Abstract
Atlantic cod (Gadus morhua), a commercially important fish species along the European coasts, displays complex behaviours, especially during the spawning season. This study offers a behaviour analysis of the cod during a period when they may be spawning, utilizing a fine-scale acoustic telemetry positioning approach in a controlled, man-made basin environment. The research primarily focuses on examining the effect of body size on swimming speed and depth use and diurnal variation in depth use. The study was conducted at the Flødevigen research station, Norway, where 45 cod varying in size from 43 cm to 80 cm were tagged and monitored in an outdoor basin from January to May 2022. The basin was about 50 m across with a maximum depth of four meters. The acoustic telemetry method enabled horizontal and vertical tracking of each cod's movements with the precision of a few tens of centimeters, revealing insights into their swimming speed and depth use. Key findings indicated that large cod tended to swim faster and occupy shallower depths compared to smaller ones, potentially due to their greater muscle mass and increased boldness. Moreover, all the individuals tended to occupy shallower depths at night compared to the day. Other results indicated that cod with lower growth rates were observed to move faster and occupy shallower waters compared to their faster-growing counterparts. All the individuals tended to swim faster at night. The study also revealed two interaction effects: the increase in swimming speed associated with larger body size was more pronounced at night compared to during the day, and the differences in depth use related to growth rate was primarily evident during daytime, with minimal differences observed at night. Body size and growth rate appear to be important factors influencing the behavior of cod during their spawning season.