Detecting rare fish species across extensive river systems can be challenging due to their low abundance. However, detection probability may increase when individuals aggregate within localised areas during the spawning season. This study investigates whether environmental DNA (eDNA) detection probability can be improved for the threatened Australian grayling ( Prototroctes maraena ) by concentrating sampling effort in the putative spawning grounds (lower freshwater reaches) during the spawning season. Eight replicate water samples were collected from each of three sites in the Tuross and Deua rivers in New South Wales (NSW), where Australian grayling occur in very low abundance, and from three sites in the Bunyip River in Victoria (positive field control)—one of the last rivers supporting a relatively high abundance of this species. Sampling was conducted on six occasions between April and June 2023 to coincide with the species' downstream migration to spawning grounds. A species-specific quantitative PCR (qPCR) assay targeting the Australian grayling COI gene was developed and applied to detect the species using six replicate qPCR reactions per sample. Australian grayling were detected or putatively detected in 78% of sampling occasions in the Tuross River and 100% of sampling occasions in the Deua and Bunyip rivers. The proportion of positive qPCR replicates was much higher in the Bunyip River, consistent with the species' higher abundance in this location. Nevertheless, there was a peak in detection probability in the NSW sites from May to early June, confirming the efficacy of a targeted sampling strategy coinciding with spawning aggregations in the lower reaches. The study's findings demonstrate the benefits of incorporating knowledge of behaviour into eDNA survey design for improved detection of low-abundance species, which can assist in informing threatened fish recovery actions.

Freshwater ecosystems and their biota are under increasing pressure from anthropogenic stressors. In response to declining fish stocks, hatchery and stocking programmes are widely implemented as core components of restoration and management strategies, with positive outcomes for some wild populations. Despite this, stocking remains contentious due to potential genetic and ecological risks to wild populations. Monitoring and evaluation of stocking outcomes are critical to ensuring the long-term sustainability of wild populations, but identification of stocked individuals post-release remains a key challenge, particularly for mobile species. In this study, we combined otolith (natal origin and age) and genomic data to identify stocked individuals and evaluate the genetic implications of stocking for a culturally and socioeconomically important and mobile freshwater fish, golden perch Macquaria ambigua (family: Percichthyidae), across Australia's Murray-Darling Basin (MDB). We also generated a chromosome-level genome assembly. Many close kin were detected across the MDB, increasing in prevalence over recent decades and mostly of hatchery origin. Rivers with many close kin were associated with low effective population sizes (Ne < 100). Genetic signatures of stocking varied according to local context, being most pronounced in but not restricted to rivers considered functionally isolated for management purposes. Where fish are stocked into rivers that are part of the connected metapopulation, there is scope to modify current stocking practices to avoid over-representation of related stocked individuals. Increased focus on the genetic diversity of stocked fish is likely to promote the long-term persistence of golden perch in the wild.

This study presents the first investigation into the oceanic spawning migrations of Australian long-finned eels (Anguilla reinhardtii), a critical and previously undocumented stage in their life cycle. In autumn 2024, twenty adult eels were collected from two estuaries in southern Australia and tagged with pop-up satellite archival tags. Individuals were successfully tracked up to five months, with some migrating as far as ~2,500 km from their release sites to the tropical Coral Sea. Migration trajectories revealed relatively consistent and direct movement pathways, with eels remaining several hundred kilometers offshore for much of their journey before venturing farther offshore during later stages. Tag data provided clear evidence of predation events, indicating that escapement to the sea does not guarantee successful spawning and highlighting the potential ecological role of eels within marine food webs. Approximately 40% of migrations were prematurely terminated due to predation, with sharks likely responsible in nearshore environments and marine mammals in offshore regions. Additionally, consistent observations of diel vertical migration, characterized by nocturnal ascents toward the surface and diurnal descents to deeper waters, offer valuable insights into behavioral adaptations during the oceanic migration phase of this enigmatic life history. Tag endpoints and reconstructed migratory trajectories generally aligned with a region between the East Australian Current and the Lord Howe Seamount Chain. Several endpoints clustered within a small region of the northeastern Coral Sea, south of the Solomon Islands. These locations align with historical leptocephali collection sites, strengthening the hypothesis that the spawning area of long-finned eels lies in the northeastern Coral Sea.

