Murray-Darling Research Articles

Archaeology on the edge: radiocarbon chronologies for Aboriginal cliff-top sites of the Murray River, South Australia

Abstract This paper presents new radiocarbon dates for two Aboriginal archaeological complexes situated on the cliff-lines of the Murray River in South Australia (SA); at Pooginook Flat and Tanamee. These dates represent the first age estimates for archaeological sites within the Upper Gorge section of the Murray River. The dates ranged from ca. 11 cal ka to the Late Holocene. The research supports previous evidence which has indicated that sites located along the Murray cliffs preserve much of the oldest evidence of Aboriginal peopling along the Murray River corridor in SA. The new dates also allow us to contribute to discussions concerning broader chronological trends in Aboriginal lifeways within the Murray-Darling Basin (MDB). Specifically, the new ages add some insight into the nature and timing of early Aboriginal occupation along the Murray River corridor in SA and further evidence that the LGM acted as a significant inhibitive factor for intensive occupation of this riverscape. The conservation of these significant and informative cliff-top sites remains precarious, however, and there is an imperative to continue to record and sample the extant sites.

Genomic Vulnerability to Climate Change of an Australian Migratory Freshwater Fish, the Golden Perch (Macquaria ambigua).

Genomic vulnerability is a measure of how much evolutionary change is required for a population to maintain optimal genotype-environment associations under projected climates. Aquatic species, and in particular migratory ectotherms, are largely underrepresented in studies of genomic vulnerability. Such species might be well equipped for tracking suitable habitat and spreading diversity that could promote adaptation to future climates. We characterised range-wide genomic diversity and genomic vulnerability in the migratory and fisheries-important golden perch (Macquaria ambigua) from Australia's expansive Murray-Darling Basin (MDB). The MDB has a steep hydroclimatic gradient and is one of the world's most variable regions in terms of climate and streamflow. Golden perch are threatened by fragmentation and obstruction of waterways, alteration of flow regimes, and a progressively hotter and drying climate. We gathered a genomic dataset of 1049 individuals from 186 MDB localities. Despite high range-wide gene flow, golden perch in the warmer, northern catchments had higher predicted vulnerability than those in the cooler, southern catchments. A new cross-validation approach showed that these predictions were insensitive to the exclusion of individual catchments. The results raise concern for populations at warm range edges, which may already be close to their thermal limits. However, a population with functional variants beneficial for climate adaptation found in the most arid and hydrologically variable catchment was predicted to be less vulnerable. Native fish management plans, such as captive breeding and stocking, should consider spatial variation in genomic vulnerability to improve conservation outcomes under climate change, even for dispersive species with high connectivity.

Risk v. reward for responsive movements by a highly mobile fish species in a flow-constrained and barrier-laden river

Context Understanding the movement behaviour of flow-dependent fish species is a foundational principle underlying the effective management of highly modified riverscapes. Aims To determine how variations in river discharge and instream barriers affect the residency, survival and movement of golden perch (Macquaria ambigua) in the degraded Gwydir River system within the northern Murray–Darling Basin. Methods We monitored the movement of 25 acoustic-tagged golden perch for up to 3 years by using a linear array spanning ~180 km of the main river channels across the lower Gwydir system. Key results Golden perch were largely sedentary for extended periods, with movements constrained by the barrier maze that now defines the system. High flows facilitated passage over instream barriers, with the highest periods of activity occurring in spring and early summer, and to a lesser extent in autumn. Conclusion Our findings are indicative of a highly constrained and isolated population of golden perch that is now likely to be neither a source nor a sink, but is in effect a false sink perpetuated by re-stocking practices. Implications The rehabilitation of the fish community in the Gwydir and other systems in similarly poor condition throughout the Murray–Darling Basin will require major institutional and societal change.

An introduction to the collection ‘Environmental flows in northern Murray–Darling Basin: what we know about the science and management after a decade of practice’

This paper is the foreword to a collection on monitoring and research outcomes related to water management in the northern Murray–Darling Basin (MDB), Australia. It provides background on the northern MDB, along with a summary of papers presented in the collection. This article belongs to the collection Environmental flows in northern Murray–Darling Basin: what we know about the science and management after a decade of practice.

