Stocking density limits for post-smolt Atlantic salmon (Salmo salar L.) with emphasis on production performance and welfare

For the development of commercial scale semi-closed sea systems for farming post-smolt Atlantic salmon (Salmo salar), further knowledge is required on the interaction between fish density, farming conditions and fish welfare. In this experiment post-smolts (115.0g±13.6) were stocked at 5 different densities (25, 50, 75, 100 and 125kgm−3), and kept at these densities for 8weeks. All treatments received an equal specific flow rate of 0.6Lkg fish−1min−1 of flow-through seawater (fully oxygenated, salinity 34‰ and temp. 9.3°C) and water oxygen (O2), pH, carbon dioxide (CO2) and total ammonia nitrogen (TAN) levels were monitored in the outlet and kept within recommended limits. Over the 8week period, specific growth rate (SGR %) was significantly reduced in stocking densities of 50kgm−3 and above. Increasing density from 100kgm−3 to 125kgm−3 lead to a 42% decrease in SGR. Between 50kgm−3 and 125kgm−3 there was a correlation between reduced feed intake and increased stocking density and there was a linear increase in feed conversion ratio (FCR) with stocking density (25kgm−3 to 125kgm−3). At the end of the 8week period primary and secondary stress responses such as elevated plasma levels of cortisol, sodium, pCO2 and decreased plasma pH were observed in the highest density treatment compared to other treatments. In combination with the reduced SGR in the highest density treatments these results indicate an allostatic overload i.e. the environment has exceeded the adaptive ability of the fish with chronic adverse effects on fish welfare. Stocking densities of 100kgm−3 or more also increased pelvic fin damage and the prevalence of cataracts was higher in the 125kgm−3 treatment. In conclusion, our results suggest that at this temperature and fish size it is feasible to rear Atlantic salmon post-smolts in densities up to 75kgm−3 without compromising performance and welfare.