A comparison of acoustic tag sizes on wild Atlantic salmon Salmo salar L. smolt migration success and behaviour

Tracking of animal migrations using telemetry technologies needs to take into consideration the burden that the tag exerts on the animal. Here, we examined the potential impacts of acoustic tags of two sizes (nominally a ‘V6’ [smaller] and ‘V7’ [larger]) on the downstream riverine migration success and behaviour of wild Atlantic salmon (Salmo salar L.) smolts. One hundred fish were tagged with either a V6 or V7 tag. Tag burden (tag: fish weight) ranged from 1.88% to 7.39% and differed significantly between fish tagged with the V6 (mean [SD] = 3.63% [0.51%]) and the V7 tags (mean [SD] = 5.84% [0.95%]). There was no significant difference in the in-river migration failure between the two groups when tested with a time-to-event analysis. There were also no differences in other elements of the migratory behaviour (rate of movement, time of detection and residency time) between the two tagging groups. These data support the use of acoustic tracking for monitoring smolt migration and highlight that tagging of smaller smolts at up to 7.39% tag burden to gain a more representative understanding of migration success and behaviours across a smolt population.