Impact of three light smoltification regimes on performance and genetic parameters of traits in Atlantic salmon

Bjarne Ragnar Gjerde^1*Solomon Antwi Boison²David Hazlerigg³Trine Ytrestøyl¹Turid Mørkøre⁴Even Hjalmar Jørgensen³Anja Stringberny¹Simen Sandve⁴

• ¹Norwegian Institute of Food, Fisheries and Aquaculture Research (Nofima), Tromsø, Norway
• ²MOWI Genetics (Norway), Bergen, Hordaland, Norway
• ³UiT The Arctic University of Norway, Tromsø, Troms, Norway
• ⁴Norwegian University of Life Sciences, As, Akershus, Norway

A total 3000 Atlantic salmon pre-smolts from 100 families were individually PIT-tagged and distributed among six tanks. Fish in duplicated tanks were put on different light regimes, i.e. six weeks on 8L:16D, 12L:12D or 24L:0D followed by six weeks on 24L:0D. One week prior to seawater transfer in June 2021, body weight and external smolt status traits were recorded. Body weight, wounds and loosers were again recorded 135 days after seawater transfer (November 2021), and body weight also 300 days after seawater transfer (April 2022). Binary survival traits were defined for seawater Period 1 (day 0 to 135) and Period 2 (day 136 to 300), and time to death for the collected dead fish. The effect of light regime on body weight in November 2021 and June 2022 was not significantly different from zero. Fish on 24L:0D showed increased mortality after sea transfer with an accumulated mortality of 8.8% during the two first months, while it was only 0.7% and 1.0% for fish on 8L:16D and 12L:12D, respectively. In Period 1 the effect of light regime on survival was not significantly different from zero, but the collected dead fish on the 24L:0D regime experienced an on average 15.7 and 17.7 less day to death than the fish on 12L:12D and 8L:16D, respectively. In Period 2 mortality was very similar for fish on the three light regimes. For the other traits, the effect of light regime was not significantly different from zero. Most traits showed moderate heritability estimates but were low for healthy fish and for fish with wounds in November 2021. For survival the heritability on the liability scale was 0.09±0.03 and 0.22±0.06, and for time to death 0.09±0.03 and 0.12±0.04, in Period 1 and 2, respectively. The traits show negligible genotype by light regime interaction. Genetic correlations of survival with other traits were low to medium and not significantly different from zero. For obtaining genetic improvement in survival in the seawater period, direct selection for increased survival and growth during the first months the sea is probably better strategy than performing indirect selection for the studied smolt indicator traits