Comparing the success and failure of the Murray–Darling Basin Plan's water recovery programs

AbstractThe Murray–Darling Basin (MDB) Plan is held up by some as an exemplar for world‐leading water policy, whilst others have called it a failure. Total proposed recovery was to return 3200 GL of consumptive (e.g. namely irrigation) water use to non‐consumptive (e.g. environmental) use in the MDB, making it one of the largest planned reallocations of water resources in the world. As of June 2023, over AUD$13 billion was committed financially from the Australian government to Basin water reform, with AUD$7.7 billion spent to date on water recovery programs. The implementation of the Plan has been mired in controversy, and it missed its first water recovery targets that had been set for 2024, requiring substantial legislation amendments. This study provides a qualitative policy overview comparison of the success and failure of the two main water recovery programs implemented—buyback of water entitlements from willing sellers and subsidising irrigation infrastructure (on and off‐farm) projects. It concludes that contrary to the popular view held by many, the buyback water recovery program has been a success—achieving the majority of the water recovered to date, at the least cost, with the least negative externalities—in comparison to infrastructure programs.Points for practitioners Assessing success and failure of MDB water recovery programs requires establishing effectiveness, efficiency and equity criteria. Greater work on the benefits of water reallocation downstream and cultural water is needed. A comparison of MDB buyback water recovery programs with irrigation infrastructure programs found higher net benefits for buyback programs. Use first‐best policy to address each water policy objective, not one policy aimed at addressing multi‐objectives. Overall, the Plan.s overall success is still to be determined, given the difficulty in judging environmental outcome success that is intrinsically linked with other external factors.

Breaking Water Laws in the Murray‐Darling Basin: Understanding Water Compliance Challenges and Stakeholder Perceptions

AbstractClimate change will put pressure on irrigated agriculture, increasing water scarcity and possible non‐compliance with water regulations. This is especially true in areas such as the Murray‐Darling Basin (MDB), Australia, an area where water resources have been overallocated. Understanding the expected penalties and probability of detection and prosecution of water theft faced by irrigators is important for understanding the incentives for water compliance. We analyze state‐level compliance action data and undertake a detailed case study of estimating the real penalty value of water theft in New South Wales (NSW), the largest MDB state. This analysis is supplemented with qualitative comment on compliance issues from 63 MDB stakeholder interviews. Although findings from random audits suggest that only a small percentage of irrigators commit water theft offenses, the average probabilities of audit detection and prosecution for water theft in NSW between 2018/19 and 2020/21 were low, leading to an average real expected penalty value of stealing water well below existing market prices. Stakeholder interviews confirmed that the majority did not believe there were serious water theft or water compliance challenges—whereas recent secondary survey data suggests that the general public perceives water theft and compliance as a serious and ongoing challenge in the MDB and more needs to be done to address it. Three recommendations for data and policy reform are proposed, namely: (a) improving compliance data and reporting; (b) increasing the probability of detection and prosecution; and (c) increasing penalties, regulator visibility, and reforming legislation. Results illustrate that even countries with highly advanced water governance require data improvements, along with sufficient levels of compliance activities and penalties for effective deterrence and stakeholder confidence.

How unusual was Australia's 2017–2019 Tinderbox Drought?

