Hypoxic events (where levels of dissolved oxygen are typically lower than 25%) worldwide have increased in both frequency and severity; this is particularly true in the Murray River System. It is believed a lot of this relates to the management of upstream water and also the prolonged periods of drought we have experienced. Included in this is a poor understanding of the physiological effects of hypoxia on our native species, with most studies showing species specific tolerance limits to low levels of dissolved oxygen. We have also not been able to determine the long term effects on fish to hypoxia, with most studies focussing on presence or absence data which only provides a snapshot of the effects of hypoxia on fish.
Oxygen levels are a key factor in the survival of all fish species but the influence of dissolved oxygen (DO) levels on fish health is often overlooked and not well understood due to the difficulties associated with testing. Past analysis of the metabolic performance of fish has shown varied results, with some species showing large tolerances to low DO, while others begin gasping at levels just below normal. This project aims to identify a tolerance limit for one of our key native species (golden perch or callop). Oxygen consumption will be measured under combined treatments of temperature and dissolved oxygen using resting respirometry.
This will allow us to determine a true physiological tolerance limit of our sensitive native species to hypoxia.
The second section of the project focuses on recreating a history of hypoxia using the earbones of fish, which can record exposure to different environmental conditions similar to the rings of a tree over the full life of an individual. Tracing modern hypoxia histories is expected to be achieved by correlating present DO data from the river with both field and laboratory validation. Chemical tracers present in recent and historic ear bones of native freshwater fish will be used to reconstruct hypoxia histories for the Murray River region. Field and laboratory experiments will allow us to validate a specific chemical tracer of hypoxia, with Manganese showing promise. The utilisation of archival collections of otoliths from native freshwater fish will provide the means to reconstruct long-term trends of hypoxia exposure in freshwater systems in Australia.
An improved understanding of the physiological tolerances and the effects of long-term exposure of native fish is necessary in order to begin to develop better preventative strategies for mass mortalities due to hypoxia exposure. Furthermore, it will allow us to potentially understand either natural or anthropogenic causes for an increase in hypoxic events over history (e.g. damming, increased land use, climatic shifts).
Kayla Gilmore is a PhD Candidate at the University of Adelaide.