Short- and long-term effects of fish density and specific water flow on the welfare of Atlantic cod, Gadus morhua

Intensive rearing of farmed fish may have potential welfare related consequences on a short-term and long-term scale. Groups of individually tagged juvenile Atlantic cod, Gadus morhua, were held in eight tanks for 9 weeks at different rearing intensities, given as combinations of fish density and specific water flow (litre water per kg− 1 fish per min− 1). The control groups (LD-HF) had low density (~ 30 kg m− 3) and high specific water flow (0.84 l kg− 1 min− 1), while the other groups had high density (~ 60 kg m− 3) and either high specific water flow (HD-HF, 0.84 l kg− 1 min− 1), or low specific water flow (HD-LF, 0.17 l kg− 1 min− 1). Oxygen was added to maintain an O2 level of > 80% saturation in all tanks. The groups were pooled after the nine week treatment period, transferred to a sea cage and reared under standard conditions for 14.5 months, until reaching an average body size of approximately 1.4 kg. A combination of high fish density and low water flow (HD-LF) resulted in accumulation of CO2 in the water. This, in turn was associated with significantly higher blood pCO2, a higher frequency of nephrocalcinosis, and a 20–30% reduction in growth, as compared with the other groups. The sub-optimal growth observed in the HD-LF groups was not fully compensated for after transfer to the sea cage, with the fish of the LD-HF and HD-HF groups being 5–10% larger than the HD-LF fish by the end of the experiment.

► Juvenile Atlantic cod were held for 9 weeks at different fish density and specific water flow.
► Combination of high fish density and low water flow resulted in accumulation of water CO2.
► The CO2-increase led to higher blood pCO2 and frequency of nephrocalcinosis.
► The most intensive groups had a 20–30% reduction in growth.
► The growth reduction was not fully compensated for after 14.5 months production in sea cages.