Australia’s Murray-Darling Basin experienced three consecutive years of meteorological drought across 2017–2019, collectively named the ‘Tinderbox Drought’. Rainfall deficits during the three-year drought were most pronounced in the Australian cool season (April to September). Deficits in both the cool season and annual total rainfall were unprecedented in the instrumental record. However, the instrumental record provides just one of a range of equally plausible climate trajectories that could have occurred during this period. To determine if the Tinderbox Drought was outside this range, we used observational data from prior to the onset of the drought to construct Linear Inverse Models (LIMs) that emulate the stationary statistics of Australian rainfall and its connection to global sea surface temperature (SST) anomalies. Overall, we find that rainfall deficits were most unusual in the northern Murray-Darling Basin, and during the final year of the drought. The global SST anomalies observed during the first two years of the Tinderbox Drought, particularly the cool anomalies in the central tropical Pacific and western Indian Ocean, are not typically associated with low rainfall across the Murray-Darling Basin. In terms of single-year rainfall anomalies, the only aspect of the Tinderbox Drought that was beyond the range of the LIMs was annual-total rainfall over the northern Murray-Darling Basin during 2019. This coincided with an extreme positive Indian Ocean Dipole event that was also beyond the range of the LIMs. When considered in terms of basin-wide rainfall over the full three years, rainfall deficits during the Tinderbox Drought were beyond the LIM range in terms of both cool-season and annual-total rainfall. This suggests an anthropogenic contribution to the severity of the drought—likely exacerbated by the 2019 extreme positive Indian Ocean Dipole event.

Seed rain as a propagule source for restoration of semi‐arid floodplain old fields

AbstractAimsRevegetation of the world's abandoned agricultural land (old fields) is vital to promote biodiversity and restore ecosystem services. Natural regeneration, whereby vegetation regrows on its own via the propagules already within the landscape, is a cost‐effective restoration approach for old fields. It is vital that we understand how different seed sources contribute to the regenerative capacity of old fields to make informed management decisions. Little is known about seed rain as a propagule source for restoration within semi‐arid floodplains of eastern Australia.LocationWithin this study, we examined seed rain and extant vegetation in old fields and paired remnant sites across four regions in the northern Murray–Darling Basin, eastern Australia.MethodsSeed rain was surveyed using seed traps that were then subjected to a seedling emergence experiment.ResultsMinimal differences in seed rain composition, species richness, abundance, and plant functional groups were observed between old‐field and remnant sites, indicating similarities in seed dispersal within both land use histories. Larger distances to remnant patches and mean annual rainfall were found to drive the composition of old field's seed rain. The study found few emerged Eucalyptus seedlings despite seed traps being placed during peak seed release periods, which may be due to poor tree conditions from drought, interference with seed traps and the episodic germination requirements of Eucalyptus coolabah.ConclusionsThis study demonstrates that seed rain can contribute to the natural regeneration of understorey vegetation, with low abundances of exotic species, unlike other seed bank types. However, seed rain composition resembled little of the extant vegetation, suggesting a species pool facing barriers to germination and establishment, but likely linked to bet‐hedging strategies. Moving forward to promote natural regeneration via seed rain in old fields, protecting existing patches of vegetation within the landscape is vital to enable biotic connectivity.

Alluvial floodplain gully erosion in dryland rivers – An overlooked source of sediment with implications for river condition and management

Dryland alluvial rivers are naturally complex systems with a range of in-stream and floodplain geomorphic units that provide habitat for aquatic and terrestrial biota. These systems are increasingly threatened by accelerated rates of sedimentation leading to declines in geomorphic complexity, habitat quality, and ponding depth. The implications of sedimentation on waterholes, or deep pools, is of particular concern, as they provide critical refugia in arid environments. However, sources of sediment entering and infilling waterholes, and the flow-on effects for habitat and water quality, are not well understood. This study addresses the potential for sediment derived from alluvial floodplain gullies to influence geomorphic change in dryland rivers. Alluvial floodplain gullies are often overlooked in comparison to more widely documented hillslope, or colluvial, gullies. Alluvial gullying is a prevalent feature of the Barwon-Darling River, one of Australia's longest and most important waterways in the northern Murray-Darling Basin (MDB). The estimated volume of sediment derived from floodplain gullies is 168 million m3, which far exceeds estimates used in past sediment budgets. Gully size and complexity varied from small, linear features to large, complex, branching gullies and the total number of gullies increased by ~40 % from the 1960s to 2000s. However, the more recent episodes of gullying (i.e., post 1960s) are limited to smaller gullies, which are likely to yield less sediment than the side walls of the older, larger, more complex gullies. Based on the average decline in maximum waterhole depth, ~19 million m3 of sediment has accumulated in Barwon-Darling waterholes over the past 120 years (equivalent to ~158,000 m3 a−1). A predictive relationship between gully volume and change in waterhole depth was expected, but not observed. Nevertheless, the role of alluvial floodplain gullies as a significant source of sediment should not be overlooked when assessing dryland river forms and processes. River management should consider the implications of changing sediment sources and in-stream loads, with associated changes in water quality and aquatic habitats, which influence river condition.

