Effects of Photoperiod, Temperature, and Fish Size on Oxygen Consumption in the Black Porgy Acanthopagrus schlegeli

The effects of photoperiod, temperature, and fish size on oxygen consumption (OC) in the black porgy Acanthopagrus schlegeli, a euryhaline marine teleost, were studied using a closed recirculating seawater system with a respiratory chamber. Fish reared in indoor recirculating seawater tanks were divided into two groups: small (15.7-55.8 g, mean 38.1$\pm$15.9 g) and large (108.7-238.8 g, mean 181.8$\pm$54.9 g) fish. The OC of the fish showed a clear diel rhythm, with higher values in the daytime and lower values at night, in accordance with light (09:00-20:59 h) and dark (21:00-08:59 h) cycles. The OC of the fish increased linearly with the water temperature. The OC was the highest at 10:00 h, one hour after the onset of daylight and was the lowest at 03:00 h, six hours after dusk. The average OC at $20^{\circ}C$ during the light period was as high as 219.8 mg $O_2$/kg/h in the small fish and 156.3 mg $O_2$/kg/h in the large fish, while during the dark period it was as low as 130.5 and 110.4 mg $O_2$/kg/h, respectively. The OC during the dark period, which showed limited variation, could be regarded as the resting OC, and was 107.6, 130.5, and 219.8 mg $O_2$/kg/h at 15, 20, and $25^{\circ}C$, respectively, in small fish, and 52.3, 110.4, and 171.0 mg $O_2$/kg/h in large fish. As the body weight of black porgy increased, the OC decreased exponentially and the relationship was expressed as OC=1,222.8$BW^{-0.567}$, OC=1,113.2$BW^{-0.448}$, and OC=1,495.3$BW^{-0.468}$ at 15, 20, and $25^{\circ}C$, respectively. At a fish density of 14.5 g/L at $20^{\circ}C$, black porgy had the highest OC per breath compared to fish at the same density at 15 or $25^{\circ}C$. This suggests that the black porgy responds to the stocking density (15 kg/$m^3$) and water temperature ($20^{\circ}C$) conditions commonly observed in intensive aquaculture with the deepest breath and the highest metabolic activity.