Interactions between age-classes of Atlantic cod and their distribution among bottom substrates

When settling out of the water column, age 0+ Atlantic cod (Gadus morhua) enter a habitat already occupied by older conspecifics. To elucidate the nature of possible interactions between age-classes, we examined the influence of conspecifics on use of two combinations of bottom substrates by age 0+ and 1+ cod: sand and cobble, and sand and gravel. Substrate use by age 0+ and 1+ cod was examined in the absence and presence of a predator (age 3+ cod), when each age-class was either alone or together in the laboratory tank. Regardless of substrate combination, age 0+ and 1+ cod used sand when there was no predator. However, with a predator, both 0+ and 1+ cod either avoided the predator (sand and gravel) or hid in the cobble. Substrate use by the age 1+ cod was the same when together with age 0+ cod as when alone; in contrast, age 0+ fish avoided age 1+ fish, resulting in a significant increase in use of gravel or cobble. When a predator was present, avoidance of age 1+ cod by age 0+ cod disappeared; the presence of age 1+ cod significantly reduced predation on age 0+ fish. Our results suggest that some level of habitat segregation should be expected between age 0+ and 1+ cod.