W1: Aquatic ecosystem health

Introduction

For information on this indicator see Background: Water.

This indicator assesses the health of rivers and streams in the ACT and includes assessments of native fish, macroinvertebrates and riparian health. These provide an indication of the health of aquatic ecosystems and demonstrate the biological impacts of degraded habitat, changes to flows and water quality, and the impacts of invasive aquatic species.

Although urban waters are included in this report, a more comprehensive assessment of the ACT’s urban waters, including lakes and wetlands, can be found in the 2022 Investigation into the State of the Lakes and Waterways in the ACT.

There are currently no available assessments for non-urban wetland condition in the ACT. However, upland bogs and fens are included in the Conservation Effectiveness Monitoring Program undertaken by the ACT Government (see Biodiversity). This monitoring program will provide data on the condition of these wetlands in the future.

Climate trends are an important consideration for aquatic ecosystem assessments. This is particularly the case for rainfall which provides the flows in waterways needed to maintain ecosystem health. The period from 2015 and 2022 saw extremes in weather conditions across the ACT, from the hot and dry conditions between 2017 to 2019 to the very wet conditions from mid-2020 to 2022 (see Climate Change). These extremes had a significant impact on river flows in the ACT (see Indicator W2: River flows) and have influenced the aquatic ecosystem assessments presented in this report.

Catchment Health Indicator Program (CHIP) assessments

This is the second inclusion of CHIP data in the ACT’s state of the environment reporting. As in the 2019 report, the data presented are averaged results for assessed reaches over a four-year period aligned to the State of the Environment reporting periods. This approach is considered to be more accurate than comparisons of annual data which often represent short-term changes rather than actual trends in condition. This 2023 report uses average scores for the 2019 to 2022 period.

It is important to note that the site selection used for CHIP has a significant influence on the overall number of reaches assigned to a particular condition score. Consequently, although figures on the number of reaches in each condition category may be useful for an overview of CHIP results, they should be viewed with caution.

Information on CHIP methodology, reaches used for assessments, individual reach condition scores and the main pressures affecting condition, can be found in annual CHIP reports.

Aquatic health

Overall aquatic ecosystem condition — the Catchment Health Indicator Program (CHIP)

Overall condition was assessed for 71 reaches in the Ginninderra, Molonglo and Southern ACT catchments, with scores averaged for the 2019 to 2022 period.

For the total reaches assessed, 52% were in fair condition across the three catchments and 41% were in good condition (Figure 1 and Figure 2). Four reaches were found to be in excellent condition and one was assessed as poor. Aquatic condition was strongly influenced by land use with reaches in fair condition mostly in urban and rural areas, and those in good and excellent condition mostly on conservation and protected lands.

All 15 reaches assessed in the Ginninderra catchment were classed as fair condition. For the 26 Molonglo catchment reaches, 12 were found to be in fair condition, 13 were classed as good and one attained an excellent condition score. The Southern ACT catchment had the highest proportion of reaches in good to excellent condition with 19 out of the 30 reaches assessed, three of these reaches had excellent condition scores. The higher proportion of good and excellent condition reaches is mainly due to the catchment having large areas of conservation and protected lands with unmodified landscapes. For urban and rural areas in the Southern ACT catchment, five reaches attained good scores, seven reaches were classed as fair and one as poor.

The 2019 to 2022 period had slightly improved overall aquatic ecosystem condition scores compared to the 2015 to 2018 period with a higher percentage of reaches in good to excellent condition (47% compared to 42%) and a lower percentage of reaches in fair to poor condition (53% compared to 58%). The small difference between the two periods is likely related to the increased rainfall and waterway flows over the 2020 to 2022 period. CHIP results show that reaches in conservation areas achieve consistently good scores regardless of rainfall, agricultural reaches score slightly better in wetter than average years and urban reaches score worse in wetter than average years.

These results show that aquatic ecosystem condition is variable across the ACT. Although results are strongly linked to land use, the presence of some reaches in good condition in urban areas shows that healthy aquatic ecosystems can be supported with effective management and water-sensitive design.

The water quality component of the CHIP assessment is presented in Indicator W3: Water quality.

Macroinvertebrate condition — Catchment Health Indicator Program (CHIP)

The CHIP assesses macroinvertebrate community health for each reach using the results of two annual surveys. For information on macroinvertebrates see Background: Water.

