Effect of chronic confinement stress on the immune responses in different sex ratio groups of Oreochromis mossambicus (Peters)

Fish live in a complex and diverse aquatic environment. Various environmental stressors continuously challenge the physiology of fish. Social stress has a crucial role to play in immunity of fish. Hence, an attempt was made to investigate whether a stressor would have differential effect on the immune system of Oreochromis mossambicus when maintained in different sex ratios. Fish were crowded by confining at three different densities, i.e., 4 g l(-1) (control), 8 g l(-1) or 16 g l(-1) and in three different sex ratios, i.e., (i) all-male, (ii) all-female and (iii) equal male and female (mixed). Confinement stress was maintained for a period of 28 days and the fish were assessed for their specific immune response in terms of antibody response to Aeromonas hydrophila assayed by ELISA and plaque forming cell assay, nonspecific immune response in terms of intracellular ROS production by peripheral blood leukocytes, serum lysozyme level and integrated immunity by host resistance. The results showed that chronic confinement enhanced specific immunity in monosex ratio groups in terms of both antibody response, in the number of antibody producing cells. The nonspecific immune parameter and serum lysozyme level showed enhancement on confinement in all-male sex ratio group, whereas the ROS production showed suppression on confinement irrespective of sex ratio. The host resistance test revealed that enhanced specific immunity and lysozyme activity were not protective by showing 56% mortality in all-male confined group. Fish maintained as equal male and female group showed marginal level of protection in control group. Hence, the social environment in terms of sex ratio does have both positive and/or negative impact on the immune system depending on the degree of confinement. (C) 2005 Elsevier B.V. All rights reserved.