The behavior of cod (Gadus morhua) in an offshore aquaculture net pen

摘要

Despite the rapid growth in cod (Gadus morhua) aquaculture, relatively little is known about the fundamental aspects of cod behavior within net pens. To investigate some aspects of their behavior, we used a high resolution ultrasonic telemetry system that makes it possible to continuously and accurately monitor the position of multiple fish, in 3D, for days at a time. The data presented in this manuscript were obtained from a total of 32 cod (29.3A plus or minus 4.9cm, range: 22.5-43.0cm), tracked under a variety of conditions, inside a net pen located 13km off the coast of New Hampshire, USA. Typically, cod exhibited clear diurnal rhythms, with the highest swimming activity during daytime hours (mean swimming speed=17.8A plus or minus 5.5cm/s; 0.6A plus or minus 0.2 body lengths per sec (BL/s)) compared with nighttime hours (6.6A plus or minus 0.5cm/s; 0.2A plus or minus 0.03BL/s). Analysis of net pen utilization revealed that: a) the entire volume of the net pen was not utilized (volumes used by individuals overlapped); and b) there was a spatial preference for the lower half of the net pen. Adding lights and increasing stocking densities elicited dramatic changes in behavior. When lights were turned on at dawn and dusk, to extend the day photoperiod, cod increased their swimming activity by 66.4% and 202.6%, compared to day and night levels, respectively. When the density of cod reached high levels (~48.5kg/m3) as they grew, their typical diurnal pattern of independent swimming changed to schooling behavior, with no significant difference in day and night activity. The results of this study could be used to optimize various aspects of cod aquaculture including: net pen geometry, feeding schedules, stocking densities, and the use of artificial lights. Implementation of these modifications could increase production efficiency and animal welfare.