Effects of Photoperiod on Growth, Reproduction, and Physiology of Nile Tilapia ( Oreochromis niloticus )

The photobiological responses of fish are strongly influenced by the surrounding photoperiod conditions. This study aimed to evaluate the effects of photoperiod on the growth performance, gonadal development, major haematological parameters, skin coloration, and proximate composition of Nile tilapia ( Oreochromis niloticus ). Four photoperiod treatments were employed: 2 h of light and 22 h of dark (02L:22D), 12 h of light and 12 h of dark (12L:12D), 17 h of light and 7 h of dark (17L:07D), and 22 h of light and 2 h of dark (22L:02D), denoted as Treatment one (T1), Treatment two (T2), Treatment three (T3), and Treatment four (T4), respectively. Fingerlings (3.21 ± 0.51 g) were reared for 90 days and provided a commercial diet (35% crude protein) twice daily at 5% of their body weight. The light intensity was maintained at 1350 lux throughout the study period. Compared with those under the other treatments, significantly greater weight gain (WG) (47.09 ± 13.89), percentage WG (%WG)(1465.05 ± 394.29), and specific growth rate (SGR) (3.05 ± 0.25), along with a lower feed conversion ratio (FCR) (1.79 ± 0.44), were observed under the 17L:07D photoperiod. The gonadosomatic index (GSI) was highest in T4 for both males (1.11% ± 0.41%) and females (7.18% ± 1.10%). Skin brightness was also greater in T4 ( L ∗ = 85.63 ± 0.31) than in T1 ( L ∗ = 71.02 ± 0.51). Red blood cell (RBC) counts were significantly higher in T3 (1.56 ± 0.06 × 10 6 /µL), whereas white blood cell (WBC) counts were highest in T4 (31.72 ± 0.16 × 10 3 /µL). Hemoglobin (Hb), hematocrit (Hct), mean corpuscular volume (MCV), mean corpuscular Hb (MCH), MCH concentration (MCHC), and monocyte levels showed no significant differences among treatments. Proximate composition showed the highest protein content in T3 (14.47 ± 0.19) and the greatest lipid content in T4 (1.82 ± 0.06). Overall, 17L:07D (T3) optimizes growth and protein deposition, whereas 22L:02D (T4) enhances reproduction, lipid accumulation, and skin pigmentation in Nile tilapia.