| Summary: | Background: In general, the length of photoperiod affects the physiology of Pangasionodon hypophthalmus. This study aimed to understand the growth and health status of this fish reared under manipulated photoperiods. Methods: The study was conducted between June to August 2020[A1] . Three treatments were applied: control (natural photoperiod); 18 hours of darkness (18D6L; tanks were placed under a dark tarp tent for 18 hours); and 24 hours of darkness (24D0L; tanks were placed under a dark tarp tent continuously). Three replications were performed per treatment. At baseline, fish were approx. 7.5 cm [A2] total length (TL) and 4 g body weight (BW), and were reared in circular plastic tanks (100 L; 30 fish/tank) with aerators and filters, and fed with commercial fish feed pellet (2 times/day to satiation). Fish growth and survival were studied once/week, and blood and tissue samples were taken at the end of the experiment (8 th week). Tissue was formalin fixed and HE stained. Results: The survival of fish in all treatment was 100%. The fish reared in 24D0L and 18D6L grew better than control, achieving mean TL of 23 cm and BW of 98 g (control = 19 cm TL and 72 g BW). There was no difference in hematology condition or tissue structure between the three groups. Tissue structure of gill, kidney and liver were normal, but light abnormality due to parasites was present in the gill of fish reared in 24D0L. Blood samples for all three groups showed mean red blood cell count of 1,800,000 cells/ml and white blood cell (WBC) count of 55,200 cells/ml. WBC consisted of lymphocyte 65%, monocyte 24%, thrombocyte 6%, neutrophil 3%, eosinophil 1% and basophile 1%. Conclusion: Data obtained indicate that a short photoperiod improves the growth of P. hypophthalmus fish and does not negatively affects their health.
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