Male agonistic behaviour on Atlantic cod spawning grounds

The cod mating system has been described as a lek, a system where male- male aggressions are expected. However, observing aggressions directly in marine environments is challenging. So far, the evidence supporting the importance of male – male aggressions for cod is mainly derived from laboratory studies with fish kept in close confinement, possibly elevating aggression levels and the perceived importance of such interactions. In natural aquatic systems, agonistic interactions between individuals may be inferred from acoustic telemetry transmitters fitted with accelerometers, given that such interactions are associated with high activity levels. We show that male-male agonistic interactions, as inferred from activity levels, are present on natural spawning grounds for Atlantic cod, especially in deeper waters, albeit at reduced levels compared to the levels exhibited in net - pen confinement. We hypothesize that the heightened activity levels of males compared to females when occupying deeper waters is related to the overall deeper distribution of males and territorial and agonistic behaviour at these depths, agreeing with the lek - description of the cod mating system.