Capel River - Yates Bridge

River condition at the Yates Bridge site (site code: CR62CAPE1, site reference: 6100288) on Capel River has been assessed as part of the Healthy Rivers program using standard methods from the South West Index of River Condition (SWIRC). The SWIRC incorporates field and desktop data from the site and from the broader catchment. Field data collected include the following indicators, assessed over about a 100 m length of stream:

• Aquatic biota: fish and crayfish community information (abundance of native and exotic species across size classes, general reproductive and physical condition)
• Water quality: dissolved oxygen, temperature, specific conductivity, and pH (logged in situ over 24 hours), as well as laboratory samples for colour, alkalinity, turbidity and nutrients
• Aquatic habitat: e.g. water depth, substrate type, presence of woody debris and detritus, type and cover of macrophytes and draping vegetation
• Physical form: channel morphology, bank slope and shape, bioconnectivity (barriers to migration of aquatic species), erosion and sedimentation
• Fringing zone: width and length of vegetation cover within the river corridor and lands immediately adjacent, structural intactness of riparian and streamside vegetation
• Hydrology: measures of flow (velocity) at representative locations (compared against data from stream gauging stations within the system)
• Local land use: descriptions of local land use types and activities (compared against land use mapping information for the catchment)

The Yates Bridge site was most recently assessed in March 2022. All known assessments are listed below:

• For a subset of the assessments above, water quality loggers were deployed for extended periods (see below). This enabled assessment of water quality responses to changes in climate, streamflow, and factors such as the intactness of vegetation within the river corridor. Amongst other things, this allows detection of sub-optimal water quality conditions which can be missed during the standard 24-hour assessment.
  • 2017‒18 summer (December to May)
  • 2015‒16 summer (December to May)

  It should be noted that considerable effort is underway to improve health in the system and wider catchment under the Revitalising Geographe Waterways program, run by the multi-agency committee Vasse Taskforce.

  Other data:
  The Capel River – Yates Bridge Gauging Station (site reference: 610219) is at the Yates Bridge site. The gauging station is owned by the Department of Water and Environmental Regulation (the department) and has been in operation from 1966 to 1976, and from 1994 to present.

  Search the site code or site reference in the department’s Water Information Reporting (WIR) system to find data for this site and nearby sampling points (flows, surface water quality, groundwater monitoring, department’s meteorological data). See also the Bureau of Meteorology website for additional meteorological data for the area.

The site had moderate to good structural aquatic habitat complexity, including dense woody debris of various sizes throughout the site. Woody debris provides aquatic species shelter from fast flows as well as protection from aquatic and terrestrial predators.

Additional habitat was provided by a small number of overhanging roots draped in water, over hanging banks, submerged macrophytes (ribbon weed species) and vegetation draped in the water. The lack of vegetation draped in the water is reflective of a reduced ground, shrub and tree layers within the riparian zone. Vegetation draped in the water can provide habitat for small fish and macroinvertebrates and can aid in providing bank stability.

The riverbed consisted of sand, silt, gravel, pebbles, cobble, boulders and bedrock. More than half of the site had a dense cover of organic material on the substrate including algae, detritus, epiphytes and leaves, which provides food for grazers and macroinvertebrates.

The species captured at the Yates Bridge site during each of the sampling periods are provided in Table 2. The table includes a list of all species previously reported in the subcatchment, which provides an indication of species that may occur at the assessment site. As differences in habitat within a subcatchment naturally influence species distributions, and variability in methods between sampling programs can affect the species caught, this list is only indicative. Below are some of the notable findings from the assessments.

Fish and crayfish assemblages have varied between the four sampling events.

Freshwater crayfish were the dominant species in most years. Smooth marron were dominant in 2016 and gilgie in 2019 and 2020. It is not uncommon for these species to inhabit the same environment, though smooth marron are typically restricted to permanent waters whereas gilgie can tolerate short periods of drying (being able burrow to avoid desiccation). Both species rely on woody debris for shelter and forage on the wood and periphyton (small organisms living on submerged structures). Catches of smooth marron have declined over time; further monitoring will assess whether this reflects a trend in population size or natural flux and/or sampling effort.

Native fish populations have typically been low across all sampling periods, with less than nine individuals spread across two or three species in any sampling event, and no juveniles collected across any species. However, during the most recent sampling in 2022, a total of eighteen western minnow was recorded. Western minnow were first observed in 2020 and have also been observed at the Capel River 1 site 13.5 km downstream at each of the sampling periods. Western minnow typically school in large numbers, are highly mobile and known to migrate long distances.

