ACTIVITY AND MOVEMENT PATTERNS OF JUVENILE ATLANTIC COD, GADUS-MORHUA, IN CONCEPTION BAY, NEWFOUNDLAND, AS DETERMINED BY SONIC TELEMETRY

Cod tracked between June and early September (summer) were wide ranging (>3 km/day), nocturnally active, and migrated daily between deep (30 m) cold water where they were inactive and shallow (<15 m) warm water where they fed. Cod tracked between mid-September and December (autumn) stayed in shallow (<20 m) water where they were active in relatively small (545.3-2581.6 m2) home ranges during daylight hours and inactive at consistent resting sites at night. Home ranges were over sand whereas resting sites were generally in rocky areas. The seasonal change in migratory behaviour coincided with the disappearance of the shallow (<30 m) summer thermocline. When the water column became isothermal over the depth range of juvenile cod, they remained in shallow feeding areas throughout the diel period. The summer diel migration is probably a strategy to increase energetic efficiency. The switch from nocturnal to diurnal activity may be an antipredator strategy. -from Authors