Hatchery-raised Atlantic salmon are transferred into seawater for commercial salmonid culture. Smoltification, the transformation of freshwater parr into seawater smolts, is a critical stage with implications for growth and survival. Prior to smoltification, parr have low tolerance to seawater, leading to reduced growth and increased mortality. This project aimed to:

1. To compare the reliability of predicting smoltification and seawater readiness between ATPase, gene expression, plasma chloride, and osmolality.

2. Compare the repeatability between the diagnostic tests.

3. Incorporate the findings into standardised procedure for Petuna Aquaculture.

The 2022 smolt year class was closely monitored during smoltification using ATPase and traditional tests. Baseline freshwater samples collected from each group. Smolt inducing photoperiod (24-h light) introduced.

• Weekly samples collected from each group.

• Groups transferred to seawater and samples collected for2 more weeks.

• Samples were analysed for Gill Na+/K+-ATPase activity, blood chloride, plasma osmolality and gene expression.

• Although some fish groups passed plasma chloride and osmolality tests after 1-2 weeks of 24-hour light, ATPase activity was still increasing in freshwater. This led to modifications in the production strategy, with several groups held in freshwater for an additional periods to allow for increased ATPase levels, better preparing the smolt for seawater transfer.

• The 2022 smolt year class showed significant improvements in growth performance and failed smolt rates compared to previous years.

• Data collected in 2022 and beyond will be used to create a model for benchmarking and tracking performance over time.

• Development of a cost-effective and commercially viable smolt status assessment tool.

• Improved performance (mortality rate and time to 0.9 RFI) of new entry smolt post-transfer.

CHRISTINE HUYNH
CHRISTIAN VAN RIJN