Diel changes in behaviour and habitat use by age-0 Atlantic cod (Gadus morhua L.) in the laboratory and field

Our understanding of the habitat requirement and spatial dynamics of juvenile Atlantic cod (Gadus morhua) is almost entirely based on daytime studies. Diel spatial patterns of age-0 juvenile cod over sand and eelgrass habitat were examined using a combination of infrared videography in the laboratory and repeated net-sampling in the field. Both lab and field data indicated that shoaling behaviour was most pronounced during the day over sand habitat. Under darkness, age-0 cod maintained less distance between themselves and a larger gadoid predator relative to light periods, regardless of habitat. The vertical distribution of prey was also dependent on light level, with prey staying below predators under high illumination and above predators under low illumination. Seine sampling conducted at night in the field also indicated a more even distribution (i.e., less aggregated) of age-0 juvenile cod across habitats compared to day collections. Together, these data suggest that age-0 juvenile cod adjust their spatial use within and across habitats on the basis of diel changes in perceived predation risk.