Insights to key operational questions in forecast-informed dam release operation: case of Hume Dam

ABSTRACT Dam operators decide on dam releases given downstream demands and other factors. Downstream tributaries complicate the problem due to significant uncertainties in their flows. Demands can also be highly uncertain. Incorporating probabilistic ensemble forecasts into the decision-making process can mitigate these uncertainties. We investigate how ensemble flow and demand forecasts can inform Hume Dam releases, in the Murray-Darling Basin, for delivery to Lake Mulwala, about four days downstream. We develop and simulate three forecast-based decision rules governing the releases. The first and second rules are based on the expectations and medians respectively of ensemble forecasts of water excess and shortage, forced by ensemble tributary inflow and demand forecasts. The third rule is based on the risks of an excess or shortage per the ensemble forecasts of water excess and shortage. We analyse the results to answer the following key operational questions: (1) Which decision rule is best for managing Hume Dam releases for water supply and under what conditions? (2) How far ahead of flow and demand forecasts should Hume Dam operators adopt? (3) How does limiting the day-to-day change in Hume Dam releases influence the value of forecasts? (4) What is the value of improved demand forecasts?

How the Resist-Accept-Direct framework is being used by communities for socio-economic climate adaptation: a case study in Australia’s Murray-Darling Basin

Globally, water governance struggles to reconcile increased demands on water resources with climate change–induced reductions in supply, making climate adaptation in water governance a pressing concern. The Resist-Accept-Direct (RAD) framework has emerged as a climate adaptation tool designed to help make adaptation decisions. However, there is limited understanding of social and political factors, which are critical in driving RAD decisions. This paper explores how communities are employing RAD to make climate adaptation decisions, using a case study of the Goulburn-Murray Resilience Strategy (the Strategy); a community-led strategy that uses a version of the RAD framework to build regional resilience in the Goulburn-Murray Irrigation District (GMID) in Australia’s Murray-Darling Basin (MDB). The Strategy focuses on building socio-economic, rather than ecological, resilience, making this research a valuable contribution to RAD literature. To apply the RAD framework to socio-economic adaptation, we adapted the framework to include IPCC language around incremental, transformational, planned, and autonomous adaptation. With the GMID considered a leader of resilience thinking in Australia, the Strategy may help decision-makers address water overallocation and contested governance in the MDB, and provide lessons for water governance globally. Data analysed from 20 semi-structured interviews with people involved with the Strategy revealed two main findings: (1) Communities and governments prioritise different actions under the RAD framework. Governments, particularly at the state level, preference incremental planned adaptation to maintain the status quo (resist), over incremental autonomous adaptation to changing conditions (accept), and transformational planned adaptation at various scales (direct). (2) Community and government actors perceive that factors driving governments’ preference for incremental-resist adaptation include electoral short-termism, linear planning, and conservative government culture.

