Metabolic responses of the Maugean skate (Zearaja maugeana) to chronic hypoxia and hyposalinity

Abstract

The endangered Maugean skate faces a variety of anthropogenic and physicochemical challenges in its endemic habitat of Macquarie Harbour, Tasmania, Australia. Among these challenges are highly stratified waters with variable temperatures, oxygen saturation, and salinity, and increasing salmonid aquaculture. It is known that the skate is capable of tolerating short-term hypoxic exposure, but much of its physiology remains unknown. Decreasing trends in oxygen saturation throughout all layers of the harbour over the last decade correspond with increases in salmonid aquaculture, and increased occurrences of major storms may exacerbate these declines. As such, I investigated the Maugean skate’s physiological responses to chronic hypoxia to understand what affect it may have on these endangered animals. I exposed the skates to either hypoxia (20% DO) or normoxia (90% DO) for 96-hours, immediately after which, I measured their metabolic rate at 20% DO. I quantified lactate and glycogen concentrations in tissue samples taken immediately following the metabolic rate test and both pre- and postexposure plasma samples. I found that skates exposed to hypoxia for 96-hours exhibited significantly lower metabolic rates than their normoxic counterparts. Plasma lactate concentrations were undetectable in the field, and there was no significant difference in tissue lactate or glycogen concentrations relative to normoxic tissues. My results indicate that the Maugean skate is extremely hypoxia tolerant and copes with chronic hypoxia by depressing its metabolism to potentially limit glycogen use and lactate production. While the skates seem to be able to tolerate chronic hypoxia through metabolic depression, this may limit their ability to forage and/or reproduce, which may inhibit population recovery.