Sublethal effects of fluctuating hypoxia on juvenile tropical Australian freshwater fish

Hypoxia in freshwater ecosystems of the Australian wet tropics occurs naturally, but is increasing as a result of anthropogenic influences. Diel cycling of dissolved oxygen (DO) concentration (fluctuating hypoxia) is common in the region. Laboratory experiments sought to identify relationships between severity of fluctuating hypoxia and sublethal effects on ventilation, feeding and growth for juvenile barramundi (Lates calcarifer), eastern rainbowfish (Melanotaenia splendida splendida) and sooty grunter (Hephaestus fuliginosus). Fish continued to feed and grow under daily exposure to severe fluctuating hypoxia treatments for several weeks. Ventilation rates increased in a significant direct quadratic relationship with the severity of hypoxia treatments and increasing hypoxia caused ventilatory behaviour changes in all species. Barramundi and rainbowfish attempted aquatic surface respiration and were more tolerant of severe hypoxia than was sooty grunter; barramundi and rainbowfish are also more likely to experience hypoxia in the wild. There was a significant quadratic relationship between growth and minimum DO saturation for barramundi. Although all three species were tolerant of hypoxia, anthropogenic stressors on tropical Australian aquatic ecosystems may increase the frequency and severity of hypoxic conditions causing a concomitant increase in fish kill events.