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The influence of environmental factors on the habitat use of Black Bream Acanthopagrus butcheri and the implications of artificial oxygenation in the Swan-Canning Estuary

Watsham, Jake (2016) The influence of environmental factors on the habitat use of Black Bream Acanthopagrus butcheri and the implications of artificial oxygenation in the Swan-Canning Estuary. Honours thesis, Murdoch University.

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Deoxygenation events within the Swan-Canning Estuary are a severe and potentially lethal threat to the resident aquatic fauna and an ongoing concern for management. Hypoxic conditions develop in the upper estuary following heavy rainfall events that deliver an influx of freshwater into the system, stratifying the water column. When tidally driven seawater and freshwater discharge meet, haloclines are formed, creating a barrier to vertical mixing. Organisms living within the stratified bottom waters can rapidly deplete oxygen reserves, eventually causing the lower layers to become hypoxic. The frequency and severity of these events are projected to increase due to climate change as associated reductions in surface runoff reduce river flow (a remediate of hypoxia) and allow deeper penetration of the marine salt wedge, causing prolonged and more intense stratification of the system. In response to this threat, artificial oxygenation plants have been installed in two regions of the upper estuary known to experience hypoxic conditions most frequently in an attempt to maintain dissolved oxygen concentrations appropriate for fauna.

Using acoustic telemetry, we tracked fifty-five Acanthopagrus butcheri over a 116-day period between autumn and winter with the aim to relate the patterns of movement (including residency, habitat use and movements on daily and seasonal scales) 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 the operation of the oxygenation plants did not significantly influence A. butcheri detection. Acanthopagrus.butcheri avoided areas wherein large proportions (>20%) of the available habitat was occupied by hypoxia, instead favouring those with <5%. Areas of high habitat complexity owing to the abundance of large woody debris were associated with high numbers of A.butcheri and salinity was a significant predictor of A.butcheri presence with the species being found to favour areas of salinities between 10-20ppt, rarely being detected in waters of <5ppt and >25ppt despite their extreme euryhaline physiology.

Thus, the results highlight the species vulnerability to hypoxia and provides further evidence supporting the hypothesis that the habitat compression of A.butcheri is heavily responsible for declines in its condition within the Swan River Estuary. Hence, ongoing management of the Swan-Canning Estuary should aim to mitigate the factors exacerbating hypoxia within the system and incorporate protection and restoration of in-stream woody habitats and riparian vegetation given its importance to native fishes such as A.butcheri as critical refugia. More research will be required to accurately quantify the benefits of artificial oxygenation to estuarine species within the Swan-Canning Estuary.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Beatty, Stephen, Gleiss, Adrian and Close, Paul
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