Effects of Mn2+ and flow speed on blood and physiological indexes of larvae GIFT tilapia (Oreochromis niloticus)

Abstract

To assess the toxicological effects of heavy metal ions on fish at different flowing speeds within aquatic environments, we investigated the impact of Mn^2+^ and flow speed on the physiological parameters of juvenile GIFT tilapia (95.2±4.3g,15.1±0.21cm). The experiment design included four different levels of Mn^2+^ concentration: control group (0mg/L), 0.1mg/L, 0.5mg/L and 1mg/L, as well as three different flow speeds: control group (0 body length/second), low speed (1 body length/second), and high-speed group (2 body lengths/second). The main research findings were as follows: Mn^2+^ had varying effects on the hematological parameters of juvenile GIFT tilapia. As the levels of Mn^2+^ increased, there was a remarkable reduction in both the red blood cell (RBC) number and hemoglobin content of the fish(*P*\<0.05). The activity of SOD, CAT, the level of MDA in the liver and white blood cell (WBC) number exhibited an initial increase followed by a subsequent decrease, with the highest levels observed mostly in the 0.5mg/L group. However, With the prolongation of Mn^2+^ exposure duration, a significant decline in levels of RBC, hemoglobin, CAT and SOD was observed. In most cases, the highest values of those, as mentioned earlier four physiological parameters were recorded at 24 hours, while the lowest values were recorded at 96 hours when exposed to the same Mn^2+^ level. However, MDA levels reached their peak at 48 hours. These findings suggested that lower levels of Mn^2+^ might enhance the immune capacity of tilapia, while higher levels could impede its immune and antioxidant functions. An increase in flow speed led to a significant elevation in both the RBC number and hemoglobin content of the GIFT tilapia at 30d(*P*\<0.05), but flow speed had no significant impact on the WBC of the fish(*P*\>0.05). Its effect on the fish antioxidant parameters also showed a certain induction, and there were significant correlations between the activity of SOD, CAT, the level of MDA, and flow speed (*P*\<0.05). At the same flow speed, the prolonging water-flow-stimulus duration could not increase the levels of three antioxidant parameters(*P*\>0.05). These findings could provide fundamental data for investigating the influence of heavy metal stress on fish ecotoxicology in aquatic environments and the artificial running water breeding of GIFT tilapia in Hebei province, China.