COMPARATIVE STUDY IN SALINITY TOLERANCE AND DISEASES RESISTANCE OF THREE NILE TILAPIA (OREOCHROMIS NILOTICUS) STRAINS CULTURED IN EGYPT
Mohamed M. Said
Aquaculture Department, Faculty of Fish Resources, Suez University, Suez, Egypt.
|Received 4/ 12/ 2016||
Accepted 3/ 1/ 2017
Salinity tolerance and resistance to Aeromonas hydrophila infection were investigated in three Nile tilapia strains cultured in Egypt: improved Abbassa, commercial Manzala and Kafr Elshiekh throughout two experimental studies. In the first experiment, 120 fish from each strain (15 g ± 0.25) were equally divided into four groups (30 fish each). Each group was transferred to three tanks (10 fish each). Fish of groups 1-4 were exposed to a direct transfer to four different concentrations of saline water 6, 12, 18, and 24 ppt for 96 hours. In the second experiment, 60 fish from each experimental strain (20 g ± 0.34) were divided into two equal groups including control group and tested group, three replicates each with 10 fish per replicate. Tested groups were challenged with pathogenic A. hydrophila. A 0.1 ml dose of 24-h broth from virulent bacterial pathogen of A. hydrophila (5×105cells/ml) was injected by intraperitoneal (IP) route. The challenge test lasted for 14 days. At the end of salinity tolerance experiment, Abbassa and Manzala strains showed significantly (P<0.001) higher survival rates (71.66 and 70 %, respectively) than Kafr Elshiekh (59.16%). Median lethal salinity (MLS96) was significantly (P<0.01) higher in Abbassa (19.97) and Manzala (19.90) than Kafr Elshiekh (17.07). Lethal concentration 50 (LC50) of both Abbassa and Manzala (19.13 and 19.4, respectively) were also significantly (P<0.05) higher than that of Kafr Elshiekh (16.26). In the second experiment, after challenging with Aeromonas hydrophila some of the tested experimental fish showed darkness in the color of the skin, detachment of the scales, and large irregular hemorrhages on the body surface. Survival rate% at tested groups was significantly (P<0.001) higher in Abbassa (93.3%) followed with Manzala (70 %) whilst Kafr Elshiekh showed the lowest survival rate (46.7%). It could be concluded from
the current study that, Abbassa selection strain is a good choice for tilapia culture in brackish water and meantime have more resistance to Aeromonas hydrophila infection than the other two strains.
Keywords: Salinity tolerance, diseases resistance, A. hydrophila, improved strain.