Abstract
The mating system of the iconic Atlantic cod (Gadus morhua) has been described as a lek, a mating system where male-male agonistic interactions are expected. However, observing such behaviors directly in marine environments is challenging. So far, the evidence supporting the importance of agonistic interactions between males is mainly derived from laboratory studies with fish kept in close confinement, possibly elevating the observed levels and their perceived importance. In natural aquatic systems, agonistic interactions may be inferred from acoustic telemetry transmitters fitted with accelerometers, given that they are associated with high activity levels. A large telemetry dataset on Atlantic cod was collected during (i) a field study at two Western Norway spawning grounds from 2019 to 2024 and (ii) a close confinement net-pen study in 2019-2020. A total of 154 sexed mature Atlantic cod were tagged with acoustic transmitters fitted with accelerometers and depth sensors, yielding ~1.5 million accelerometer detections for subsequent activity analyses. In the net-pens, male cod showed much higher activity levels than females. Overall activity levels were reduced in the field, but male activity levels remained higher than those of females during the spawning period. A prominent feature of coastal cod spawning along the Norwegian coast is that males tend to dwell deeper than females. We therefore explored whether depth influenced activity patterns of males and females. This was indeed the case with males being more active than females at fish depths > 20 m and no sex-difference in activity levels closer to the surface (< 20 m depth). No sex difference in activity levels was found outside the spawning period. Our data suggest that male-male agonistic interactions, inferred from activity levels, are present on natural spawning grounds for Atlantic cod, albeit at reduced levels compared to levels exhibited in net-pen confinement. We hypothesize that the heightened activity levels of males compared to females when occupying deeper waters are related to their overall deeper distribution and territorial and agonistic behavior at these depths, agreeing with the lek description of the cod mating system.