Variation in somatic growth plays a critical role in determining an individual’s body size and the expression of its life history. Understanding the environmental drivers of growth variation in mobile organisms such as fishes can be challenging because an individual’s growth expression integrates processes operating at different spatial and temporal scales. Traditionally, otolith (ear stone) based growth analyses have focussed on temporal environmental variation by assuming an individual spends its whole life at its capture location. This approach ignores the movement potential of individuals and thus the role of spatio-temporal variation in conditions experienced. Here, we develop a modelling framework that incorporates individual movement information reconstructed via the analysis of chemical tracers in otoliths. We assess whether consideration of movement histories is important to estimating growth of a mobile freshwater fish, golden perch (Macquaria ambigua) at three spatial resolutions: basin-scale, reach-scale (movement-exclusive), and reach-scale (movement-inclusive). The predictive capacity of annual growth models slightly improved from the basin to the reach spatial scales (inclusive or exclusive of movement histories). Contrary to expectations, incorporating individual movement information, did not improve our ability to describe growth patterns. Golden perch growth was linked to the magnitude of and variation in spring, summer, and previous-year (antecedent) discharge, and spring temperature. The direction and magnitude of these effects was, however, dependent on life stage. Adults benefitted strongly from any increase in discharge or temperature, whereas juveniles benefitted only from increased summer discharge and grew slower in years characterised by wetter and warmer springs. We suggest that, for highly mobile fish like golden perch and in the absence of fine, ‘within reach’ scale biological data, coarser ‘reach-scale’ environmental variation may adequately describe individual growth trajectories.

Anguillid eel populations are under threat globally. A particularly vulnerable life-cycle stage is the migration of mature adult eels downstream from freshwater habitats through estuaries into the sea to spawn. This study investigated the factors associated with downstream migration of the short-finned eel Anguilla australis (Richardson 1841) from a coastal wetland (Lake Condah) in south-east Australia, using acoustic telemetry. Migration was associated with time of the year, higher water level and river flows, decreasing water temperature, and darker moon phases. Larger individuals and those in better condition were more likely to migrate from the wetland. Downstream migration peaked in spring, in contrast to the typical autumn migration period for other temperate anguillids. Variable responses, in comparison to other studies, highlight how migration cues may not be universal. In south-east Australia, short-finned eels may have evolved to migrate in multiple phases by first migrating to the estuary during typical seasonal spring flow pulses (e.g., to avoid being stranded in upland reaches during dry summer periods) and then migrating into the ocean in autumn. More research is needed to unravel these processes and causes, especially considering that the relationship between migration and hydrology may be complex and confounded (e.g., by human-induced disruptions to migratory pathways).

1. Seeking out appropriate habitat or food resources is one of the key reasons why animals move. Despite the benefits of movement, some individuals in a population remain resident or express alternative movement phenotypes. Movement, however, is energetically costly and can result in resources being diverted away from growth and reproduction. The presence of multiple movement phenotypes within populations suggests that each can have commensurate levels of performance depending on the underlying environmental conditions. 2. Here we explore the context-dependent costs and benefits of upstream and downstream juvenile dispersal in a large river system. We expect that if the energetic costs of moving (upstream against a current or downstream with a current) exceed any benefits to growth, then residents (who do not move) will have faster growth. Alternatively, movement costs may be offset if individuals move to more favourable environments, resulting in dispersers benefiting with faster growth. 3. We used biological information naturally archived in the otoliths of a potamodromous fish, golden perch Macquaria ambigua, to quantify how movement phenotypes affect growth across individuals exposed to spatiotemporally varying environmental conditions. 4. We found that juvenile growth differed considerably among dispersal phenotypes (resident, upstream, downstream, stocked): in general, surviving wild-spawned downstream and upstream dispersers and hatchery-stocked fish all grew faster than individuals that remained resident within their natal reach. Further, juvenile growth was sensitive to local environmental conditions and had carryover effects from an individual's natal-year. Juvenile growth of all dispersal phenotypes was higher in years with below average natal-year summer discharge (when fish are similar to 2-3 months old), likely because of increased concentrations of food resources. In contrast, the effects of natal-year spring discharge (around the time individuals were spawned) were dependent on dispersal direction, with positive effects for downstream dispersers and upstream dispersers, and negative effects for resident individuals. 5. Our results suggest that an individual's growth can benefit from early-life movement, although the magnitude of this effect depends on local environmental conditions and the direction travelled. Our study reinforces the importance of heterogeneous and connected riverscapes that foster a diversity of individual growth responses.

