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Abstract
The present study was planned to investigate the clinicopathological changes (hematological and biochemical findings) and histopathological alterations of Nile tilapia reared in farms under different densities of stocking rate together with investigating the effect of stocking rate on the growth and survival and ability of fish to resist diseases.
Seven hundred and twenty O. niloticus fries were divided into 3 equal groups each subdivided into three equal replicates. The fish in group (1) were reared in stocking rate 2fish /m3as control, fish in group (2) were reared in stocking rate 4fish/ m3 and fish in group (3) were reared in stocking rate 6fish/ m3 for 5 months (1st of April to end of August 2007). Three replicates were used in each group and were randomly assigned to hapas. Growth performance was determined; the body weight gain was determined where the fish of all the replicates were weighed individually after 5 month of the experiment. Specific growth rate (SGR) and condition factor (CF) were calculated and survival rate was recorded during the course of the feeding experiment and blood samples were collected at the end of experiment for the determination of some hematological and biochemical parameters.
The specific growth rate and average weight were decreased while total weight was increased when fish reared at high density.
There was no significant effect of stocking density on survival rate as non-significant increase in survival rate was seen by the increase in the stocking density.
The erythrocytic parameters revealed non-significant changes where non-significant increase of RBCs count, Hb and PCV% was seen in fish by increasing the stocking density.
The leukocytic parameters revealed non-significant changes where non-significant increase in the total leukocytic count, neutrophils, monocytes and basophils were noticed in fish by increasing the stocking density. Also, there were non-significant decrease in lymphocytes and non-significant increase in eosinophils to 3rd group.
A marked elevation in glucose was seen in fish reared at high density. The liver enzymes (ALT, AST) were increased when fish reared at high density.
Histopathological changes revealed normal tissues architecture and cellular details in the internal organs of control group (stocking density 2fish/ m3). The internal organs and gills of examined fish in second group (stocking density4fish/ m3) showed similar picture to those notes in control group. However in some cases the intestine exhibited mucinous degeneration in the epithelial lining. The gills of examined fish in 3rd group (stocking density6fish/ m3) showed congestion in the branchial vessels, focal hyperplasia in secondary lamellae was evident and other showed focal desquamation and liver showed focal to diffuse vacuolar degeneration in the hepatic cells perevascular and preglandular melano macrophages were seen.
When the fish injected intra protenially with pathogenic strain Aeromonas hyDROPhila, the fish died 1-4 days after inoculation in group(2 and3). The maximum mortality was obtained 4 days post injection after which no mortality was observed. Fish reared at high density recorded higher mortality rate, where clinical signs and postmortem lesions were more prominent in that group (3).