Macroinvertebrate condition was assessed for 71 reaches in the Ginninderra, Molonglo and Southern ACT catchments, with scores averaged for the 2019 to 2022 period. For the 71 reaches assessed, 17% were found to be in poor to degraded condition, with 46% classed as fair (Figure 3 and Figure 4). A further 37% of reaches assessed were found to have good to excellent macroinvertebrate condition. Only one reach in the Ginninderra catchment had good macroinvertebrate condition, the Molonglo catchment had 12 reaches classed as good, and the Southern ACT catchment had 12 reaches classed as good and one as excellent.

The 2019 to 2022 period saw improved macroinvertebrate condition scores compared to the 2015 to 2018 period with a higher percentage of reaches in good to excellent condition (36% compared to 25%) and a lower percentage of reaches in fair to poor condition (61% compared to 72%). The difference between the two periods is likely in response to the increased rainfall and waterway flows over the 2020 to 2022 period.

The CHIP scores for macroinvertebrate condition are strongly linked to land use, with urban areas generally having fair to poor condition. In such areas, high levels of pollution, alteration to natural flows and loss of riparian vegetation are likely to have had a negative effect on macroinvertebrate communities.

Riparian condition — Catchment Health Indicator Program (CHIP)

The CHIP assesses riparian condition for each reach with surveys conducted every two years. For information on riparian vegetation see Background: Water. 

It should be noted that the current riparian assessment methodology results in poor scores for sites without extensive tree canopy cover. Consequently, riparian condition assessments for naturally treeless ecosystems, such as swamps, bogs and fens, are unlikely to reflect their true condition. The riparian condition methodology is being reviewed to improve assessments of naturally treeless ecosystems.

Riparian condition was assessed for 68 reaches in the Ginninderra, Molonglo and Southern ACT catchments. Riparian condition scores are the worst of all the CHIP assessment components (macroinvertebrates, riparian and water quality). Only 13% of reaches were assessed as having good to excellent riparian condition and 34% were found to be in fair condition. Over half the reaches (53%) were assessed as having poor to degraded riparian condition (Figure 5 and Figure 6), in comparison only 17% of reaches had poor to degraded macroinvertebrate condition and no reaches had poor to degraded water quality.

The Ginninderra catchment had no reaches with good riparian condition, the Molonglo catchment had three reaches classed as good and one as excellent, and the Southern ACT catchment had three reaches classed as good and two as excellent.

The 2019 to 2022 period saw a further decline in riparian condition compared to the 2015 to 2018 period with a higher percentage of reaches in poor to degraded condition (53% compared to 48%), although the number of reaches in excellent condition did increase slightly in the 2019 to 2022 period. The reasons for the changes are unclear but may reflect the stress on riparian vegetation caused by the drought prior to 2020.

As with overall CHIP scores and macroinvertebrates, riparian condition is strongly linked to land use, with urban and rural areas generally having fair to degraded condition due to vegetation clearing. However, there were also fair and poor reaches in conservation and protected areas demonstrating that riparian health can be degraded regardless of land use due to impacts of changed flow regimes and pest plants. However, some of these lower scores also may reflect the methodology used.

The restoration of riparian health is perhaps the most easily achievable of actions required to improve aquatic ecosystem condition. The replanting of native species in cleared riparian zones and the removal of weed species would greatly improve aquatic health and the amenity of aquatic ecosystems for the ACT community.

Native fish

The ACT Government undertakes fish surveys in the Cotter River at five sites above Corin Dam and five sites between the Bendora and Cotter Dams (above Cotter Reservoir), and seven sites in the Murrumbidgee River. Fish survey results for these rivers are presented below for abundance and biomass (a measurement of the relative size of species). It should be noted that the assessments of fish abundance and biomass can be affected by the numbers of schooling species, the presence of juvenile fish as compared to adults and the presence of larger bodied species.

Native fish abundance and biomass – Cotter River

In 2023, the proportion of native fish numbers to alien fish in the Cotter River was the lowest since 2015 with native species accounting for just over 50% of the total abundance (Figure 7). This is much lower than between 2015 and 2022 when native fish accounted for 65% to nearly 90% of the total fish abundance. The 2023 decrease in the Cotter River’s native fish is also shown for biomass with native species only accounting for 26% of the total biomass (Figure 8). As with fish abundance, this was much lower than between 2015 and 2022 when native fish accounted for 55% to nearly 80% of the total biomass.

The Cotter River fish surveys have found that native fish have been significantly impacted by the 2019 drought and especially the 2020 bushfires. This is particularly the case for the five survey sites above Corin Dam where both the abundance and biomass of native fish has decreased since 2019 (Figure 9 and Figure 10). The main reason for native fish decline has been the combination of sediment and ash from the burnt catchment and high flows from above average rainfall since 2020. These have decreased the abundance of Two-spined Blackfish (Gadopsis bispinosus) above Corin Dam to the lowest numbers since monitoring began in 2000.