Improved knowledge of habitat use and movements is needed to support recovery of threatened riverine fish species. The trout cod (Maccullochella macquariensis) is a nationally threatened fish species endemic to rivers of the southern Murray–Darling Basin (MDB), Australia. This study investigated diurnal and nocturnal habitat use and movements of trout cod in the Goulburn River, using radio-telemetry. Trout cod typically occupied small reaches (<300 m) of stream during the late spring–summer study period. Larger-scale movements (mostly home-range shifts) and movements onto inundated riparian habitats also occurred during a period of high discharge. Trout cod occupied deeper water further from the bank during the day and shallower water closer to the bank during the night and used areas with slower velocities more than was their availability, particularly during the earlier hours of the day. They also made extensive use of in-stream cover (i.e. wood). Our information on diel habitat use provides a platform to guide informed management (e.g. availability of preferred habitats under different river-discharge scenarios) to improve trout cod population sustainability.

We conducted the first comprehensive global assessment of the extinction risk of Australia's native freshwater fishes. Using International Union for Conservation of Nature (IUCN) criteria, 37 % (88 species) of the 241 assessed species were threatened (Critically Endangered, Endangered or Vulnerable), with one being Extinct. Lepidogalaxiidae and Neoceratodontidae had the highest level of threat (100 %, both single species families), followed by Galaxiidae (78 % of 40 species), Percichthyidae (62 % of 22 species) and Melanotaeniidae (53 % of 19 species). Northern Australia supported greater species richness, while a concentration of threatened species occurred in the more human-populated areas across southern and eastern drainage divisions, including South West Coast (55 % of species assessed as threatened), Tasmania (54 %) and South East Coast (Victoria) (45 %). Most threatened freshwater fishes qualified for listing based on their restricted geographic ranges (Criterion B: 70 % of all assessments; Criterion D2: 7 %) although population size reduction (Criterion A) was identified in 21 % of species assessments. Key threats to species included invasive and other problematic native species, genes and diseases (92 % of threatened, Near Threatened or Data Deficient species), natural system modifications (82 %), and climate change and severe weather (54 %). Despite the high level of extinction risk, implemented conservation measures for threatened species are presently very limited. A further 17 species were assessed as Near Threatened. This study highlights the imperilled nature of Australian native freshwater fishes and emphasises that targeted conservation measures are urgently needed to avoid imminent extinctions.

Many migratory species are under threat globally. Management actions to conserve these species require a quantitative understanding of their life-history requirements, such as cues for migration. Migrations to spawn are a critical component of the life-cycle of diadromous fishes but are often poorly understood. Downstream migration patterns of common galaxias (Galaxias maculatus) were investigated in the Bunyip–Tarago River system, southern Australia, using passive integrated transponder technology. Fish undertook rapid (typically 1–3 days) downstream migration (up to 50 km) from the upper reaches to the lowland reaches near the estuary. Migration occurred from March to May during the known spawning period, with peak movement between late April and mid-May. Migration was also related to increased river discharge, with fish being more likely to migrate on days when flow was high compared with the previous 2 days, and a waxing moon (i.e. increasing illumination). Our results fill an important knowledge gap in the life history of this widely distributed diadromous species. Importantly, the knowledge about the likely drivers of movement can inform management actions such as providing environmental flows in austral autumn and at appropriate moon phases to trigger the downstream migration of adult common galaxias from freshwater reaches to estuarine spawning areas.

Outcomes from restoration and reintroduction programs can be enhanced by understanding the habitat use of animals, and how likely they are to move and why? River blackfish are native to southeastern Australia where their range and abundance have declined. We reintroduced 27 river blackfish into two sites, one where rehabilitation had been undertaken (with more vegetation, instream woody habitat, and undercut banks) and an unmanipulated control site. We monitored fish for 9 months after release to assess site occupancy, habitat use, and drivers of movement. We modeled the likelihood of fish remaining at either site, their habitat use, and predictors of movement (site type, habitat, time, and river discharge). While more fish remained at the treatment site, this difference was not statistically significant. Future trials incorporating increased numbers of fish and replicate treatment and control release sites would allow a more detailed test of whether fish are more likely to remain at restored sites. Fish spent more time in deeper areas with more riparian vegetation. Habitat, flow, and time since release were significant predictors of fish movement, with fish more likely to move from release sites with less suitable habitat, during rainfall events, and during the initial stages of our study. Our results illustrate that blackfish are a suitable candidate for translocations, retaining their largely sedentary behavior and should be easily measurable if restoration has provided suitable habitat and their presence is an indicator of success. Our work is also a useful case study for translocation of other nonmigratory fish worldwide.

