Triploid Atlantic salmon in aquaculture - Consequences for fish health and welfare under farming conditions. Scientific Opinion of the Panel on Animal Health and Welfare

Sammendrag

Background and terms of reference Farmed Atlantic salmon (Salmo salar) that escape into the wild could interbreed with native fish, posing a potential risk to the genetic diversity of wild Atlantic salmon populations. The Atlantic salmon in aquaculture are diploid, meaning the fish has two sets of chromosomes. To mitigate the genetic impact on wild populations, the concept of producing sterile triploid farmed Atlantic salmon has been suggested as a solution. However, it is important to ensure that the utilization of triploids in commercial farming aligns with the regulations set forth in the Norwegian Animal Welfare Act. The Norwegian Food Safety Authority (NFSA) requested the Norwegian Scientific Committee for Food and Environment (VKM) to do an assessment about health- and welfare consequences in triploid Atlantic salmon under commercial farming conditions, as compared to diploid counterparts. VKM was also requested to describe the underlying physiological mechanisms concerning consequences of triploidy as well as address potential measures to reduce the negative impacts on the health and welfare of the fish. Methods A working group consisting of members with expertise in salmonid biology, aquaculture systems, veterinary medicine, fish health and welfare, virology, bacteriology, parasitology, breeding and genetics has drafted this opinion. To answer the Terms of Reference as mandated by the NFSA, the authors addressed fish health and welfare as a unified concept in this report. Two external experts have reviewed and provided their opinion before it was assessed and approved by the VKM’s Panel on Animal Health and welfare. The literature used in this work was peer-reviewed studies retrieved from a search in four databases as well as non peer-reviewed reports. Selection of studies was conducted independently by two members in the working group and based on predefined inclusion and exclusion criteria. Conclusions Under commercial farming conditions, triploid Atlantic salmon are often found to have lower standards of health and welfare compared to diploids. For example, field and experimental studies have found triploids to be more prone to skeletal and heart deformities, and cataracts, while field studies suggest that under commercial farming conditions they cope less well with handling and are more susceptible to skin ulcers. However, research has indicated that some of the effects of triploidy can be mitigated through specialized diets or environmental adjustments. There is a noticeable tendency across farm studies and experimental trials for triploid salmon to be equal or larger in size at the end of freshwater phase, but equal or smaller in size at the end of the seawater phase. Most publications conclude that within what is considered the optimal temperature range of diploids, oxygen consumption rate, oxygen binding capacity, and aerobic swimming capacity do not significantly differ between triploid and diploid Atlantic salmon. However, findings from experimental trials suggest a lower optimal temperature range for triploids, and data consistent across studies indicate that triploids possess lower tolerance to hypoxia at elevated temperatures. Triploid Atlantic salmon are less robust to higher water temperatures than diploid, and have other nutritional needs than diploids, especially regarding phosphorus, and histidine. There are few studies on the susceptibility of triploid salmon to infectious agents and diseases. Field observations indicate that triploid fish are more susceptible to primary infectious salmon anaemia (ISA) outbreaks than diploids under commercial farming conditions at the level of the farm, and at cage level within farms that experience an ISA outbreak. A higher susceptibility to the ISA virus would potentially affect not only the health and welfare of the triploid fish at the farm with an outbreak but may potentially spread to other farms. .............