The aim of this study was to evaluate the impact of reduced water column in different periods on health and welfare of the Nile tilapia during harvesting. As a common practice for gathering and harvesting of tilapia in Egypt, the water column was reduced gradually until reach the lowest level (30 cm). The water column was reduced from 150 to 30 cm in different periods as follow; Group1 (G1, control group), at which the water column was reduced from 150 to 30 cm in 72 hour (h). Group 2 (G2), at which the water column was reduced from 150 to 30 cm in 48 h. Group 3
(G3), at which the water column was reduced from 150 to 30 cm in 24 h. The results showed that, there was a significant (P˂0.05) decrease in muscle total bacterial count (TBC), fish injuries, heterophil /lymphocyte (L/H) ratio, serum
glucose, Lactate dehydrogenase, alkaline phosphatase and an increase in water, skin TBC, dissolved oxygen (DO), unionized ammonia (UIA), and total leucocyte count (TLC) in G3 compared to G2 and G1.
In conclusion, the process of reduced water column before gathering and harvesting needs to be carried out within 24 h to reduce injuries and bacterial load of fish. In addition, it improves the immune system and health of fish that reducing the
risk of infection. Therefore, slow reduction of water column height during harvesting produces crowding stress altering health status and welfare of Nile tilapia (Oreochromis niloticus).