New research at NMBU reveals how and why precise water temperatures are important for proper development of Atlantic salmon organs during the early stages of their lives, with strong implications for the aquaculture industry.
Carefully controlled temperatures play a pivotal role in all sorts of production processes.
Take, for example, baking. There are certain baked goods that, even for the most skilled chef, remain notoriously complex and difficult to master without pinpoint precision and carefully monitored oven temperatures.
This includes the French macaron, an intricate meringue-based delicacy where small temperature deviations at the wrong moment can be the difference between patisserie-level success and a disappointing tray of deflated shells. Croissants, soufflés and chocolate are other examples where temperature control during preparation can have a major impact on the taste, look and quality of the finished product.
Beyond food, there are many complex production systems where temperature and timing are critical to success or failure. Heating and cooling cycles determine whether steel becomes flexible and resilient or brittle and fracture-prone, with obvious implications for the integrity of structures with steel components. Subtle thermal variations during glass making can create structural weaknesses that aren’t visible by eye but affect the quality and strength of the finished glass product.
Early salmon development: complex and temperature sensitive
The processes that guide the early developmental stages of Atlantic salmon, where a single fertilized fish egg is transformed into a multicellular embryo that will go on to become an adult fish, are incredibly complex. Even macaron baking and steel production look simple in comparison. Early stem cells in the embryo divide and become more specialized to perform specific roles in a carefully controlled and coordinated manner, ultimately producing separate collections of specialised cell types which combine to form functional organs.
The Salmocode project, a collaborative Norwegian research initiative coordinated at NMBU, is revealing that carefully controlled temperature and growth conditions are critical for these early developmental stages in salmon and have major consequences for adult salmon later in life.
The project also includes partners Nofima, the Arctic University of Norway, NCE Aquaculture, Aqua Kompetanse, Mowi and Benchmark Genetics. Its findings have important implications for the aquaculture industry, where higher water temperatures are often used in early life stages (egg and larvae), as this is known to speed up fish growth.
Tweaking temperatures in early salmon development
As for most fish species, salmon egg fertilization and embryo development occur ‘externally’, i.e. in an exposed environment, meaning these processes are particularly sensitive to alterations in surroundings – much more so than in animals that spend these first stages cut off from their environment prior to birth. This can be advantageous to salmon in the wild, enabling fish to adapt to the environments they will face in later life. It is also something the aquaculture industry has learnt it can use to its advantage by tweaking water temperatures to optimize growth rates.
Unfortunately, many salmon produced in aquaculture today are lost before they reach slaughter size (more than 90 million each year). This scale of loss reflects a serious ethical problem that limits the environmental sustainability of aquaculture, as well as restricts economic gains. Many such losses are thought to be caused by poor organ health, including the heart, kidney and gills.
Studies have indicated that increased water temperatures in early development are correlated to abnormal organ development and weaker immune responses in adult salmon. However, the precise timing of organ development during early salmon growth is not fully understood, and so the exact effects of the farming industry’s ‘artificial’ growth conditions on the development of different organs has been difficult to pinpoint.
A ‘roadmap’ for early salmon development
Salmocode has been running since 2023 and aims to build the first developmental ‘roadmap’ for Atlantic salmon, plotting how and when different cells and organs arise and grow as mature salmon develop, and how these processes are disrupted by increased water temperatures. Salmocode thereby offers a route to answer key questions facing the aquaculture industry today:
• What early production conditions, such as increased water temperature, affect organ development in salmon?
• When and where can detrimental effects in organ development first be detected?
• What is the longer-term impact of suboptimal organ development on the robustness of the fish – does this affect salmon welfare and ability to reach maturity or does it have little to no effect?
Ultimately, answering the above questions will help to solve the most important issue of all; how can we maintain optimal salmon production to meet the demands of a growing world population, whilst ensuring we give farmed salmon the best possible start in life?
Cell-by-cell analyses help paint a better and more detailed picture of developmental processes
To track the highly complex processes guiding organ development in salmon, the Salmocode project uses an approach called single cell sequencing, which allows cell-by-cell analysis of gene expression patterns in nearly all cell types in the developing embryo. Project leader Dr Christiaan Henkel (NMBU) explains the advantages to this approach:
“Single cell sequencing allows us to track the development of different organs, one cell at a time. It gives us a resolution of the developmental picture that isn’t possible using more conventional approaches, where large numbers of cells are processed in bulk at the same time, so it’s impossible to distinguish different cells destined for different roles in separate organs.”
Using single cell sequencing has enabled Salmocode researchers to assess with greater precision which production conditions affect organs at different stages in their development. In turn, this will help to better understand the long-term effects of various growth conditions on salmon health and robustness.
The most detailed picture yet
As the project reaches its conclusion in 2026, a number of important achievements have been made, Dr Henkel continues:
“We have now created a roadmap covering 30 developmental stages – from fertilized to ‘eyed’ eggs (so-called because you start to see the developing embryo’s eyes, which happens prior to egg hatching). This roadmap represents data from 140,000 cells, which belong to the approximately 50 broad cell types that emerge during early development."
This is the most detailed picture of the temporal development of organs in salmon available, and will be highly valuable to future research efforts towards improved salmon farming.
When comparing different eggs grown under different conditions, the project’s results align with earlier findings on the effects of increased water temperatures. Project partner Dr Erik Burgerhout (Nofima) explains:
“Where eggs were grown at close to the ‘natural’ temperature they would experience in the wild (4°C), survival was high. However, in experiments performed at the same time at elevated temperatures, mortality was very high, and concentrated around specific developmental time points, which we see as developmental bottlenecks where only the most robust salmon survive."
The researchers are now analyzing organ development in survivors of these bottlenecks, with the hypothesis that they might showcase temperature-induced deviations in healthy development. Such deviations could, in turn, have a negative impact on resilience of the salmon later in life.
Translating developmental insights into better salmon farming routines
The results achieved so far open up a range of exciting possibilities for the final phase of the project, and beyond. With more data generated across more developmental stages, collected using different combinations of temperatures and other environmental factors such as light exposure, this will identify with greater clarity the key bottleneckswhere water temperatures cause the mis-regulated development of different organs.
In addition, the cellular roadmap can immediately be applied for diagnosing developmental problems arising from, for example, broodstock treatment protocols. In future efforts, it could even be used to inform breeding strategies.
Project partner Ann-Cecilie Hilling (NCE Aquaculture) concludes: “This would enable the establishment of protocols that will aid the industry in finding optimal conditions for egg rearing, preventing developmental problems and maintaining economically favorable production conditions. But most of all, ensuring that salmon has a good quality of life, right from the beginning."
The researehrs are are working closely with industry, who have been involved as partners in the project and are keen to incorporate the results into their farming practices.
Much like the precise recipes and methods that must be followed to get the best results in man-made production systems, clearly the aquaculture industry can use insights from the Salmocode project to help finetune their farming practices and safeguard healthy fish development from the very first stages.
The Salmocode project is funded by the Norwegian Seafood Research Fund (FHF). You can learn more on the FHF website, and watch a short popular science video about the project on the Nofima Youtube channel.