, vol. , 55, no.3 , pp. 687-717, 2007.
The present study was carried out to assess the effects of blue light as antistressor and its effect on susceptibility of Nile tilapia (Oreochromis niloticus) to Aeromonas hydrophila (A. hydrophila) infection. A total number of (120) O. niloticus was divided into 3 experimental groups (40 fish / group). Group 1 (G1), was exposed to normal day light and considered as control group. Group 2 (G2), was exposed to blue light. Group 3 (G3), was exposed to white light (fluorescent illumination). After 15 days of light exposure, fish in groups 2 and 3 were confined into small area and then challenged with A. hydrophila infection. The obtained data revealed that during exposure to blue light, the fish of (G2) recorded significant lower proportion in behavioural parameters including factors of fright behaviour "creeping, oblique plan position and air-gulping" than those of﴾G3﴿. On the other hand, (G2) showed lower behaviour alterations for fright behaviour during confinement and infection than those of﴾G3﴿. Contrarily, it also had significantly higher proportions of aggressive behaviour during exposure to light, confinement and infection than﴾ G3﴿. SO blue light improved behaviour and fish become more comfortable in (G2) than (G3). Moreover, fish of (G2) showed higher significant growth performance during exposure to blue light and little reduction in growth performance during exposure to both confinement and infection than those of (G3). The clinicopathological studies revealed significant increase in the stress response indicators after exposure to confinement stress in (G.3). Marked elevation in the serum cortisol, glucose levels and significant leukocytosis associated with heterophilia and lymphopenia was observed in group 3 (G3). On the other hand, non significant changes were observed in the measured stress parameters in group (G 2) which exposed to the blue light. Exposure of stressed fish to A. hydrophila infection induced significant leukocytosis with heterophilia, monocytosis and lymphopenia in both (G2&G3). Analysis of serum biochemical constituents showed significant elevation in ALT, AST, BUN, creatinine and glucose while significant reduction in the total proteins, albumen and globulin was achieved. These changes were more pronounced and early detected in A. hydrophila infected stressed fish exposed to white light compared to those exposed to blue light From pathological point of view, fish group which exposed to blue light and infection revealed early positive immune response "activation of melanomacrophage center of spleen" but with time dependent adverse pathological lesion. The lesions were noticed in spleen and gills after 2 weeks. Regeneration of the most observed lesions were detected after 4 weeks. On the other hand, the fish group exposed to white light and infection showed more advanced lesions including diffuse necrosis in hepatic tissue with additional changes in the kidneys. So, the blue light has a protective effect aganist stress in Nile tilapia.
