Field observations of the movements of locally adapted Atlantic cod (Gadus morhua) living in zero and sub-zero centigrade temperatures for half the year

Atlantic cod (Gadus morhua Linnaeus, 1758) can experience mortality at sub-zero (<0°C) temperatures in ice-infested waters, and many populations migrate to deeper and warmer areas during winter. An exception is the resident population in Gilbert Bay, Labrador, which annually experiences six months of sub-zero temperatures. We used the VEMCO Positioning System (VPS) to estimate fish locations to determine depth utilization, and seasonal movements of tagged Gilbert Bay cod ranging in size from 43 to 75 cm. This is the first study to directly monitor the movements of adult Atlantic cod for extended periods (months) in sub-zero temperatures. Gilbert Bay cod remained active under an ice-covered surface during the six months of sub-zero temperatures. During both warm (>0°C) and cold (≤0°C) periods, fish spent similar amounts of time not moving, moving slowly, and moving at medium and fast rates. They tended to utilize shallow depths, <10 m, beneath surface ice during winter, while utilizing deeper depths as well during other times of the year. Surprisingly, fish that resided within our tracking array year-round had greater cumulative swimming distances during sub-zero periods compared to warmer periods. No tracked fish died during winter further highlighting the cold-adaptiveness of this population, and its importance to biodiversity conservation.