Abstract
The rapid development of cod aquaculture has raised concerns about how this industry will affect local coastal cod. One area of concern is that escapees may hybridise with wild cod, causing genetic introgression, which can cause fitness depressions in wild populations. The risk of hybridisation will depend on the escapees overall performance in mating competition with wild cod. Sperm traits will clearly influence males' reproductive success. We therefore analysed sperm velocity, percentage of motile cells and spermatocrit of wild and farmed males at the onset and the end of the spawning season. Wild and farmed males were also paired in in vitro crosses. Wild males had higher sperm velocity, percentage of motile and progressive cells and spermatocrit values at the start of the spawning season. The in vitro crosses showed that sperm velocity was positively correlated to fertilization success and concurrent with their faster sperm, wild males had higher fertilization success. At the end of the spawning season there was no difference between males in percentage motile or progressive cells or spermatocrit, but wild males still maintained a higher sperm velocity. Our results indicate that farmed males have limited reproductive success in sperm competition with wild cod. This likely reduces the risk of hybridisation between male escapees and wild females. Lack of social structure in fish farms or nutritional deficiencies might cause these differences between wild and farmed cod.