Rainbow trout (Oncorhynchus mykiss) sperm collected post-mortem could be revitalised by short-term storage

Post-mortem semen collection in fisheries is a frequently used breeding method for wild fish species with high mortality rates after capture. Moreover, breeders are interested in the possibility of using milt from fish that have died during the spawning season. This study aimed to investigate the possibility of revitalising rainbow trout (Oncorhynchus mykiss) sperm with the use of short-term storage of undiluted and diluted semen samples collected from the fish post-mortem. We found that the pH decreased rapidly in the semen within 8 h of the fish's death, while lactic acid concentration increased simultaneously. We found that nearly immotile rainbow trout sperm collected from the fish body 2, 4, 6, and 8 h after its death regained their motility as a result of short-term storage. In the samples taken 10 h after the fish's death, sperm revitalisation was significantly lower compared to the samples taken 2, 4, 6, and 8 h after the death of the fish. After this time, only samples diluted with artificial seminal plasma (ASP) were able to regain about 20% motility. Completely recovered motility was observed when the semen was collected within 2 h after the fish's death and the samples were diluted in an ASP solution. Our in vitro study showed that sperm exposed only to low pH (7.5) did not decrease their motility within hours, as it was observed in vivo. We found that the addition of different lactic acid concentrations to semen samples in ASP with low pH (7.5) caused a significant and rapid decrease in sperm motility, which is reversible, however, only when not more than a 10 mM concentration of lactic acid was used. In conclusion, we believe that the combined effect of pH and lactic acid accumulated in the gonads after the fish's death may be responsible for the permanent immobilisation of rainbow trout sperm.