Deoxygenation events within the Swan-Canning Estuary are severe and potentially lethal threats to aquatic fauna and an ongoing concern for management. Hypoxic conditions develop in the upper estuary following heavy rains that deliver influxes of freshwater into the system, stratifying the water column and creating barriers to vertical mixing. Organisms living within the bottom waters rapidly deplete oxygen reserves, causing this region to become hypoxic. The frequency and severity of these events are projected to increase due to climate change following reductions in surface run off and reduced river flows (a remediate of hypoxia). In response, artificial oxygenation plants have been installed in two regions of the upper estuary known to experience hypoxic conditions most frequently to maintain oxygen levels appropriate for fauna.
Using acoustic telemetry, we tracked fifty-five Acanthopagrus butcheri over a 116-day period over autumn and winter with the aim to relate patterns of movement (including residency and habitat use) to a suite of environmental variables commonly associated with the movement of fishes in estuaries, and determine whether the spatial residency of this species is influenced by the operation of the artificial oxygenation plants. The study revealed that detection of A. butcheri was significantly influenced by hypoxia, habitat complexity, salinity and flow, while operation of the oxygenation plants did not significantly influence A. butcheri detection. Although, due to the necessity for the plants to operate in a way that prevents hypoxia from forming, an effective study design to specifically answer this question wasn’t able to be achieved.
Ongoing management of the Swan-Canning Estuary should aim to mitigate the factors exacerbating hypoxia within the system and incorporate protection of instream woody habitats and riparian vegetation given its importance to native fishes as refugia. More research will be required to accurately quantify the benefits of artificial oxygenation to estuarine species.