Abstract
Bangladesh is one of the most vulnerable countries for sea level rise. The
intrusion of salinity triggers the aquaculture towards the coast. That is why the
need for the salinity tolerant fish could be the solution for the coastal
aquaculture. Oreochromis niloticus might be a potential candidate to develop a
saline tolerant strain. The Lethal concentration 50 (LC50), behavioral response,
survival rate, growth, hematological, biochemical and histological alterations of
gill and kidney of tilapia were observed; and the selective breeding, interspecific hybridization and growth comparison among different breeds were
carried out in this study. Probit analysis showed that the LC50 of the 5 and 15
day after hatch (DAH) tilapia were 16.53 and 18.65ppt, respectively. The mean
initial weight of 5DAH and 15DAH tilapia was 0.02±0.00g and 0.17±0.00g,
respectively, while the mean weight gain was 0.20±0.01 and 0.38±0.07g,
respectively. Increasing salinity, tilapia was become very weak, settle at bottom
and died but in the control no abnormalities were observed. High salinity
expressed increased opercular movements accompanied with excessive
secretion of mucus. Blood hemoglobin decreased slightly up to 10ppt (11g/dl)
salinity than fluctuates without any significant differences. Red blood cell
(RBC) count significantly decreases with the increasing of salinity level up to 10
ppt (2x 108/mm³) then fluctuated unexpectedly. White blood cell (WBC) count
decreased significantly up to 15ppt (3x 108/mm³) salinity then followed
upward trend up to 15ppt (17x 108/mm3) salinity. Significant difference of the
blood glucose level between 5DAH and 15DAH fries were 2-4 mmol/1 and 4-10
mmol/1, respectively. As salinity increases, the gill showed hyperplasia of the
epithelium, fusion of secondary lamellae and necrosis, while the kidney severe
hydropic degeneration, edema and necrosis. Selective breeding in 10ppt
salinity produces viable offspring. However, in 15ppt condition around thirty
months of diligent trial no breeding occurred due to fatty urethras preventing
the eggs to release in time. No significant increase in mean survival rate of the
crosses (70.5%) of O. niloticus () and O. mossambicus (3) than the selective
breed fries (67.5%) of GIFT tilapia. Comparisons among different strains the
highest specific growth rate (SGR) were shown in selective breed (6.12±0.04)
while the lowest value was for cross breed strain (5.07±0.54). The highest
survival rate in cross breed strain (83%) followed by selective breed strain
(80%) and the lowest value was for GIFT strain (60%). The breed with the
selective breeding strain might be more applicable where the salinity is below
(10ppt) the coastal region approximation. Whereas the crossbreed strain could
be more salt tolerant and suggested to culture in comparatively higher saline
zone than the coast. The blue economy of Bangladesh may flourish by cage
culture in marine environment through introducing saline tolerant crossbreed
strain.