Large-scale serial replacement of invasive tench (Tinca tinca) by invasive carp (Cyprinus carpio) in the presence of redfin perch (Perca fluviatilis) in the Murray-Darling River system, Australia

Ecosystems worldwide are increasingly being invaded by multiple species, and the rate of biological invasion is accelerating, leading to more interactions among invasive species. One such interaction that has received little attention is the phenomenon of ‘serial replacement’ or ‘over-invasion’, where an established invasive species is supplanted by a second invasive species. Understanding this interaction is important as controlling the second species could inadvertently lead to an increase in the abundance of the first species. We used a hierarchical state-space model to analyse changes in annual abundances (commercial catch-per-unit-effort) of three invasive fish species, tench (Tinca tinca), common carp (Cyprinus carpio) and redfin perch (Perca fluviatilis), in the Murray-Darling River system (MDRS), Australia between 1954–2002. Tench were present at low abundances until the mid-1970 s, before declining to the point of no commercial catch post-1989. This rapid decline coincided with a significant increase in carp abundance, suggesting that carp may have driven the tench decline through habitat modification (particularly the destruction of aquatic plants), consistent with findings from European studies that show the disappearance of tench from ponds with intensive carp farming. Redfin perch populations, were inferred to be much less impacted by the invading carp population. While carp were present in the MDRS for the duration of the study, the rapid increase in carp abundance in the early 1970 s coincided with the introduction of a specific genetic lineage—the “Boolarra strain”. Our analysis provides compelling evidence of serial replacement of long-established tench by invading common carp triggered by the introduction of a novel carp strain.

The phenology of Eucalyptus camaldulensis (Dehnh, 1832) and Eucalyptus coolabah (Blakely & Jacobs, 1934) in the northern Murray–Darling Basin and implications for recruitment on floodplains

Context Published evidence suggests recruitment of floodplain trees is declining in the northern Murray–Darling Basin (MDB) and that this is linked to river flow. Aims We investigated the reproductive phenology of Eucalyptus camaldulensis and Eucalyptus coolabah to better understand tree reproductive cycles and the influence of rainfall and flow on seed production. Methods We surveyed phenological events (bud, flower and fruit production) over 6 years, at nine sites along the Condamine–Balonne River and examined correlations between these events and meteorological parameters. Key results For both species, event timing was consistent across sites and years and event relative abundance scores were different between years. Bud abundance was correlated with rainfall, flooding and warming temperatures and E. coolabah produced additional bud crops after floods. Conclusions The timing of events did not differ significantly from published results for the southern MDB. Rainfall, river flows and flooding in spring resulted in more abundant bud crops in E. camaldulensis and E. coolabah, and subsequently more abundant fruit crops. Fruit dehiscence coincided with summer–autumn flooding, which provides seed for germination. Genetic and physiological factors may also influence the observed pattern. Implications These results will inform water planning in Queensland, Commonwealth Environmental Water Holder watering actions and long-term watering plans in the northern MDB.

Aboriginal earth mounds and ENSO on the Calperum floodplain, Murray-Darling Basin, South Australia

This paper presents new data derived from pollen, starch and radiocarbon samples that were collected from six Aboriginal earth (oven) mounds and middens on the Calperum and Pike floodplains, Murray-Darling Basin (MDB), South Australia (SA). Analyses of these samples were undertaken in order to enhance our understanding about the Holocene lifeways of Aboriginal people living in this region. The results from these analyses, combined with published data about the mounds’ contents, relevant ethno-historical information and climate data, allow us to infer that Aboriginal people adopted, in this region, an innovative food production system about 3800 years ago. Further, we argue that the timing of the change suggests this was in response to adverse El Niño Southern Oscillation (ENSO) related weather patterns and consequently fluctuations in both terrestrial and aquatic food resources.

Movement patterns of Murray cod (Maccullochella peelii) and golden perch (Macquaria ambigua) in a northern Murray–Darling Basin dryland river

Context Dryland rivers are unique ecosystems, where drought and flood play an important role in shaping the ecosystem. River regulation has altered the natural flow regime in many of these systems, affecting migration cues and connectivity for many species. Aims To quantify the discharge-related movements of Murray cod and golden perch within the Condamine–Balonne River subject to differing levels of river regulation. Methods We quantified flow regime variability, river regulation and fish movement to develop generalised additive mixed models to predict movement probability for Murray cod and golden perch. Results Both species showed strong positive relationships between discharge and movement. Murray cod did not show any association with river regulation; however, medium-sized individuals were significantly more likely to move than were smaller or larger fish. Golden perch movements varied among levels of regulation, were more likely to move as body weight increased and showed seasonality of movement, moving less during winter. Conclusions This study presents the largely unobserved movement behaviours of fish across a gradient of river regulation and environmental conditions in the northern Murray–Darling Basin. Implications This information is valuable for informing policy and management decisions that may affect species’ life-history requirements in analogous river systems.

