Survival improves but big challenges remain in Norway, warn fish vets

The mortality rate in Norwegian salmon farms reduced last year for the first time since 2020, according to new figures from the Norwegian Veterinary Institute (NVI), but health and welfare challenges remain serious.

In the sea phase, 57.8 million salmon (15.4%) were categorised as “dead fish”, down from 62.7m salmon (16.7%) in 2023, the NVI says in its annual Fish Health Report. However, the number of fish lost and registered in the “other” category increased from previous years and was 4.6m fish (excluding reported counting errors).

The number of dead fry (over 3 grams) was reported to be 45.8m salmon and 3.4m rainbow trout in 2024, the highest recorded figures since such data were made available. NVI said the way the data is reported makes it difficult to calculate annual percentage mortality in the fry phase.

“There are still serious health and welfare challenges in Norwegian salmon farming, even though mortality in 2024 has decreased slightly,” said NVI acting director Ingunn Sommerset.

“Injuries as a result of lice treatments, wound problems and gill disease are the biggest challenges. However, we are seeing a positive development for several viral diseases. However, some bacterial diseases for which there are no good vaccines have had a negative development. We must monitor this closely.”

Diseases and lice treatments

The Fish Health Report is based on official data, disease statistics from diagnostic laboratories, results of an annual survey and data on causes of death for farmed salmon in Norway through the industry initiative AquaCloud. The latter is based on voluntary sharing of data from approximately half of all farming locations in Norway made available through a collaboration with industry body Seafood Norway and AquaCloud.

For fish farming locations that share data, “Infectious diseases” accounted for approximately 33% of the total, “Injuries” approximately 27%, “Unknown cause” approximately 21%, “Environmental conditions” approximately 9%, while the other categories accounted for a smaller share. “Environmental conditions” accounted for a much larger share in 2024 than in 2023 (approximately 3%). This increase can be largely related to damage caused by jellyfish, especially in Northern Norway, said NVI.

An annual survey of coastal fish health personnel, as well as inspectors and advisors at the Norwegian Food Safety Authority, ranked injuries after delousing as the most important cause of mortality, with gill disease, jellyfish and wound infections also being important causes.

Disease statistics show a continued decline in the number of cases of pancreas disease (PD) in 2024. Furthermore, the number of confirmed outbreaks of infectious salmon anaemia (ISA) is somewhat below the average for the last ten years. Both PD and ISA are viral diseases.

Winter sores

Of the bacterial diseases, winter sores continue to be a major problem, and although pasteurellosis has declined in Western Norway, outbreaks have been registered in Northern Norway in 2024. Furthermore, piscirickettsiosis (also known as salmon rickettsial septicaemia, or SRS) has been detected for the first time in several locations in Northern Norway. In addition, problems with bacterial kidney disease continued in Central Norway in 2024.

"The detections of piscirickettsiosis are probably related to high sea temperatures. If sea temperatures continue to rise, there is a risk that bacterial problems in general will increase in scope," said Torfinn Moldal, editor of the Fish Health Report.

Less ISA and PD

ISA was confirmed at 13 sites in Norway in 2024 and suspected at a further nine sites. In 2023, ISA was confirmed at 18 sites and suspected at a further five sites.

There were 48 new cases of PD compared to 58 cases in 2023 and 98 cases in 2022. All of last year's PD cases were within the endemic zone. Bacterial kidney disease (BKD) caused by Renibacterium salmoninarum experienced an upsurge in 2023 with detection at 12 sites. In 2024, BKD was confirmed at eight sites.

Sea lice remain the biggest parasite challenge for Norwegian aquaculture, said the NVI. Lice levels were lower in early 2024 than in previous years, but lice numbers increased sharply throughout the summer and autumn. For four weeks in September and October, the average was over 0.3 adult female lice per fish, which has not been recorded since 2014. High lice levels were particularly observed in Central and Northern Norway and were associated with abnormally high sea temperatures. The report said increased biomass of farmed fish in the same areas has also contributed to more lice and increased production of lice larvae.

Fish welfare

The number of welfare incidents at salmon hatchery facilities reported to the Norwegian Food Safety Authority increased in 2024 compared to 2023 and many were considered serious. Often these will involve human error, water quality and equipment failure, said the NVI.

There was a reduction in the number of welfare incidents reported from food fish or broodstock facilities in 2024 compared to the previous year. The three most common categories of welfare incidents at sea were "Medication-free delousing with handling", "Other" and "Unexplained mortality".

In the survey, damage in connection with handling-intensive delousing operations is once again ranked highest as a cause of reduced welfare in salmon food fish, while gill disease and jellyfish come in joint second place as a cause of reduced welfare. Measured in number of treatment weeks, there were more delousings in 2024 than in 2023. Mechanical and thermal delousings separately or in combination account for well over half of all treatments.