June 2023: Oyster Harbour Catchment Health Update

Our local harbour, rivers, and streams are highly valued for the wide variety of social, cultural, recreational, environmental, and economic services they provide. These include water sports, fishing and marroning, nature-based recreation and ecotourism, and water supply for a range of agricultural, industrial, and urban uses. 

A properly functioning ecosystem supporting native biodiversity is critical to maintaining the intrinsic values of rivers and the essential services they provide. However, because of our use of rivers and the development pressures they face, aquatic ecosystems are exposed to significant and increasing stress.  Notable impacts include changes to natural flow patterns and connectivity (interrupting species migrations and flushing), poor water quality, reduced instream and fringing vegetation and habitat, and introduction of exotic species (which predate and/or compete with native species and bring disease and parasites).

The more stress our environmental systems face, the less they are able to tolerate and adapt to a changing climate, pressures from expanding and intensifying development, and increased frequency and magnitude of bushfires.    

Since 2008, a total of eight river health assessments have been carried out throughout the Oyster Harbour catchment under the Department of Water and Environmental Regulation’s (the department) ongoing program. This includes targeted assessments during the 2017-20 Regional Estuaries Initiative to investigate key areas of the catchment that previously had little or no data. Information from these assessments was examined under the Regional Estuaries Initiative to assess the status of values and threats.

Based on historical data, the Oyster Harbour catchment was found to have reasonably high native biodiversity, with 15 species of native fish and crustaceans observed. This included eight species of native freshwater/freshwater-estuarine fish, the pouched lamprey, which migrates from the ocean into the headwaters of rivers to spawn, and four species of freshwater crustaceans (crayfish and shrimp). Ten of these species are endemic to the South West of Western Australia (WA), which means that these species are not naturally found anywhere else.
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At the sites sampled in the 2020 assessments on Chelgiup Creek, King River and Mill Brook, the biodiversity was found to be in generally good condition, as was the water quality and aquatic habitat. In contrast, significant issues were identified for the Yakamia Creek site, many of which are known and are the subject of ongoing rehabilitation and restoration efforts as part of.

The 2020 assessments highlighted important ecosystem values, but also several pressures on the health of the river ecosystems, including:

• Historic clearing reflecting urban, cropping and grazing land uses in the catchments. The loss of vegetation, particularly through the stream corridor, is having an impact on water quality and habitat. Issues include:
  • Low dissolved oxygen concentrations
  • High nutrient and metal loads
  • Loss of complex in-stream shelter particularly woody habitat
• Presence of exotic fish and crayfish species, which influence native species through competition, predation, and changes to habitat
• Access to rivers by livestock resulting in:
  • Erosion, contributing to sedimentation
  • Loss and trampling of fringing vegetation
  • Potential water quality issues and an increase in nutrients entering the waterway.

Of note was that the Yakamia Creek site was dominated by exotic fish and crayfish species. Native species are adapted to natural conditions and under pristine conditions are highly resistant to introduced species, whereas invasive species are typically able to tolerate a wider range of conditions and have a significant advantage in impacted systems. The exotic species in Yakamia Creek is a clear reflection of suboptimal conditions and suggests the creek is under higher stress in comparison to the other river systems in the Oyster Harbour catchment. The habitat and water quality within Yakamia Creek were observed to be in poor condition, which supported this assessment.

Recent assessments were not an exhaustive survey of the current status of the entire catchment. New data now adds to our knowledge base of how WA river systems function in different circumstances. Data also provides a baseline for ongoing monitoring of river health in the catchment.

The Healthy Estuaries WA website has just finished being updated to include all this content and reports.