Tag retention and survival of Atlantic salmon (Salmo salar) smolts surgically implanted with dummy acoustic transmitters during the transition from fresh to salt water

Acoustic telemetry of surgically tagged fish has become a powerful tool for quantifying the survival of fish as they migrate. However, when a transmitter is not retained within the body cavity, survival estimates will be underestimated. In this study, we quantify transmitter retention rates and mortality of hatchery Atlantic salmon smolts tagged with various sized transmitters during the transition from fresh to salt water. Final retention rates ranged from 34 to 85% depending on transmitter type and the surgeon performing the tagging. Transmitter expulsion occurred for all transmitter types and predominantly occurred over a 40-day period from 25 to 65 days post-tagging. Expulsion rates were significantly higher for transmitters that weighed >7.5% of total smolt weight. Mortality was only significantly higher than sham treatments for transmitters over 12% of total smolt weight. Accounting for transmitter expulsion when modelling survival for tagged fish is important, otherwise, there is potential for biased estimates. However, the degree of bias will depend on transmitter size, life stage tagged, and study duration.