Researched what temperature does to salmon health and development

The aim was to investigate how temperature affects the health and development of salmon throughout their lifecycle. Temperature is a master regulator in fish biology, shaping their behaviour, physiology, health, welfare, and growth. Before salmon undergo smoltification — the physiological adaptation for life in seawater — temperature is a highly effective tool for managing growth rates.

In recent years, concerns have grown that the temperatures used during rearing may be too high. Possible adverse outcomes include poor heart health, and it has also been claimed that fast-growing parr (juvenile salmon) may not thrive after transfer to seawater.

The Norwegian aquaculture industry demanded answers, so the scientists conducted a study to address these points. "There was a lack of long-term experiments assessing the effects of temperature from smolt to harvest. We needed this to determine whether high temperatures early in life would impact salmon growth and health later on," says Anja Striberny, a scientist at Nofima – the Norwegian Institute of Food, Fisheries and Aquaculture Research.

The trial

Striberny and colleagues set up the experiment at Nofima’s aquaculture research station in Sunndalsøra at the west coast of Norway. This research station offers unique facilities for trials on both small and large salmon in environments with varying water quality.

Atlantic salmon were followed from 10 grams up to the smolt stage (up to 200 grams), with groups reared in water at 8, 12, or 14 degrees Celsius. To ensure industry relevance, both flow-through and recirculating aquaculture systems (RAS) were tested. Once smoltified, the fish were transferred to the seawater facility at Gildeskål in Northern Norway. Throughout the trial, the research team measured, weighed, sampled, and documented both the external and internal health and welfare status of the fish.

Most findings were as expected:

• All fish developed good seawater tolerance.
• In all groups, changes in day length influenced smolt development. This supports previous findings that photoperiod may matter more than temperature for smoltification.
• Survival in seawater was high in all groups.
• Not surprisingly, fish held at higher freshwater temperatures were significantly larger at seawater transfer: the average weight for the 14-degree group was 140 grams, compared to just 70 grams for the 8-degree group.

However, what did surprise the scientists was that, during the sea growth phase, fish from the higher temperature group continued to grow faster than those from the lower temperature groups.

It remains unclear if this is due to a higher growth rate during smolt production or simply a higher body weight at sea transfer.

Growth and survival are not everything, and the team is still analysing tissue samples and investigating molecular responses to look for "invisible" effects. Based on outer welfare indicators, there were only minor differences between the groups, but the negative tendencies observed in fish that had experienced high temperatures are important to highlight. There was a tendency for mild cataracts at the end of the seawater phase, and the hearts were slightly smaller than in other fish. Smaller hearts were also seen in fish reared in RAS.

Must be viewed in context with other findings

The Norwegian Seafood Research Fund (FHF) has financed the research, and Head of Research, Sven Martin Jørgensen, emphasises that these results must be seen in relation to findings from other projects funded under FHF’s research area “Robust Fish”:

“When several of the projects have delivered their recommendations, it will be important to compile the results so that the aquaculture industry can gain updated knowledge about which early production conditions produce a farmed salmon with the best possible health and performance,” says Jørgensen.

The main objective of salmon farming is food production, so the scientists also investigated whether early rearing conditions could affect the final product.

“Interestingly, the 12 and 14-degree groups showed a higher proportion of fish with superior quality, with a tendency towards more intense fillet colour”, says Striberny.

In summary, results from conventional smolt production between 10 and 200 grams show that temperatures up to 14 degrees are themselves unproblematic — and may even be advantageous for long-term performance. However, a word of caution for those who believe higher temperatures guarantee rapid growth in salmon:

“We only manipulated temperature in this trial. Density and water quality were kept well within recommended levels, and changes to those parameters could produce different results,” says Striberny.

The research was conducted in the project Temp-Intens in collaboration with the Arctic University of Norway (UiT), University of Bergen (UiB), Norwegian University of Science and Technology (NTNU), Mowi, Grieg Seafood, and Cermaq.