Standard Talk (15 mins) Australian Society for Fish Biology Conference 2022

Assessment and applicability of fish otoliths as in-situ monitors of chronic sub-lethal hypoxia (#48)

Sam J Lewis 1 , Alison J King 1 , David A Crook 2 , Luke M McPhan 1 , Jonathan Marshall 3
  1. Centre for Freshwater Ecosystems, La Trobe University, Wodonga, VIC, Australia
  2. NSW Department of Primary Industries, Fisheries, Narrandera, NSW, Australia
  3. Department of Environment and Science, Dutton Park, QLD, Australia

Many Australian inland waterbodies experience extreme variations in hydrologic and water quality conditions, with high rainfall periods often being separated by extended drought. Large-scale drought conditions over recent decades in the Murray-Darling Basin (MDB) have resulted in declining water quality and aquatic habitat availability, and acute hypoxic events and associated ‘fish kills’. These events are highly visible to the public and have come under increasing public scrutiny and media attention. Despite this recent attention, there is little understanding about the effects of chronic sublethal exposure of fish to hypoxic conditions. Otolith elemental concentrations have proven to be a useful and convenient tool for reconstructing past environmental conditions experienced by fish. Several recent studies from the northern hemisphere have identified the potential use of otolith Manganese (Mn) concentrations as an indicator of marine hypoxia.

In a pilot study, we analysed trace elemental concentrations in otoliths from individuals of three common fish species from the northern MDB, to investigate the utility of Mn as a proxy to identify hypoxia histories in the Australian freshwater context. Two proxies were explored: Mn:Ca and Mn:Mg (with  magnesium (Mg) concentrations thought to reflect metabolic activity but not hypoxia). We used generalised linear models to predict hypoxia proxy concentrations against an environmental proxy for hypoxia (annual cease-to-flow (CTF) days). Models showed significant differences between individuals, species and locations; suggesting little synchrony exists between individuals of each species even at the same locations. However, Mn concentrations generally increased as CTF days increased, though this relationship was not statistically significant. While our study suggests there is potential for otolith chemistry to trace hypoxia histories, further research with observed dissolved oxygen concentrations is needed. A greater understanding of hypoxia and fish responses in dryland rivers will improve the future management of fish and ecosystems under drought conditions.