Sustainable management of estuaries depends on understanding the synergistic effects of nutrients and hydrological factors on estuarine food webs. We examined how phytoplankton, zooplankton and selected fish larval counts (Acanthopagrus butcheri, black bream) vary in relation to groundwater inputs and environmental flow releases (EFRs) in a small, highly flow-regulated estuary (Werribee, Victoria, Australia). We found that groundwater-derived nutrients and an EFR were associated with the community structure of phytoplankton and zooplankton and abundance of fish larvae. Elevated phytoplankton concentration was associated with increased flagellate abundance (genus Euglena), followed by increased abundance of diatoms (Cyclotella sp). The latter increase was primarily associated with groundwater-derived nitrate entering the estuary. Cyclotella sp. abundance collapsed following an EFR and the subsequent increase in ammonium (NH4+) concentrations in the estuary. In addition, the strong halocline and increased bottom water NH4+ concentrations resulting from the EFR appeared to stimulate population growth of flagellate taxa and calanoid copepods. The abundance of calanoid copepodites and nauplii, which are the preferred food for larval A. butcheri, greatly increased due to the EFR, and was associated with an increase in larval A. butcheri abundance at this time. This effect of the EFR on the cascading trophic interactions in the estuarine food web has rarely been shown. Our study reinforces the ecological linkages between freshwater inputs, including groundwater and riverine discharge, and estuarine biotic community structure and function. Importantly, this study underscores the potential food web consequences of freshwater flow manipulation into estuaries and provides evidence for using environmental flows to enhance and/or manage the recruitment of estuarine fishes.

Ecological responses to changing riverine flows are often evaluated by describing the relationship between river discharge and response. However, aquatic organisms experience the hydraulics (i.e. velocity, shear stress, depth) of a river, not its discharge. Hydraulic characterizations of riverine habitats may improve our ability to predict ecological responses. We used two-dimensional hydraulic models to translate river discharge into flow velocity. We used discharge and reach-averaged velocity, along with water temperature and 8 years of field observations of fish spawning, to develop predictive models of the spawning of golden perch (Macquaria ambigua) in the Goulburn River, south-east Australia. Probability of spawning was positively related to both discharge and reach-averaged velocity. Water temperature was critical for enabling the flow response, and antecedent flows prior to spawning had a weak positive effect. Against expectations, there was little difference in predictive uncertainty for the effect of flows when reach-averaged velocity was used as the main predictor rather than discharge. The lower Goulburn River has a relatively simple channel and so discharge and velocity are monotonically related over most flows. We expect that in a more geomorphically complex environment, improvement in predictive ability would be substantial. This research only explores one example of a hydraulic parameter being used as a predictor of ecological response; many others are possible. The extra effort and expense involved in hydraulic characterization of river flows is only justified if our understanding of flow-ecology relationships is substantially improved. Further research to understand which environmental responses might be best understood through different hydraulic parameters, and how to better characterize hydraulic characteristics relevant to riverine biota, would help inform decisions regarding investment in hydraulic models. Regardless, hydraulics offers a more process-based assessment of ecological responses to changing flows, has the potential to facilitate mechanistic understanding rather than just associations, and provides the opportunity to translate hydraulic metrics that drive ecological responses across river systems of differing sizes. However, while considering ecological responses in terms of river hydraulics is more physically realistic, our results suggest that average hydraulic conditions may not result in an improved ability to predict the effects of changing flows.

