Effect of stocking density on water quality and (Growth,Body Composition and Plasma Cortisol Content) performance of pen-reared rainbow trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>)

The goal of the study was to examine the effect of stocking density on the water quality of culture area, as well as the growth, body composition and cortisol content of rainbow trout (Oncorhynchus mykiss). Pen-reared trout were stocked in densities of 40, 60, 80 fish individuals m^?3 (4.6, 6.6, 8.6 kg m^?3, SD1, SD2 and SD3 groups, respectively) for 300 days. Compared to the water from SD1 and SD2, that from SD3 exhibited significantly higher NH ₄ ⁺ -N content and COD (chemical-oxygen-demand), and a significant reduction of dissolved oxygen in day 180 (40.6 kg m^?3). Stocking density was significantly associated with body weight, standard length, VSI (viscerosomatic index), CF (condition factor) and FC (food coefficient) in group SD3, particularly in day 240 and day 300 (45 or 49.3 kg m^?3). Increased crude fat and decreased crude protein were displayed in high density group when the density reached to 36 kg m^?3. As a cumulative effect of density-related stress, VSI, CF, FC, moisture, and crude protein content varied over time in each density group (SD1, SD2, and SD3). In summary, trout exhibited a better growth performance in low density (26.3 kg m^?3) than those reared in high densities (36 and 45 kg m^?3). The results indicate that rainbow trout (114.44 g ± 6.21 g, 19.69 cm ± 0.31 cm) initially stocked in 6.6 or 8.6 kg m^?3 should be lightened to less than 36 kg m?3 after an intensive rearing for 240 days.