Charles Robert Todd

The Australian lungfish, Neoceratodus forsteri, is one of the world’s oldest vertebrate lineages, with a slow life-history and threatened status, requiring immediate conservation efforts. The main threats to lungfish populations are degradation and availability of key macrophyte habitats, water regulation and flow modification. As this long-lived species (at least 77 years) has delayed maturity (mature at 10 years), field monitoring alone will not be enough to inform the challenge of ensuring sustainable populations. A stochastic metapopulation model was developed for the Burnett River (Southeast Queensland, Australia), an important habitat for the lungfish that is a highly regulated system with extensive water infrastructure. The model consists of three interacting populations, where the ecology and biology of the species were translated into an 80-year-class population projection matrix for each population, each with post-development streamflow, habitat and movement rules. The model highlights the longer-term interaction between dams and stream flows on habitat availability and subsequent recruitment. Through a pre-development streamflow, we quantify the impact of high regulation and development on the lungfish population in the Burnett River: a minor decline in the upstream population (e.g., 9.8% decline), a large decline in the middle population (64.2% decline), virtually no change in the downstream population (e.g., 1.2% decline) and a moderate decline in the overall metapopulation (e.g., 22.3% decline). The loss of spawning and feeding habitat remains the main reason for population decline, with implications that the loss will lead to greater pressure on remaining downstream habitat due to combined flow and dam effects and, in turn, to extended periods of recovery of spawning habitat. Our modeling approach substantially advances conservation management of this species, as it can be adapted to suit other populations in other river systems and used to test sensitivity to recovery actions.

Australian lungfish, Neoceratodus forsteri, has a slow life-history, are one of the world’s oldest living vertebrate lineages, are listed as a threatened species and requires immediate conservation management. The main threats to lungfish populations are degradation and availability of key macrophyte habitats, and water regulation and flow modification. As this long-lived species (at least 77 years) has delayed maturity (mature at 10 years), field monitoring alone will not be enough to inform the challenge of ensuring sustainable populations. A stochastic metapopulation model was developed for the Burnett River (south-east Queensland, Australia), an important habitat for the lungfish that is a highly regulated system with extensive water infrastructure. The model consists of three interacting populations, each with post-development streamflow, habitat and movement rules and is underpinned by an 80 year-class population projection matrix. The model highlights the longer-term interaction between dams and stream flows on habitat availability and subsequent recruitment. Through a pre-development streamflow, we quantify the impact of high regulation and development on the lungfish population in the Burnett River, with 95% decline in one population and a metapopulation decline of 31%. Our modelling approach substantially advances conservation management of this species as it can be adapted to suit other populations in other river systems and used to test sensitivity to recovery actions.

Freshwater fish are facing an extinction crisis on a global scale, with increasing demand for human water consumption driving the regulation and degradation of freshwater ecosystems. Flow regulation especially poses a threat to small‐bodied floodplain and creek specialist species through increasing population fragmentation and isolation, loss and degradation of habitats, and interactions with predators and competitors, resulting in reductions in species' range and abundance. Conserving and recovering many small‐bodied fish species will likely require translocation from wild habitats to refuge habitats to reduce extinction risk and provide buffers against catastrophic natural events (e.g., drought, bushfires). We assessed the value of semi‐artificial farm dams, an abundant feature in the Australian landscape, as interim refugia for the threatened southern pygmy perch Nannoperca australis (Percicthyidae). We compared the relative abundance, population size–structure and body condition of fish introduced (3–4 years prior) into three farm dams with those of three nearby creeks to assess the feasibility of farm dams as a resource to assist small‐bodied native fish conservation and recovery. Farm dams had higher abundance of fish, and equivalent size structure and body condition compared with creek populations, highlighting that suitable farm dams are a valuable and underutilized asset for threatened species' conservation globally.

Context: Handling of wild animals can have adverse effects on individuals that scale up to affect populations and ultimately species. Limited mobility and high harvest pressure expose the threatened Australian freshwater crayfish (Euastacus armatus) to multiple handling events by fishers that cause egg loss. Aim: Our aim was to assess the effects from harvest pressure on the reproductive output of an E. armatus population. Methods: A stochastic model based on empirical data and estimated harvest pressures was used to predict the effect of repetitive handling of individuals on a population of E. armatus subject to harvest and consequent egg loss. Key results: Egg loss resulted in substantial predicted population-level declines, most pronounced in populations subject to high and extreme harvest pressures. Conclusions: We advocate educating fishers to visually determine the reproductive condition of female crayfish by inspecting gonopore setation, eliminating the need for unfurling the tail. Implications: Maximising species reproductive output through limiting handling loss supports the management and conservation of E. armatus populations.

Context: Temperature regulates most ecological processes in freshwater ecosystems. Anthropogenic changes to natural thermal regimes, especially cold water released from stratified impoundments (cold-water pollution, CWP), is a widespread and major threat to fish populations globally. Aims: Because mitigation options for CWP are often expensive, we aimed to provide robust ecological evidence to inform expenditure. Methods: We modelled population responses to CWP remediation for two warm-water fish species (Murray cod and golden perch) downstream of two impoundments (Copeton and Pindari dams) in the semi-arid northern Murray–Darling Basin, Australia. Key results: Predicted populations of both species were severely affected by CWP at both sites; however, impacts were species- and site-specific. Effects were greater on golden perch than Murray cod because of their higher temperature requirements for spawning. Predicted spawning opportunities decreased by 77–100% for golden perch and by 38–92% for Murray cod. The larger Copeton Dam had greater impacts than did the smaller Pindari Dam. Conclusions: Remediation of CWP can help restore populations and meeting the needs of golden perch may benefit a range of other species. Implications: Globally, CWP is a serious threat to warm-water fishes that may compromise and even nullify other restoration efforts. However, restoration may be impeded by current low abundances, angler harvest and restrictions to fish passage.

