The effects of stocking density on hemato-immunological and serum biochemical parameters of rainbow trout (Oncorhynchus mykiss)

In the present study, the effect of stocking density on hematological, biochemical and immunological parameters of rainbow trout (Oncorhynchus mykiss) was investigated. Experiments were performed in a 30-day period at two different stocking densities: low density (LD 10 kg/m3) and high density (HD 45 kg/m3). Fish held at HD showed significant higher levels of hemoglobin and red blood cells (12.76 mg/dl and 1.8 × 106 mm3, respectively) compared with those held at LD (11.43 mg/dl and 1.23 × 106 mm3, respectively; P < 0.05). Stocking density had no significant effect on mean corpuscular hemoglobin and mean corpuscular hemoglobin concentration levels. The level of HCT was higher in fish reared at HD, but it was not statistically significant. In the case of biochemical parameters, plasma glucose and cortisol increased significantly in HD compared with LD (P < 0.05), but stocking density caused no significant changes in total protein, globulin and albumin levels. Our results also revealed that the levels of plasma cholesterol and triglycerides were significantly lower in LD compared with HD (P < 0.05). Results of some immune parameters of O. mykiss reared at two different stocking densities revealed that at HD, the level of white blood cells was lower significantly compared with LD (P < 0.05). The percentage of lymphocytes, neutrophiles and monocytes showed no significant changes between two groups. In addition, the level of serum lysozyme was significantly higher in animals maintained at HD compared with LD (P < 0.05). Overall, our results show that high stocking density caused chronic stress in rainbow trout and consequently altered the levels of some hematological, biochemical and immunological parameters compared with fish kept at low stocking density.