Predicting trajectories of dryland wetland vegetation transformation under climate change: a case study of the northern Murray–Darling Basin, Australia

Dryland wetland vegetation is, paradoxically, both highly sensitive and extremely resilient to environmental change. In the short-term, rapid responses of wetland biota and ecological processes to wetting and drying, which influence ecosystem structure and function, enable rapid reproduction which enhances long-term resilience. However, over longer periods, mechanisms such as seed dormancy and persistent seed and egg banks enable dryland wetland ecosystems to recover after major environmental perturbations such as mega-droughts or wildfire. Climate change is likely to have a significant effect on key drivers of ecological response in dryland wetlands, including hydrology and its interface with other climatic stimuli, e.g. temperature, as well as the frequency and intensity of extreme events. Like species, wetland ecosystems may respond to climate change in three main ways, namely, disappear, persist without significant adjustment, or transform. Here, we consider the conditions under which each of these responses may eventuate for dryland wetlands, by using wetland vegetation of the northern Murray–Darling Basin Australia, as a case study. We also explore what wetland transformation might entail in this region in terms of wetland ecological character and the key values this supports. Finally, we consider the implications for wetland policy and management at present and as trajectories of wetland change unfold.

Integrating Sentinel 2 Imagery with High-Resolution Elevation Data for Automated Inundation Monitoring in Vegetated Floodplain Wetlands

Monitoring inundation in flow-dependent floodplain wetlands is important for understanding the outcomes of environmental water deliveries that aim to inundate different floodplain wetland vegetation types. The most effective way to monitor inundation across large landscapes is with remote sensing. Spectral water indices are often used to detect water in the landscape, but there are challenges in using them to map inundation within the complex vegetated floodplain wetlands. The current method used for monitoring inundation in the large floodplain wetlands that are targets for environmental water delivery in the New South Wales portion of the Murray–Darling Basin (MDB) in eastern Australia considers the complex mixing of water with vegetation and soil, but it is a time-consuming process focused on individual wetlands. In this study, we developed the automated inundation monitoring (AIM) method to enable an efficient process to map inundation in floodplain wetlands with a focus on the lower Lachlan floodplain utilising 25 Sentinel-2 image dates spanning from 2019 to 2023. A local adaptive thresholding (ATH) approach of a suite of spectral indices combined with best available DEM and a cropping layer were integrated into the AIM method. The resulting AIM maps were validated against high-resolution drone images, and vertical and oblique aerial images. Although instances of omission and commission errors were identified in dense vegetation and narrow creek lines, the AIM method showcased high mapping accuracy with overall accuracy of 0.8 measured. The AIM method could be adapted to other MDB wetlands that would further support the inundation monitoring across the basin.

The propensity for negative media reporting of the Murray-Darling Basin Plan in Australia

