Effects of stocking density on growth and non-specific immune responses in juvenile soft-shelled turtle, Pelodiscus sinensis

Abstract

To investigate the influence of stocking density on the growth and non-specific immune responses of juvenile soft-shelled turtle, Pelodiscus sinensis, three groups of turtles (initial body weight 74.2±12.2 g) were reared in 80 cm (L) × 40 cm (W) × 30 cm (H) tanks for 35 days at three stocking densities (StD), which were 1 individuals (ind.) tank−1 (3.13 ind m−2) for D1, 4 ind. tank−1 (12.5 ind m−2) for D2 and 6 ind. tank−1 (18.75 ind m−2) for D3 respectively. D1 was the control group. Turtles were weighted individually at the beginning and at the end of the trial. At the end of the rearing trial, non-specific immune parameters were determined. The results showed that the heterophil:lymphocyte ratio (HLR) increased and the total plasma protein decreased with increasing StD, suggesting a stress response. Specific growth rate (SGR) significantly decreased with increasing StD. Non-specific immune indicators such as blood cell phagocytic activity, serum haemolytic activity and serum bacteriolytic activity increased clearly at a higher StD, indicating a pattern of immunoenhancement. The results indicated that at StDs of 4 and 6 ind. tank−1, non-specific immune responses were not suppressed compared with the control, but were inversely enhanced at the cost of growth reduction. A possible energy-based trade-off between growth and constitutive immunity may exist in soft-shelled turtles.