An understanding of population demographics and life history processes is integral to the rehabilitation of fish populations. In Australia’s highly modified Murray–Darling Basin, native fish are imperilled and fish deaths in the Darling River in 2018–19 highlighted their vulnerability. Golden perch (Macquaria ambigua) is a long-lived percichthyid that was conspicuous in the fish kills. To guide population rehabilitation in the Darling River, pre-fish kill age structure, provenance and movement of golden perch were explored using otolith microstructure and chemistry (87Sr/86Sr). Across the Lower and Mid-Darling River, recruitment was episodic, with dominant cohorts associated with years characterised by elevated discharge. There was substantial variability in age structure, recruitment source and movement patterns between the Lower and Mid-Darling River. In the Mid-Darling River, tributaries were an important recruitment source, whereas in the Lower Darling fish predominantly originated in the Darling River itself. Downstream movement of juveniles, upstream migration of adults and return movements to natal locations were important drivers of population structure. Restoring resilient golden perch populations in the Darling River will be reliant on mitigating barriers to movement, promoting a connected mosaic of recruitment sources and reinstating the hydrological and hydraulic factors associated with spawning, recruitment and dispersal. Globally, increasing water resource development and climate change will necessitate such integrated approaches to the management of long-lived migratory riverine fishes.

Recognition that many species share key life-history strategies has enabled predictions of responses to habitat degradation or rehabilitation by these species groups. While such responses have been well documented for freshwater fish that exhibit ‘periodic’ and ‘opportunistic’ life-history strategies, this is rare for ‘equilibrium’ life-history, due largely to their longevity and by comparison, more regular and stable levels of recruitment. Unfortunately, this limits the confidence in using life-history strategies to refine water management interventions to rectify the negative impacts of river regulation for these species. We addressed this knowledge gap for Murray cod Maccullochella peelii, a high-profile, long-lived recreationally popular equilibrium species in south-eastern Australia. We used monitoring data collected across a gradient of hydrologically altered rivers over two decades, to test various hypotheses linking recruitment strength with key attributes of the flow regime. Although Murray cod recruited in most years, as expected for an equilibrium species, responses to flow varied among and within rivers among years. We found links between recruitment strength and the magnitude and variation in discharge during the spring spawning period, as well as flows experienced by juvenile fish during summer and winter - the hydrological components most affected by river regulation. However, the specific slopes and directions of some of these links varied idiosyncratically across rivers. Our results emphasise the importance of accounting for flows that influence each of the key life stages during the recruitment process and lend support for managing rivers in accordance with the natural flow regime. It also shows the need for waterway-specific studies and further refinement of existing flow metrics to allow more credible transferability of results. The approach used in this study can also be applied to other species sharing life-history strategies for which long-term monitoring data has been compiled and length-at-age relationships established.

Anguillid eel populations have declined dramatically over the last 50 years in many regions of the world, and numerous species are now under threat. A critical life-history phase is migration from freshwater to distant oceans, culminating in a single life-time spawning event. For many anguillids, especially those in the southern hemisphere, mystery still shrouds their oceanic spawning migrations. We investigated the oceanic spawning migrations of the Australasian short-finned eel ( Anguilla australis ) using pop-up satellite archival tags. Eels were collected from river estuaries (38° S, 142° E) in south-eastern temperate Australia. In 2019, 16 eels were tracked for up to about 5 months, ~ 2620 km from release, and as far north as the tropical Coral Sea (22° S, 155° E) off the north-east coast of Australia. Eels from southern Australia appeared to access deep water off the Australian coast via two main routes: (i) directly east via Bass Strait, or (ii) south-east around Tasmania, which is the shortest route to deep water. Tagged eels exhibited strong diel vertical migrations, alternating between the warm euphotic zone (~ 100–300 m, 15–20 °C) at night and the mesopelagic zone (~ 700–900 m, 6–8 °C) during the day. Marine predators, probably lamnid sharks, tuna, or marine mammals, ended many eel migrations (at least ~ 30%), largely before the eels had left the Australian continental shelf. The long and risky marine migrations of Australasian eels highlight the need for better information on the processes contributing to eel mortality throughout the life cycle, including the impacts of future changes to oceanic currents, predator abundance and direct anthropogenic disturbances.

