Introduction
Rainbow trout is Norway's second largest aquaculture species, yet it experiences substantial production losses in both freshwater and seawater phases, comparable to those observed in Atlantic salmon. Healthy development of rainbow trout is established during early embryogenesis, when temperature critically regulates organ formation. Deviations from an optimal temperature in this phase increase deformities and mortality, yet the effects of temperature fluctuations during key developmental windows remain insufficiently understood. Although high dietary lipid levels and selection for rapid growth promote increased fat deposition and altered body composition, the role of egg-stage temperature and whether males and females respond differently to these early-life conditions in terms of muscle development and fish quality, remains largely unknown. This study examines how embryonic temperature and early dietary lipid levels interact to shape growth, metabolism, and physiological outcomes in all female and mixed sex rainbow trout groups.
Material and methods
Fertilized rainbow eggs were incubated at two temperatures (6°C and 10°C) until hatching, after which all fish were reared under the same temperature regime. From first feeding to ~20g body weight, fish were assigned to three diets differing in lipid content (12%, 18%, and 24%) and to fish groups with all female or mixed sex. Growth, proximate body composition, and physiological parameters were monitored from early stages through seawater transfer and up to harvest size. The mitochondrial respiratory capacity in heart was evaluated at 20g and ca 3000g using a Seahorse Analyzer, while gene expression of muscle markers was investigated in muscle at 20g and 100g. Lipid deposition in different fat depots was analyzed by OilredO staining at 5, 20 and 100g.
Results and discussion
Temperature during the egg stage significantly affected growth and metabolic programming of rainbow trout. Prior transfer to seawater when the fish was approximately 100 gram, the all-female group had a higher SGR than mix sex group, and the 10°C group exhibited higher SGR than the 6 C group. There was no significant dietary effect on growth. However, body composition analyses revealed differences in lipid and protein deposition across dietary, temperature, and all���female versus mixed���sex groups, with the high���protein/low���fat diet resulting in higher protein deposition than the high���fat diets, particularly at 20 g.Both temperature, diets and all-female/mix sex affected gene expression of muscle markers in trout at 20 and 100g. The 6°C group showed enhanced mitochondrial oxidative capacity in cardiac tissue at 100 g, suggesting developmental programming effects. Dietary lipid levels had modest effects on early growth in freshwater but influenced performance in a temperature-dependent manner in sea cages. Increased dietary lipid levels enhanced growth in the 10°C group, in particular in the mixed sex group, while the opposite trend was observed in the 6°C group at slaughter. Differences between all-female and mixed-sex groups were also observed, with all-female fish generally exhibiting higher condition factors and lower average weight than the mixed sex group.
Conclusion
Early-life nutritional and temperature conditions influenced long-term growth, fat deposition, and physiological traits differently in mixed sex and only female groups. These findings demonstrate that embryonic temperature and early dietary lipid levels play a critical role in shaping long-term performance in rainbow trout. The results highlight the importance of early-life programming and suggest that optimizing incubation conditions and early feeding strategies can improve growth efficiency and product quality.
Acknowledgment
This work was funded by Norwegian Seafood Research Fund (FHF 901899).