Annie Brook Pools

River condition at the Annie Brook Pools site (site code: MB15ANNI1, site reference: 6104073) on Annie Brook 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 Annie Brook Pools site was most recently assessed in December 2024. All known assessments are listed below:

Other data:
There are currently no monitored gauging stations on Annie Brook.

Search the site code or site reference in the Department of Water and Environmental Regulation’s (the department) 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.

Aquatic habitat was similar in character between the years, although the diversity and amount of habitat varied depending on level of inundation. The site has poor to moderate structural aquatic habitat complexity, including sparse woody debris of similar sizes. Larger sized woody debris was more available in 2017 with the higher water levels inundating the areas where larger wood pieces occurred. Woody debris provides aquatic species shelter from terrestrial predators as well as protection from fast flows.

Habitat was also provided by a small proportion of undercut banks as well as submerged and overhanging tree roots and terrestrial vegetation draped in the water.

Triglochin, a submerged aquatic macrophyte, covered a small portion of the assessment site during the 2017 survey but was not observed in 2022 and 2024. The density of Triglochin varies naturally with seasonal conditions, as it inhabits seasonally inundated environments and can rapidly establish from the sediment seed bank following initial flows. Macrophytes provide additional habitat as well as food resources for aquatic fauna, including for species that forage directly on the plant material as well as the associated periphyton and free-swimming invertebrates.

The riverbed substrate consisted of pebble, gravel, sand and silt. In 2017, there was a sparse to moderate cover of organic material (algae and detritus), compared to a dense cover of mostly detritus and leaves in 2022 and 2024. This is mostly related to the remnant habitat in 2022—24 being limited to a small river pool (due to lower water levels), which is a deposition site (naturally higher in organic material compared to other habitats). The pool was also turbid which restricts algae growth (less light penetration). Organic material provides food for grazers and macroinvertebrates.

Water quality measured differed between the different sampling seasons.

Dissolved oxygen (DO), measured over the 24-hour collection period, was near anoxic in March 2022, well below the guideline of 4 mg/L used in the South West Index of River Condition. Beatty et al (2013) found that local freshwater fish species are typically found in river systems with DO levels greater than 4 mg/L. Encouragingly, some freshwater fish species were found in the river system (species discussed ‘Species found at site’ section). Some fish are known to tolerate low DO conditions for short periods of time, however they are expected to be under stress and therefore at risk if conditions are prolonged, worsen or if exposed to other stress factors.

Dissolved oxygen concentrations were optimal in 2017 and 2024; however, the diurnal range (over a 24-hour cycle) in 2024 was slightly greater than the recommended 4 mg/L. The sudden dip in dissolved oxygen in the early evening is most likely attributed to a localised rainfall event.

The mean water temperatures were within optimal ranges expected for South West streams (SWIRC) guidelines, based on the recommended upper range for temperature of 25°C. The below graph shows the relationship between dissolved oxygen and temperatures at the assessment site over the 24-hr sampling period in December 2024.

Footnote:

¹ A lower limit of 4 mg/L is taken from the SWIRC where it is used as a guideline limit for condition scoring. This level aligns with Beatty et al. (2013) where south-west native fish species were shown to typically reside in sites with levels above 4 mg/L.

² An upper limit for temperature is nominally set at 25°C based on the range recorded in sites where south-west native fish species are typically found to occur (Beatty et al. 2013).

The mean salinity at this assessment site has varied between the fresh (2017 and 2024) and marginal (2022) salinity categories. It is expected for salinity levels to increase during the dry season through increasing levels of evapotranspiration.

Nutrients from the December 2024 assessment (total nitrogen, 0.874 mg/L and total phosphorous, 0.111 mg/L) are considered to be indicative of moderate to very high disturbance based on the SWIRC categories. The SWIRC categories are based on analysis of departure from natural local conditions across a range of sites across south-west WA within different land use settings.

Land use in the upper Annie Brook catchment is comprised mostly of large rural properties with pockets of native vegetation and agricultural use (mostly beef grazing). Please visit the Toby Inlet – Revitalising Geographe Waterways website for more information on water quality and nutrients for this catchment.

The species captured at the Annie Brook Pools 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. Notable findings from the assessments are discussed below.

Four native freshwater fish (nightfish, western minnow, western pygmy perch, and freshwater cobbler) and one non-native freshwater fish (eastern gambusia) were present at each sampling occasion. Freshwater crustaceans (smooth marron and gilgie) were present in October 2017 and December 2024 but not detected in March 2022.

The number and proportion of the non-native eastern gambusia varied over time. In 2017, only a small number of the exotic eastern gambusia were detected (less than five per cent), in 2022 they dominated the catch, comprising 63 per cent of all fish, and in 2024, gambusia comprised only a small proportion of the catch (less than five per cent).

Native fish were well represented in all sampling periods. In 2017, western pygmy perch were the dominant species, comprising over half of total catch, whereas nightfish were the dominant species in 2024. Juveniles of both species were also present within the catch during both of these assessments. The presence of juveniles suggests successful recruitment and suitable habitat for this species within the subcatchment. Both species were observed in 2022, but the absence of juveniles most likely be a reflection of the anoxic water quality conditions during this dry season.

The high abundance of native fish found in this pool also highlights the importance of refuge areas for native fish during the dry season, with this small pool supporting a relatively large population of native species.

Neither smooth marron nor gilgie were observed during the March 2022 assessment but both were detected in low abundances in 2017 and 2024. This variation is likely a function of sampling time and effort, as well as detectability (i.e. individuals may remain within burrows and be less active when water levels are low). The relatively low number of crayfish is not unexpected given the limited extent of aquatic habitat at the site, particularly noting the small area of permanent water that is expected to be maintained through the dry season, as seen in 2022.

Gilgies have been recorded in higher abundance further upstream at the Upper Annie Brook site (no smooth marron detected at this site). This site dries over the summer months, gilgie create burrows for shelter and will retreat to these during the dry season to avoid desiccation; as such, it would not be surprising to see gilgie emerge when water levels increase at the site.

Smooth marron require permanent, good quality water and habitat, which may explain the general low abundance at this site. Future assessments should consider assessing other habitats in the area to assess the status of these species in the system and focussing sampling on early in the dry season when more habitat is available and species are more active.

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:

• Motorbike frog (Litoria moorei)
• Diving beetle (Family Dytiscidae)
• Water scorpion (Family Nepoidea).

Motorbike frog at the Annie Brook Pools site, December 2024

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

References and further reading

Beatty S, Morgan D, Keleher J, Lymbery A, Close P, Speldewinde P, Storer T & Kitsios A 2013, Adapting to climate change: A risk assessment and decision-making framework for managing groundwater dependent ecosystems with declining water levels. Supporting document 4: Environmental variables in the habitats of southwestern Australian freshwater fishes: An approach for setting threshold indicator value, National Climate Change Adaptation Research Facility, Gold Coast.

Mayer XM, Ruprecht JK and Bari MA 2005, Stream salinity status and trends in south-west Western Australia, Department of Environment, Salinity and Land Use Impact Series, Report No. SLUI 38.

Revitalising Geographe Waterways – RGW (https://rgw.dwer.wa.gov.au)

Vasse Taskforce – RGW (https://rgw.dwer.wa.gov.au/about-vasse-taskforce/)