28 DECEMBER 2023 • WORLD AQUACULTURE • WWW.WAS.ORG conditions (Crouse et al. 2021). Triploids are also used occasionally for rainbow trout (Oncorhynchus mykiss) and Arctic char (Salvelinus alpinus) production in Europe and Canada, inasmuch as these species tend to mature before they reach harvest size. Making triploids is relatively simple as numerous methods exist. The most common is subjecting eggs to a pressure shock for several minutes shortly after fertilization. This works because the unfertilized salmon egg contains 100 percent of the female’s DNA (two sets of chromosomes). Under natural conditions, 50 percent of the female’s DNA (one chromosome set) is then ejected from the egg shortly after fertilization. However, a pressure shock prevents this process, and it can be done to thousands of eggs simultaneously and optimized to be >99 percent efficient. In rainbow trout, it is possible to produce tetraploid individuals (individuals with 4 complete chromosome sets) and cross these with diploids to make triploids. The same method is currently not available for Atlantic salmon as tetraploid broodstock are not available, although we are trying, currently. Norway’s Experiment with Triploid Salmon: Small Scale Successes Related to the environmental concerns around escapees, the Norwegian salmon industry is being pushed to farm sterile fish, but triploidization is the only method currently capable of mass-producing them (Benfey 2015). New technologies are being developed, but they are classified as being GMO (Kleppe et al. 2022) and it is currently not possible to commercially farm these in Europe. Instead, there was a renewed interest in the late 2000’s to try triploids again with the hope of using our increased understanding of salmon biology to solve production bottlenecks. This work has been supported by funding from numerous governmental and industry sources (Table 1). Reports from the 90’s indicated triploids had more bone deformities and ocular cataracts than diploids (Figures 3A, B). The latter is when the lens of the eye becomes cloudy leading to visual impairment. Work in diploids suggested both were linked to nutritional dietary deficiencies, and bone deformities could also be linked to incubation temperature. So maybe it was the same in triploids? This turned out to be true. Triploids need a lower incubation temperature than diploids to prevent the occurrence of bone deformities (Fraser et al. 2015). They also need more dietary phosphorus, an important mineral for maintaining bone health in fish and mammals (Fjelldal et al. 2016). And they need more dietary histidine, an important amino acid, to prevent cataracts (Sambraus et al. 2017). If you correct for these, bone deformities and cataracts are no more of a problem in triploids than diploids. This comes at a price though, as both phosphorus and histidine are expensive dietary ingredients, excess phosphorus from waste feed TABLE 1. Projects funded which are either focused on, or include, the feasibility of using triploid salmon in Norwegian aquaculture. Project titles: 1Minimising the interactions of cultured and wild fish: A comprehensive evaluation of the use of sterile, triploid, Atlantic salmon. 2,3Feasibility study of triploid salmon production. 4Farmed escapees and interactions with wild conspecifics: quantification of genetic differences and simulating long-term fitness consequences. 5Side effects resulting from vaccination of salmon. 6,7Solving bottlenecks in triploid salmon production - a way to strengthen the sustainability of the salmon aquaculture industry. 8The molecular physiology of aquaporin-related cataract in farmed Atlantic salmon. 9Large-scale triploid. 10Mitigating the challenges in the Atlantic salmon aquaculture caused by salmonid alphavirus by unveiling the underlying immune mechanisms. 11Development and validation of methodologies for parentage assignment and traceability of Atlantic salmon using high-density genomic tools. 12Triploid salmon – susceptibility to infectious diseases. 13Production of triploid salmon from research to commercialization. 14Role of nutritional status for PD infection and recovery. 15Effects of the regulatory framework on fish welfare and health. 16Effects of salinity on growth and welfare of triploid salmon. 17Welfare of triploid salmon in Northern Norway. 18The influence of inbreeding and aberrant inheritance on welfare in gynogenetic, triploid, and tetraploid Atlantic salmon. Funders: EU, European Commission. RCN, Research Council of Norway. FHF, Norwegian Seafood Research Fund. AG, AquaGen AS. NRS, Norway Royal Salmon AS.
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