Size matters with respect to the effects of suboptimum summer conditions on Atlantic salmon production

Aquaculture of Tasmanian Atlantic salmon occurs in a geographical area where water temperatures occasionally approach the upper thermal tolerance of the species and where climate change effects are ahead of global trends. This represents a concern for the local producers as well as being of global interest, particularly in the context of evolving aquaculture practices. In this study, we investigated the impact of long-term suboptimum conditions on seawater production characteristics of both small (Experiment 1; ∼420 g average size) and large (Experiment 2; ∼2600 g) Atlantic salmon from the same breeding cohort and genetic lines (∼170 families). Each experiment was undertaken in recirculating aquaculture systems and consisted of three periods: 14 days of acclimation (15 °C and 100 % dissolved oxygen -DO-); 67–72 days of exposure to suboptimum conditions (19 °C and 80 % DO); and a 40–42 day recovery period (15 °C and 100 % DO). All fish were PIT-tagged and individually measured at the pre-acclimation, post-suboptimum conditions, and post-recovery at the end of the experiment. Feed intake decreased during exposure to suboptimum conditions in both fish sizes but recovered during the recovery period. There were significant differences in specific growth rate, feed conversion ratio and condition factor due to experimental conditions and fish size. Machine learning analyses identified a significantly greater impact of long-term exposure to suboptimum conditions on large fish as compared to small individuals. The translational approach used in this study improves our understanding of the effects of suboptimum summer conditions on Atlantic salmon and provides insights on practical size-based mitigation strategies and a baseline to produce predictive models.