Common carp (Cyprinus carpio; hereafter carp) is a highly invasive freshwater fish that has colonised river basins across the world causing ecological degradation. In regulated rivers, restoration of natural flow regimes to rehabilitate aquatic ecosystems is a primary focus of environmental management, yet some actions (e.g., environmental water delivery or artificial floodplain inundations) may have unintended consequences including favouring carp. We developed a stochastic population model to represent carp population responses to hydrology and management actions. The model uses current understanding of carp life history, including different survival rates of early life-stages for a range of habitat types, to estimate population responses to various flow/inundation scenarios. Using case studies from two major regions of a large, regulated dryland river, we demonstrate that large natural floods and prolonged droughts accounted for the greatest amplitude of projected carp population increases and declines, respectively. Within-channel flow pulses and small floods, which are a primary focus of environmental flows, appeared to have little influence on long-term carp population trajectories. Artificial inundations generated by floodplain infrastructure, however, caused significant carp recruitment compared to baseline scenarios. Large natural floodplain inundations also maintained high carp biomasses and thus likely high impacts on natural ecological values. The carp population model captures the response of carp to flows and artificial floodplain inundation and will inform environmental managers on the likely trade-offs between achieving restoration of natural ecological values and suppressing a globally invasive fish.

Context: Where water is used to support ecosystem processes under a changing climate, it is vital that it is done efficiently. The Australian bass (Percalates novemaculeata) is a freshwater fish endemic to south-eastern Australia that has been adversely affected by anthropogenic disturbances. This has prompted investigations as to the use of environmental water in support of the species in addition to widespread stocking. Aims: We investigate whether environmental flows and stocking in isolation support a population of Australian bass more effectively than they do in combination, under various climate-change scenarios. We also determine the cost efficiency of each strategy. Methods: We used an age-based stochastic population model to determine outcomes of strategies. Key results: Our model suggested that the application of two or three large environmental flows per year was highly effective in all but the most-extreme climate-change scenarios. Alternatively, the value of stocking increased with an increasing severity of climate change. The cost of delivery of environmental flows far exceeds that of stocking. However, stocking directly benefits only a single species (in this case Australian bass), whereas the provision of environmental water has multi-species benefits, in addition to being vital to maintaining the river ecosystem itself. Conclusions: Under severe climate-change scenarios, stocking, and the use of environmental water in support of populations, may be successful only to a point. Implications: Extreme climate change may therefore negate any attempts to halt the decline of populations of some species.

The release of cold-water from hypolimnetic zones of impoundments sharply reduces downstream riverine water temperature. This cold-water pollution (CWP) can extend for hundreds of kilometres, severely challenging the physiological ability of aquatic fauna, particularly ectotherms such as fish, to maintain essential processes such as metabolism, development and growth and survival. The impact of CWP on native fish, especially early life stages, is poorly known. We investigated the effect of a 24-hour exposure to a range of environmentally-related water temperatures (8, 10, 12, 14, 16, 18 and 20 ?) on three age-classes (< 24-hour-old, 7-day and 14-day-old larvae) of two Australian native fish species: Murray cod (Maccullochella peelii) and Macquarie perch (Macquaria australasica). Overall, larvae of M. peelii were more sensitive to lower water temperatures and hence CWP than M. australasica, indicated by higher rates of equilibrium loss. Larvae of M. peelii were most sensitive to exposure at seven days old whereas M. australasica larvae were most sensitive at < 24-h-old. Using our results, we modelled pre-and post-impoundment temperature scenarios and estimated the downstream CWP footprint for both species in an Australian river reach. Larvae ofM. peelii were predicted to be absent from the first 26 km of river downstream of the impoundment compared with no impact on the distribution of M. australasica. Managing riverine water temperature below impoundments is fundamental to promoting positive outcomes for endemic fish on not only a local, but global basis. This study emphasises the differential impact of CWP among the critical early life stages and fish species and highlights the urgent need to better manage hypolimnetic water releases to improve downstream river ecosystems.

The Murray–Darling Basin (MDB) is Australia’s food bowl, contributing 40% of agricultural production and supporting a population of over 4 million people. Historically, the MDB supported a unique native fish community with significant cultural, subsistence, recreational, commercial and ecological values. Approximately one-quarter of the MDB’s native species are endemic. Changes to river flows and habitats have led to a >90% decline in native fish populations over the past 150 years, with almost half the species now of conservation concern. Commercial fisheries have collapsed, and important traditional cultural practices of First Nations People have been weakened. The past 20 years have seen significant advances in the scientific understanding of native fish ecology, the effects of human-related activities and the recovery measures needed. The science is well established, and some robust restoration-enabling policies have been initiated to underpin actions. What is now required is the political vision and commitment to support investment to drive long-term recovery. We present a summary of 30 priority activities urgently needed to restore MDB native fishes.