dynamics and for estimating survival rates of many fish species, including juvenile stages of wild Atlantic salmon in both fresh and salt water [1,2,3,4,5]. The release of Atlantic Salmon (Salmo salar) in acoustic telemetry studies has occurred within several hours post-tagging to days [2, 5, 15]. There has been limited research examining the consequences of increasing post-tagging release time [16] in order to minimize the negative effects of the surgical procedure on fish behaviour and survival. Panther et al [17] recommended holding fish until the surgical incision from tagging is closed (28–70 days), reducing the negative effects and potential biases, but this would preclude any research work on actively migrating wild fish such as Atlantic salmon smolts. We examine the effect of surgery and post-tagging recovery time on the survival and migration rate of acoustically tagged wild Atlantic Salmon (Salmo salar) smolts through freshwater, estuarine and ocean phases of migration. Four treatment groups were used: pre-smolt captured in the fall that overwintered in a hatchery and were tagged either 75 days prior to release (winter hatchery) or within 24 h prior to release (spring hatchery) and smolt captured during the spring smolt run, tagged 24 h prior to release and released during the day (day-released) or night (night-released)
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