Is cod lekking or a promiscuous group spawner?

The mating system of cod (Gadus morhua) has been described as promiscuous with few options for exercising mate choice. We, on the other hand, argue that cod fulfil the prerequisites of a conventional lekking mating system. A review of papers on cod mating behaviour suggests that mature males aggregate and possibly defend small territories on the spawning grounds, where male male interactions are common. Females visit males at the aggregations when ready to spawn and males show an extensive courtship display including dance, fin display, and acoustic calls. This may allow females to discriminate between males and both fin size and mass of drumming muscles, the cod's acoustic organ, are related to a male's fertilisation potential. This, together with the presence of sexual dimorphism in drumming muscles, suggests that sexual selection by females is at work. Different courtship display combined with different mate preferences may be important premating mechanisms reducing or preventing interbreeding between groups of cod.