The absence or very low numbers of nightfish and western pygmy perch is unusual. Both species are common across rivers of south-west Western Australia and highly catchable (i.e. if they are present, they are expected to be detected). Freshwater cobbler were also only detected in low abundance, and only in 2016 and 2019, though this species is known to undertake localised migrations to access spawning and nursery grounds, which could explain variations in catches at different times.

An adult pouched lamprey was found during the 2019 assessment. This species is only known to inhabit a handful of systems across the South West. The absence of lamprey from most SWIRC assessments is not a significant concern as this species typically migrates in brief periods around peak winter flow (which is largely outside of the usual assessment periods) and only in high-flow years. Outside of the short migration periods, lamprey is unlikely to enter traps. The pouched lamprey is a Priority 3 species (poorly known) under the Biodiversity Conservation Act 2016 (WA). Priority 3 refers to possibly threatened species with insufficient data to assess threatening processes that could affect them. This species is not expected to be under imminent threat based on available data. In the early stage of their life cycle, lamprey burrow in sand in upper reaches of river systems. They migrate to the ocean where they may remain and feed for several years. Eventually lamprey will migrate back up river systems to spawn and die.

Redfin perch (non-native freshwater fish) were also found in low abundance, including three juveniles in 2022. Greater abundances of this species have previously been found further downstream. Redfin are an exotic species and typically grow to about 40 cm and are considered one of the most significant predators of native fish and crayfish species in the South West. More information on the redfin perch can be found on our website.

Note: sampling in 2020 by Murdoch University was done on the behalf of the department to investigate fish assemblages within the Whicher Allocation Plan Area (between Capel and Blackwood Rivers). This work was undertaken in the wet season in response to the absence or low numbers of some species at several of the Healthy Rivers sites. One of the concerns for the Yates Bridge site was the low numbers of western pygmy perch, western minnow, nightfish, and freshwater cobbler, including the absence of juveniles for all species. The assessment detected western minnow and western pygmy perch for the first time, which suggested sampling effort (timing) was a factor in results; however, numbers were still low and no juveniles were detected.

It should be noted that individuals captured during each of the assessments appeared to be in good condition, which is generally an indication that water quality is not a significant concern. The low abundance and lack of juveniles could be a function of predation pressure from redfin perch combined with reduced habitat availability (e.g. including loss of draping vegetation because of reduced shrub layer and rushes/sedges as well as loss of connectivity through the wider river system because of artificial barriers and/or reduced flows [drying climate and water use]). Further assessments at different sites within the reach are recommended to better understand species distribution and needs.

Note: collection of fauna from inland aquatic ecosystems across Western Australia requires a licence from the Department of Primary Industries and Regional Development (DPIRD) and the Department of Biodiversity, Conservation and Attractions (DBCA). All species collected must be reported to these agencies as part of licence conditions.

Although not directly targeted as part of the Healthy Rivers assessment, the following species were recorded:

• Carter’s freshwater mussel (Westralunio carteri) – listed as vulnerable under the Biodiversity Conservation Act 2016; also recorded during the 2019 assessment
• south-western snake-necked turtle (Chelodina oblonga) – also recorded during the 2016 and 2019 assessments
• purple swamphen (Porphyrio porphyrio).

Carter’s freshwater mussels play an important role in freshwater ecosystems as they maintain water quality by removing sediments and pollutants through filtration (Beatty et al. 2017).

Purple swamphen (Porphyrio porphyrio) at the Yates Bridge site, March 2022

For more info on these and other aquatic species, please see the River Science fauna page.

References and further reading:

Beatty SJ, Ma L, Morgan DL and Lymbery A (2017). Baseline assessment of Carter’s Freshwater Mussel, Westralunio carteri at proposed bridge construction sites on the Lower Vasse River. Centre for Fish and Fisheries Research Group and Fish Health Unit (Murdoch University).

Beatty, SJ and Morgan DL (2020) Status of aquatic fauna in systems on the Whicher Scarp: Summary report. Harry Butler Institute. Centre for Sustainable Aquatic Ecosystems. Prepared for the Department of Water and Environmental Regulation.

Revitalising Geographe Waterways – RGW (dwer.wa.gov.au).

Vasse Taskforce – RGW (dwer.wa.gov.au).