The Atlantic salmon is a keystone species for natural ecosystems and human communities, but wild stocks have declined by ~90% to their lowest recorded levels. One cause of this collapse is salmon farming, the world's fastest growing form of food provision. At least 95% of Salmo salar on our planet are now reared in farms, but hundreds-of-thousands escape each year to the wild. These escapees can survive, disperse hundreds of miles, enter wild spawning populations, and ultimately reproduce. Farm fish are different to wild salmon, having been intensely domesticated since the 1970s for rapid growth and feed conversion in cages. If they reproduce with wild salmon, farm fish erode and disrupt wild-adapted gene pools, causing ecological destabilisation through loss of important locally-adapted traits like disease resistance, phenology and growth. This impact from aquaculture is described by the BBSRC-NERC call as 'the most controversial contemporary issue in Atlantic salmon farming.' Big numbers of farm fish have been found in some wild salmon spawning populations, and evidence from across the Atlantic shows that rivers near farms have had the greatest wild salmon declines, by about 50% each generation. We propose to scientifically test and verify a potential solution to the problem of farm x wild reproduction: triploid sterilisation. Triploid induction of just-fertilised fish eggs, by applying pressure to cause retention of the second set of maternal chromosomes, produces fish that are likely to be reproductive dead-ends. Triploidy is routinely applied in trout farming, to prevent stocked fish from introgressing non-native ecosystems. Although triploid fish try to spawn, gametogenesis in most species is usually disrupted and females are often sterile. However, triploid induction does not necessarily sterilise males. Detailed studies on plaice and tench reveal that triploid males produce fully motile sperm that can fertilise haploid eggs almost as effectively as sperm from diploid equivalents. It is therefore essential that triploidy is fully verified in both sexes of a species, including under sperm competition, before we can be confident that triploid males pose no reproductive threat to wild salmon spawning populations. Even if triploids cannot produce viable offspring, large numbers of escapes could impact on wild fish by 'occupying' eggs and sterilising the reproductive potential of wild females. We will therefore fully evaluate the reproductive function of triploid farm Atlantic salmon, thereby proving biosecurity. We know that male triploid salmon show normal breeding behaviour, can induce females to spawn, and release milt. However, information on the fertilisation and reproductive potential of triploid salmon is lacking. The only scientific study of triploid adult Atlantic salmon reproduction examined just a single male, showing that it was fertile but its offspring had poor survival. We will therefore conduct detailed scientific trials on triploid male fertility, using established techniques that measure sperm and egg performance in a range of relevant conditions to assay triploid reproductive function. We will trial the performance of triploid males in sperm competitions (both in vitro and between competing males), because the salmon mating pattern is naturally promiscuous. Our experiments will generate meaningful results that will allow a full and detailed assessment of the reproductive impact of triploid farm salmon when they escape into wild spawning populations. The salmon farming industry is now in a position to embrace triploidy, since research reveals that triploids can perform as well as diploids under the right farm and diet conditions. Triploid salmon are just starting market trials, so our project is perfectly timed to assess this solution to farm x wild introgression. We will ensure that our research achieves impact, by disseminating findings to the public, policy-makers, NGOs, and salmon farmers.