Growth and welfare of germ cell-free salmon- the sustainable solution for genetic containment of farmed fish.

Using germ cell-free (GCF), sterile, dnd-knockout farmed salmon for farming could solve the problems associated with precocious maturation and genetic introgression of farmed breeds into wild populations. However, prior to using GCF fish in the salmon farming industry, it will be crucial to understand if, or how, the GCF phenotype differs from wild type (WT) counterparts in terms of growth and welfare. To elucidate characterize the GCF phenotype in relation to WTs throughout a production cycle, we have in this project reared GCF and WT salmon in indoor common garden tanks for 3 years, until harvest size. Regarding body size, smoltification markers (mRNA levels of gill Na+/K+-ATPase [NKA] subunits), plasma stress indicators (pH, glucose, sodium, chloride, calcium), relative heart size, prevalence of vertebra deformities and fillet proximate composition, GCF fish could not be distinguished from WTs. Transient differences were detected in plasma concentrations of lactate and osmolality, and only a few genes were differentially expressed in WT and GCF transcriptomes of muscle and pituitary. Towards harvest size, body growth rate, condition factor and relative heart size were higher in WT than in GCF fish, probably relating to initiation of puberty in WTs. Since GCF salmon never become sexually mature, it might be beneficial to postpone the time of harvest to exploit the growth potential uninhibited by sexual maturation. Interestingly, at harvest, the relative content of omega-3 fatty acids were higher in GCF compared to WT males. In conclusion, GCF salmon performed to a large extent similarly to their WT counterparts, but had the clear advantage of never maturing.

- Vi har observert at den kjønnscellefrie laksen har god velferd i forhold til smoltifisering, vekst og deformiteter - Den kjønnscellefrie laksen går ikke i kjønnsmodning og er derfor en god løsning å bruke RAS system der tidlig kjønnsmodning er et stort problem - Den kjønnscellefrie har god filetkvalitet

The genetic impact of escaped farmed salmon on wild populations is regarded as a major environmental risk, being one of the reasons preventing expansion of the Norwegian salmon industry. The use of sterile fish would solve this problem. Currently, functionally sterile triploid salmon are tested for commercial use. However triploid salmon are more prone to develop deformities and triploid males still go through puberty. Clearly, alternative models should be evaluated. We have recently produced germ cell-free salmon by editing a single gene, and vaccination/knockdown in salmon targeting the protein product of this gene is currently being tested with the aim of developing an approach to generate germ cell-free salmon for industrial use. This should be accompanied by assessing key features of the performance of germ cell-free fish in a larger scale production system. This includes evaluating growth, robustness and welfare of these fish. Complete loss of germ cells in all vertebrates studied so far results in sterility, while other gonadal functions, such as sex steroid production and hence development of secondary sexual characteristics and sexual behaviour, remain intact. Interestingly, germ cell-free salmon different from all previous observations in other germ cell-free vertebrates do not enter puberty. Both sexes will be studied since somatic gonadal tissue in germ cell-free salmon still differentiates into testis or ovary. One of the prerequisites for the commercial success of a farmed fish is an excellent growth performance. Previous studies in teleosts showed that the gonad stimulates somatic growth by locally producing growth hormone. It is therefore essential to monitor if the loss of germ cells affects somatic growth. Initial results suggest that both growth and development of secondary sex characteristics are affected in germ cell-free salmon. Importantly, we observe low sex steroid levels throughout life in germ cell-free fish.