Salmonids are fish of utmost social and economic importance but their numbers have considerably diminished over the last decades following overharvesting and habitat degradation. Humans have therefore initiated artificial reproduction of these fish very early and this practice is at the core of many conservation measures. However, some aspects of their reproduction are still to be fully understood, especially at the gametic level. In this thesis, we therefore studied sperm characteristics of males of three species (European grayling Thymallus thymallus, brown trout Salmo trutta and lake char Salvelinus umbla) in order to i) understand better the link between sperm quality and male reproductive strategies, female choice and offspring performance ii) study variation in sperm traits and signalling among populations iii) compare the sperm characteristics of wild and captive males and iv) optimize a sperm cryopreservation protocol. We also tested whether the use of captive fish for restocking and the cryopreservation of sperm produce genetic effects in the offspring, and whether natural populations have the potential to evolve tolerance to micropollutants. We found that, in all the species studies, males signal their sperm quality either through melanin- or carotenoid-based colouration. However, we showed in char that more ornamented males produce sperm that is better designed to swim in ovarian fluid, which confer them an advantage under sperm competition. Our findings are thus in accordance with theories that predict males displaying more ornaments invest less in sperm because their sperm are more suited to swim in the vicinity of female eggs. Interestingly, we found that signalling does not vary among population of brown trout but that their sperm characteristics differ and the distance that separate them explains these differences. In a captive brood stock used for restocking, we found that sperm of males was of better quality that of wild males and its quality was signalled in a opposite direction. These males did not produce offspring that perform worse that their wild counterparts, suggesting that relaxation of natural selection may not have already happened in one generation. We also showed that a recent sperm cryopreservation protocol yielded higher fertilisation success than a commercial widespread alternative, but that offspring produced with cryopreserved sperm were nonetheless less performant at early stage than those produced with fresh sperm. Finally, we showed that seven population of brown trout do not have the genetic potential to evolve tolerance to two common pesticides, that embryos are affected even by tolerated concentrations and that their susceptibility is positively linked to egg size. Some of the findings reported in this thesis are novel; there was no report of the relationship between sperm and male phenotype in grayling before, we report for the first time sperm phenotypic isolation by distance, and we show for the first time detrimental effects of sperm cryopreservation on offspring performance while controlling for parental effects with a high level of replication. Alteration of sperm characteristics and sperm competition dynamics under artificial reproduction may have consequences that are underestimated in restocking programs. Our findings highlight that understanding sperm biology of salmonids is therefore crucial to minimize unintended selection during artificial breeding.