The Murray-Darling Basin (MDB) Plan, or simply ‘Basin Plan’, represents one of the world's largest returns from consumptive (irrigation) water extraction to environmental use. The Plan has been mired in controversy from when its first draft was burned by irrigators in 2010, and as such has been a source of extensive media reporting. Given the ability of journalism in both informing and influencing public debates, as well as the political deliberation process, we sought to understand the themes and representation of the Plan in media newspapers. This study analysed 6694 media articles referring to the Basin Plan, across 135 outlets, from 2008 to 2022. All articles were sentiment coded using four different natural language processing (NLP) models, with additional manual coding of randomly selected 552 articles. Three-quarters of the total articles were coded as negative using the best-fitting DistilBERT NLP sentiment analysis, while the manual coding suggested almost half were negative in tone. Both methods found that negative stories far outweighed positive stories, suggesting very little ‘balance’ in media Basin Plan reporting. A regression analysis found that rural media outlets were more likely to report negative MDB stories, and that the states of South Australia and Queensland were more likely to report positive stories. Themes of irrigator victimisation and community economic decline dominated news articles (especially negative stories); followed by Basin Plan process, outcomes and governance issues; fights between states; and environmental issues. Environmental themes were relatively less discussed, and Indigenous issues were only referred to in a tiny fraction of all media stories. The study's results provide evidence that the media plays critical roles in water policy debates, giving prominent coverage - with little critical discernment of actual economic impacts - to certain actors with clear ideological and vested interests in the outcomes. Far less media coverage was given to more complex issues such as climate change, long-term structural change for irrigation communities, ecological crises, and sharing water with those who have little.

Contemporary Trends in the Spatial Extent of Common Riverine Fish Species in Australia’s Murray–Darling Basin

As one of the world’s most regulated river basins, the semi-arid Murray–Darling Basin (MDB) in south-eastern Australia is considered at high ecological risk, with substantial declines in native fish populations already identified and climate change threats looming. This places great importance on the collection and use of data to document population trends over large spatial extents, inform management decisions, and provide baselines from which change can be measured. We used two medium-term data sets (10 MDB basin-wide fish surveys from 2004–2022) covering the 23 catchments and 68 sub-catchments of the MDB to investigate trends in the distribution of common riverine species at the entire basin scale. Fifteen native species were analysed for changes in their contemporary range, and whilst short-term changes were identified, all species showed no significant continuous trend over the study period. We further analysed the native species extent relative to their historic records, with bony herring and golden perch occurring in 78% and 68% of their historic river kilometres, respectively, whereas southern pygmy perch, northern river blackfish, silver perch, mountain galaxias, and freshwater catfish were all estimated to occur in less than 10% of their historic extent. Six established non-native species were also analysed and were very consistent in extent over the years, suggesting that they are near the available limits of expansion of their invasion. We provide effect sizes for the spatial extent index which can be used as baselines for future studies, especially those aiming to monitor changes in the spatial extent and population status of native species, or changes in the spatial extent of new or existing non-native species.

Contribution of invasive carp (Cyprinus carpio) to fish biomass in rivers of the Murray–Darling Basin, Australia

Following the introduction of the ‘Boolarra’ strain to Australia in the mid 1960’s, Common carp (Cyprinus carpio) rapidly spread in waterways across the Murray–Darling Basin (MDB) in southern-eastern Australia. A previous study of the population expansion based upon data up to the early 2000s suggested the carp population may have reached, and was fluctuating around, the ecosystem carrying capacity. It has been widely stated in the scientific literature and media that carp comprise up to 90% of fish biomass in MDB rivers; however, this figure is largely based on sparse data collected in the 1990s and the accuracy of this estimate is uncertain. Here, we analyse three decades (1994–2023) of electrofishing data to estimate the median proportion of total fish biomass contributed by carp across > 1,400 sites in lowland rivers (< 700 m elevation) of the New South Wales (NSW) portion of the MDB. The proportion of sites with fish biomass dominated (> 90%) by carp has declined from ≈ 33% of sites in the mid-1990s to ≈ 20% of sites. The proportion of biomass comprised by carp varied depending on the scale of analysis. Across the NSW MDB, carp accounted for a median of 57% (95% CI: 42.5–70.4) of fish biomass at the site level. At the river catchment scale estimates ranged from ≈ 45–66%. In contrast, the median proportion of carp within key rivers ranged from ≈ 46–80%, suggesting that carp can dominate total fish biomass in some rivers for short time-periods. The proportion of biomass contributed by carp at the NSW MDB scale was remarkably stable across decades, suggesting that the carrying capacity of carp may have been reached. Our revised estimates of the contribution of carp to overall fish biomass can facilitate informed debate around the dominance of carp in MDB rivers and provide important information for carp management in the future.