The influx of sediment and ash, and the increase in flows, have also impacted the most recent breeding seasons. This is reflected in the 2023 survey results with native species accounting for just 17% of the total fish abundance and 7% of the total fish biomass. The lack of breeding success now means that most of the remaining Two-spined Blackfish are very large old adults. In contrast, the population of the alien fish Rainbow trout has increased since the fire and has dominated fish abundance and biomass in the Cotter River above Corin Dam from 2021, and particularly in 2023.

These results show that river flow and bushfires are significant drivers of native and alien fish abundance and biomass in the Cotter River allowing alien species to dominate fish populations and impact on native fish.

For more information on the fire impacts on Two-spined Blackfish see Two-spined Blackfish After the Orroral Valley Bushfire.

Native fish abundance and biomass – Murrumbidgee River

Fish surveys in the Murrumbidgee River from 2011 to 2023 show that native fish account for around 20% to 35% of total fish abundance with alien species accounting for the majority of fish in the river (Figure 11). The biomass of native fish was similar, accounting for between 20% and 40% of total fish biomass (Figure 12). The dominance of alien species in the Murrumbidgee River is mainly due to high numbers of European Carp (Cyprinus carpio).

The abundance of native fish, and to some extent alien species, declined in the high river flow years since 2019. However, there were increased numbers and size classes for Golden Perch (Macquaria ambigua) which is a species that benefits from high flows that increase connectivity.

Native fish abundance and biomass have been assisted in some recent years with the wider distribution of Murray Cod (Maccullochella Peelii) populations and occasional years of recruitment resulting in large numbers of juvenile Murray Cod. However, this has primarily occurred in sites upstream of Point Hut and into NSW.

Although alien fish abundance and biomass is higher in the Murrumbidgee River compared to the Cotter River, results for both rivers demonstrate that alien species can dominate fish populations and impact on native fish when river systems are disturbed or degraded by pressures such as poor water quality and regulated flows. River modification has especially created favourable conditions for alien species to thrive in the Murrumbidgee River.

The greater biomass of alien species has implications for native fish communities, particularly in relation to competition for food and habitat resources, as well as the spread of disease and habitat availability.

For more information on the Murrumbidgee River fish surveys undertaken by the ACT Government see 2021 ACT Murrumbidgee Fishery Survey.

Constructed barriers to native fish passage in the ACT

Connected waterways are critical for the survival of native fish. Structures such as dams, weirs and road crossings create barriers that can prevent fish passage to locate critical resources such as food, shelter, nursery sites and spawning grounds. The most recent assessment of constructed barriers to fish passage in the ACT conservation estate found:

• a total of 234 potential barriers to fish passage
• seven of these barriers were beneficial barriers that restricted the movement of pest fish species, preventing competition and the spread of disease
• 18 sites were assessed as significant barriers to fish movement, six sites had a medium level of blockage and 70 sites had a low level of blockage and/or were in low quality aquatic habitat, and
• of the total of 234 potential barriers,133 were assessed as not preventing the movement of native fish.^[1]Unpublished study conducted by the Environment, Planning and Sustainable Development Directorate.

These findings show that there are numerous constructed barriers to fish passage in the ACT. The remediation or removal of barriers, particularly those assessed as significantly restricting the movement of native fish, will improve the connectivity of ACT waterways and increase native fish populations.

Native fish stocking

The ACT Government stocks juvenile Murray Cod (Maccullochella peelii) and Golden Perch (Macquaria ambigua) into Canberra’s urban lakes and selected ponds. Because they generally do not breed successfully in these environments, ongoing stocking is required to maintain populations. However, successful reproduction of smaller native species, such as Western Carp Gudgeon (Hypseleotris klunzingeri), is common in urban lakes and ponds.

Between 2015 and 2023, nearly 222,000 Golden Perch and 230,000 Murray Cod were stocked to Canberra’s lakes and larger ponds (Figure 13). Water bodies that have been stocked include Lake Burley Griffin (funded by the National Capital Authority), Lake Ginninderra, Lake Tuggeranong, the Yerrabi, Upper Stranger, Isabella, Point Hut, West Belconnen and Coombs ponds. Stocking is undertaken to increase the abundance of native species and also to provide recreational fishing to reduce the angling pressure on natural riverine populations, such as the Murrumbidgee River fish population.

Data gaps

• Although current monitoring sites provide good coverage of the ACT’s urban areas, there needs to be more comprehensive monitoring for aquatic ecosystems in conservation and rural areas to provide a broader picture of aquatic health across the region. 
• Little information exists on wetland health.