Behavioural variation in cultivated juvenile Atlantic cod (Gadus morhua L.) in relation to stocking density and size disparity

Abstract The aim of this study was to examine the behaviour and activity of Atlantic cod ( Gadus morhua ) juveniles (∼65–120 mm total length) in relation to varying stocking density (∼500–2000 fish per m 3 ) and size disparity under both experimental and commercial hatchery conditions. This was done by using stereovideography to measure fish size and to quantify fish behaviour in situ . The swim pattern was more synchronized at higher fish densities (both experimental and commercial conditions), indicating a more pronounced schooling behaviour and possibly also indirectly reduced activity costs and stress levels with increasing fish density. Furthermore, the swim pattern became less synchronized and the relative swim speed increased as fish size disparity increased. This might indicate both elevated energetic activity costs and an augmented stress level with increasing size disparity. However, overt agonistic behaviour was nearly absent at the examined fish densities and size disparities, and negative social interactions are thus unlikely to represent a major cost under such conditions. The general absence of overt agonistic behaviour and the lack of behavioural variation indicative for extreme levels of stress or activity might suggest that the welfare status of the juveniles was not compromised under the examined conditions.

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