Genetic variation in growth pattern within a population of farmed Atlantic salmon (Salmo salar) during a standard production cycle

Selection for faster growth to market size has a long history in selective breeding programs for aquaculture species. In Atlantic salmon, selection for faster growth has been carried out for more than four decades, i.e. about ten generations. Genetic improvement of growth rate is indicated as favourable correlated to improved feed utilization and, from an economic perspective, it can be argued that improved feed utilization is the main breeding objective: improved fish production per unit of nutrients (protein or energy) provided. However, also reduced production time in challenging grow-out environments, i.e. reduced exposure to sea lice and other disease agents, may be highly valuable from an animal welfare and environmental perspective. Reduced production time in challenging grow-out environments may also be economically favourable by reducing the need for mechanical or chemical treatments of the fish.Body weights at stocking and harvest are commonly recorded in breeding programs for Atlantic salmon. Selection for faster growth rate is usually based on recorded harvest weights, which represent the accumulated growth during the entire production cycle. Alternatively, selection could be based on other growth parameters, i.e. Thermal Growth Coefficient (TGC), which specify growth in specific production periods. Production of Atlantic salmon in open sea cages entails large variation of environmental parameters, i.e. water temperature, water quality, salinity, day light etc., that influences the growth.In this study, growth of individual Atlantic salmon in an open sea cage were investigated (as a part of the project BEHAVEGENES, NFR 2014–2017) to describe potential GxE for growth in different production periods. Test fish were transported to sea in autumn, 8 months after first feeding, and harvested after 16 months at a mean harvest weight of 5.5 kg. Individual body weights were recorded at five different time points allowing estimation of growth during a total of four subsequent production periods. Sexual growth dimorphism was discovered well before onset of sexual maturation; already 4 months after sea transfer, at average population body weight ~500 g, males were measured on average 2.8% heavier than females. Heritability estimates for body weight were all high (0.42–0.72), while heritability estimates expressed by growth calculated as TGC were mediate to high (0.35–0.56). Genetic correlations between growth (TGC) in different production periods varied between −0.20 and 0.82. The correlation between growth during an 8-month summer period (average body weights increase from 500 g to 3.9 kg) and a 4.5-month winter period (average body weight increased from 3.9 kg to 5.8 kg) was not significant (rg 0.16 ± 0.17), suggesting genetic variation for growth pattern and that different genes are involved in different growth periods.