An understanding of population demographics and life history processes is integral to the rehabilitation of fish populations. In Australia’s highly modified Murray–Darling Basin, native fish are imperilled and fish deaths in the Darling River in 2018–19 highlighted their vulnerability. Golden perch (Macquaria ambigua) is a long-lived percichthyid that was conspicuous in the fish kills. To guide population rehabilitation in the Darling River, pre-fish kill age structure, provenance and movement of golden perch were explored using otolith microstructure and chemistry (87Sr/86Sr). Across the Lower and Mid-Darling River, recruitment was episodic, with dominant cohorts associated with years characterised by elevated discharge. There was substantial variability in age structure, recruitment source and movement patterns between the Lower and Mid-Darling River. In the Mid-Darling River, tributaries were an important recruitment source, whereas in the Lower Darling fish predominantly originated in the Darling River itself. Downstream movement of juveniles, upstream migration of adults and return movements to natal locations were important drivers of population structure. Restoring resilient golden perch populations in the Darling River will be reliant on mitigating barriers to movement, promoting a connected mosaic of recruitment sources and reinstating the hydrological and hydraulic factors associated with spawning, recruitment and dispersal. Globally, increasing water resource development and climate change will necessitate such integrated approaches to the management of long-lived migratory riverine fishes.

Understanding connectivity is crucial for the effective conservation and management of biota. However, measuring connectivity directly is challenging and it is often inferred based on assumptions surrounding dispersal potential, such as environmental history and species life history traits. Genetic tools are often underutilised, yet can infer connectivity reliably. Here, we characterise and compare the genetic connectivity and genetic diversity of three diadromous Australian fish species: common galaxias (Galaxias maculatus), tupong (Pseudaphritis urvillii) and Australian grayling (Prototroctes maraena). For each species, we investigate the extent of genetic connectivity across a study region in south-eastern Australia (~700km). We further determine the potential roles of contemporary ocean currents in shaping the patterns of genetic connectivity observed. Individuals across multiple rivers were sampled and >3000 single nucleotide polymorphisms were genotyped for each species. We found differences in genetic connectivity for the three species: common galaxias were highly connected, and Australian grayling and tupong exhibited patterns of isolation by distance. The degree of genetic connectivity for tupong and Australian grayling appeared unrelated to oceanic currents. This study indicates that the degree of connectivity for different diadromous species can vary greatly despite broadly similar life history strategies, highlighting the potential value of genetic tools for informing species-specific management plans.

River regulation has degraded aquatic biodiversity globally, and the effects can be pronounced for diadromous species, whose life history processes can depend on flow conditions, such as cues for adult migration, spawning, attracting recruits into coastal rivers and promoting upstream dispersal. Environmental flows are being used to mitigate the effects of river regulation, and understanding their effectiveness is required to improve management practices. This study examined the effects of targeted environmental flows on the upstream dispersal of three temperate catadromous fish species, namely common galaxias Galaxias maculatus, tupong Pseudaphritis urvillii and short-finned eel Anguilla australis. Fyke netting was used to capture fish moving upstream before and during environmental flows in summer and autumn in two coastal rivers. We found significant increases in the catch of young-of-the-year (YOY) common galaxias (6-fold higher) and juvenile short-finned eel (26-fold higher), relative to control sites, during environmental flow pulses compared with stable, regulated base-flow conditions. A significant response was not detected for YOY tupong, despite a 39% increase in the catch. These results demonstrate that environmental flows enhance the upstream movement of juvenile diadromous fishes, a critical process governing population persistence or recovery. The findings provide managers with confidence in the use of environmental flows to support populations.

Conservation management of freshwater ecosystems often focuses on mitigating or reversing the negative effects of altered patterns of river discharge. Assessments of management interventions frequently focus on direct, short-term responses to discharge without consideration of underlying population trends that span multiple years. We sought to determine the effects on fish populations of annual variation in river discharge and water temperature after accounting for underlying population trends. We used data on five native and one non-native fish species, collected over 7-20 years in seven rivers in the Murray-Darling Basin, south-eastern Australia. Population trends explained 3.4%-24.6% of the total variation in abundance and biomass of our six study species, while discharge and water temperature explained a further 1.2%-11.4% of this variation. However, population trends confounded the effects of discharge and water temperature, which suggests that the effects of annual discharge conditions may be contingent on past conditions and factors intrinsic to populations (e.g. age structure). Failing to account for population trends led to a combination of plausible and implausible associations with discharge and water temperature. Plausible associations included positive associations with the magnitude of spring discharge and negative associations with the number of days where discharge was below the long-term 10th percentile. Determining whether estimated associations are real or artefactual requires a greater focus on the processes that underpin multiyear population trends. Our results highlight the potential for underlying trends in populations to confound the short-term effects of discharge. Potential confounding of short- and long-term changes in populations underscores the need to assess responses to river discharge in the context of overarching environmental conditions, including factors other than discharge.