Effects of stocking density on growth performance and welfare-related physiological parameters of Atlantic salmon Salmo salar L. in recirculating aquaculture system

Stocking density, representing a potential source of long-term stress, is widely recognized as a critical factor in aquaculture husbandry. Atlantic salmon were reared at low-density (LSD, similar to 9.80-18.41kgm(-3), initial to final density), medium-density (MSD, similar to 19.62-36.96kgm(-3)) and high-density (HSD, similar to 28.79-53.54kgm(-3)) stocking levels for 66days to investigate the stress-induced changes in fish growth and welfare. At the end of the trial, the salmon in HSD group showed significant lower final weight and higher ration level than those in MSD and LSD groups (P<0.05). The salmon farmed in LSD group had higher special growth rate (SGR) and lower feed conversion ratio (FCR) (P<0.05). However, stocking density could not affect the mortality, condition factor and coefficient of variation of salmon. The pectoral fin index of salmon in LSD group was significantly higher than that in HSD group (P<0.05). Fifteen haematological and serum parameters were detected to assess the stress and welfare levels of salmon. The salmon farmed in HSD group had higher glucose (GLU) level and lower Cl-, haemoglobin (HGB) and red blood cell (RBC) concentrations (P<0.05). Contrast to the pattern of superoxide dismutase (SOD) activity, the cortisol, maleic dialdehyde (MDA) and alkaline phosphatase (ALP) levels of HSD group were higher than those in other two groups (P<0.05). All these findings will provide a reference for selecting suitable stocking density in Atlantic salmon farming industry.