Assessment of Hypoxia Tolerance and Survival Mechanisms in Heterobranchus longifilis Under Low Oxygen Conditions

This study assessed the hypoxia tolerance and physiological survival mechanisms of Heterobranchus longifilis (vundu catfish) under experimentally induced low dissolved oxygen conditions, conducted at a fisheries research laboratory in Kogi State, North Central Nigeria. Hypoxia is a significant stressor in tropical aquaculture ponds and natural water bodies, and understanding species-specific tolerance thresholds is critical for stocking and management decisions. Juvenile Heterobranchus longifilis (mean weight 45.2 grams) were subjected to five static dissolved oxygen treatments: normoxia (7.5 mg/L), mild hypoxia (4.0 mg/L), moderate hypoxia (2.5 mg/L), severe hypoxia (1.5 mg/L), and critical hypoxia (0.8 mg/L) in controlled laboratory tanks over 48-hour exposure periods. Physiological indicators including haematocrit, haemoglobin concentration, lactate accumulation, gill morphology, and surfacing behaviour were recorded at 6, 12, 24, and 48 hours. Results confirmed that Heterobranchus longifilis exhibits remarkable hypoxia tolerance, maintaining normal feeding behaviour at dissolved oxygen levels down to 2.5 mg/L. Below 1.5 mg/L, aerial respiration using the accessory breathing organ was consistently observed, reducing reliance on aqueous oxygen. Haematocrit and haemoglobin increased significantly under moderate hypoxia, consistent with haemoconcentration as a compensatory response. Lactate levels rose sharply below 1.5 mg/L, indicating a switch to anaerobic metabolism. The critical oxygen threshold for the species was estimated at approximately 0.9 mg/L. The study contributes physiological baseline data for the species and recommends that pond dissolved oxygen management targets be set at or above 2.5 mg/L to prevent sublethal stress impacting productivity in Heterobranchus longifilis aquaculture. Keywords: hypoxia tolerance, Heterobranchus longifilis, dissolved oxygen, aquaculture physiology, catfish