Skjæraasen, J.E.; Mayer, I.; Meager, J.J.; Rudolfsen, G.; Karlsen, Ø; Haugland, T. and Kleven, O. (2009). Sperm characteristics and competitive ability in farmed and wild cod. Marine Ecology Progress Series 375:219-228

The development of cod aquaculture has raised concerns about its effect on wild stocks. One risk is hybridisation between escapees and wild cod, causing genetic introgression, and, potentially, fitness depressions in wild populations. The potential for hybridisation depends on escapees´ success in mating competition with wild fish. Cod have a complex mating system, with males likely to adopt either dominant or sub-dominant roles, the latter typically achieving reproductive success through sperm competition. Studies on salmonids indicate that domesticated males predominantly adopt sub-dominant roles. We therefore analysed sperm characteristics of wild and farmed cod around the onset and ending of the natural spawning season. Wild and farmed males were also paired in in vitro crosses to assess their reproductive success in sperm competition. In the early spawning season wild males had higher sperm velocity, percentage of motile and progressive cells, and spermatocrit. Sperm velocity was the main determinant of fertilisation success in the vitro sperm competition and, accordingly, wild males had higher reproductive success. At the end of spawning the percentage motile or progressive cells and spermatocrit were similar between wild and farmed males, but wild males maintained higher sperm velocity. Our results indicate that farmed males have limited reproductive success in sperm competition with wild cod. This presumably reduces the risk of genetic introgression from escapees. We hypothesise that the impaired sperm quality, and lower reproductive fitness, of farmed cod is due to inhibition of the full behavioural repertoire and lack of social structure under culture conditions, and/or nutritional deficiencies.

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