Abstract
The use of reproductively sterile triploid salmonids would enhance the environmental sustainability of the aquaculture industry by preventing genetic exchange between escapees and wild conspecifics. To this end, we assessed smoltification and early seawater performance (241 days) following a yearling production cycle (i.e. spring smolts) in diploid and triploid female Atlantic salmon (Salmo salar) × male brown trout (Salmo trutta) hybrids compared to purebred diploid and triploid salmon. During freshwater rearing (n = 180/group), hybrids demonstrated a degree of bimodality in body size, significantly (p < 0.05) more so in diploid than triploid hybrids (11 and 37% in the lower mode, respectively) that was not seen in purebred salmon of either ploidy. This resulted in diploid hybrids being 66% smaller on average at sea transfer, whereas no hybridisation effect was seen in triploids, and both triploid groups were significantly heavier (16–43%) than diploid salmon. Irrespective of ploidy, lower mode hybrids grew poorly and showed low survival in seawater, suggesting they had failed to undergo smoltification. However, the upper mode diploid hybrids showed a similar Na+/K+-ATPase (NKA) enzyme activity surge during the spring as in diploid and triploid salmon, despite a higher ratio of the freshwater to seawater mRNA abundance of the NKA subunits (nkaα1a and nkaα1b) and a reduced plasma cortisol surge. At the end of the experimental period, both hybrids weighed significantly less than their salmon counterparts although the hybrid effect was again greater in diploids (71% smaller) than triploids (6% smaller). In addition, both triploid groups were on average heavier (15–22%) than diploid salmon. As such, both triploid Atlantic salmon and triploid hybrids can show enhanced growth performance from juveniles up to post-smolts compared to diploid salmon in an aquaculture setting.
Highlights
Due to artificial selection for on-farm performance and the subse quent loss in suitability for a life in the wild, escapees from aquaculture threaten both the genetic integrity and survivorship of natural pop ulations
Data on the time the individual was within the experiment in days, group (4 levels; diploid salmon, diploid hybrid, triploid salmon, triploid hybrid), and the outcome at the time of leaving the study were included
Initially we modelled body size based on all individuals measured at a given timepoint, whereas in a second analysis we assessed body size at all time points in only those fish that survived until the end of the study
Summary
Due to artificial selection for on-farm performance and the subse quent loss in suitability for a life in the wild, escapees from aquaculture threaten both the genetic integrity and survivorship of natural pop ulations. The technology to mass produce trip loids has existed since the 1970’s, their farm performance has often been more inconsistent than the natural diploid state with greater frequencies of skeletal deformities and higher mortalities (e.g. McGea chy et al, 1996; Fraser et al, 2013). This inconsistency is generally attributed to the physiological differences between the two ploidy, as triploids have lower temperature optima (Sambraus et al, 2017a, 2018) and altered nutritional requirements (e.g. phosphorus Fjelldal et al, 2016 and histidine Taylor et al, 2015). There has been a general reluctance by the industry to use triploids without further im provements to their farm performance (Benfey, 2015), they have been used for several decades in all-female stocks in Tasmania (Australia) to prevent the growth reduction that occurs with sexual maturation (Amoroso et al, 2016)
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