Adaptation strategies of juvenile grass carp (Ctenopharyngodon idella) facing different dissolved oxygen concentrations in a recirculating aquaculture system

Fishes exposed to different dissolved oxygen concentrations over prolonged time periods have different susceptibility and adaptation mechanisms. In this study, healthy grass carp with an average weight of 214.6 ​± ​18.7 ​g were selected for long-term culture in recirculating aquaculture systems at three different dissolved oxygen concentrations, i.e., 3.00 ​mg/L [(2.57 ​± ​0.33) mg/L, DO_2.57), 5.00 ​mg/L [(4.61 ​± ​0.11) mg/L, DO_4.61], which was the control group, and 7.00 ​mg/L [(6.50 ​± ​0.48) mg/L, DO_6.50]. Blood, gill, muscle, and liver samples were collected after 60 ​days. Using hematoxylin-eosin (HE), alcian blue periodic acid Schiff (AB-PAS) staining, determination of respiratory metabolism enzyme activity, and fluorescent quantitative PCR, the adaptability of juvenile grass carp facing different dissolved oxygen levels were explored. The results showed that the respiration rate increased, and hemoconcentration and hemoglobin (Hb), electrolyte and pH changed at DO_2.57. Furthermore, gill lamellae became thinner and longer, the distance between them increased, the mucus on them decreased and the interstitial cell mass decreased. In addition, the relative expression of apoptosis-related genes bcl-2 and bcl-xl in gill tissue decreased, while caspase-3 and bax increased. In muscles and liver tissues, lactate dehydrogenase (LDH) activity and lactic acid content (LA) increased, pyruvate dehydrogenase (PDH) activity decreased, and genes related to the respiratory metabolic enzymes PDH-A4 and PDH-E1α also changed accordingly. The oxygen consumption rate decreased significantly (P ​< ​0.05). At DO_6.50, the distance between gill lamellae, numbers of red blood cells (RBC) and the content of hemoglobin decreased wheras the mucus on gill lamellae increased. The activity of PDH, the expression of PDH-E1a in liver and oxygen consumption rate increased. Different dissolved oxygen concentrations affected the respiratory behavior, gill tissue structure, blood transport capacity and tissue enzyme activity of juvenile grass carp. In summary, juvenile grass carp showed different adaptation strategies when facing different dissolved oxygen concentrations over prolonged periods in recirculating aquaculture systems.