Ecology and population dynamics of golden perch in a fragmented, flow-impacted river: implications for conservation and management

In rivers worldwide, human demands for water resources have fundamentally altered flow regimes and aquatic habitats, leading to profound impacts on ecosystem integrity and biodiversity. Riverine fish are prominent indicators of the impacts of river regulation because of fundamental links between flow, life histories and population dynamics. In order to effectively manage and rehabilitate riverine fish populations, there remains a considerable need to understand the life history processes and associated environmental variables that influence population structure and resilience. In Australia’s Murray-Darling Basin, native fish populations have declined in association with river regulation, yet few studies have considered the age demographics and dynamics of populations, and the processes that influence these, in an integrated manner. In this thesis, I have explored the population structure and dynamics of the migratory, pelagic spawning, golden perch (Macquaria ambigua), in the flow regulated and fragmented River Murray. My overarching aim was to investigate the population dynamics of golden perch and the processes (e.g. recruitment and movement) that influence population structure, including the potential effects of flow and river regulation. To understand how flow and connectivity influence population dynamics, I characterised temporal variability in age demographics over a period of hydrological extremes (drought–flood), then to elucidate the processes promoting these temporal patterns, I investigated spawning, recruitment and movement. From 2001 to 2010, the Murray-Darling Basin (MDB) experienced severe drought. Throughout this period, golden perch age structure in the lower River Murray was characterised by intermittent recruitment and a few dominant cohorts. These distinct cohorts were predominantly recruited prior to the drought, in association with overbank floods or increased flow contained within the river channel (Chapter 2). Despite a depauperate age structure at the end of the Millennium Drought, population growth of golden perch in association with flooding in 2010 was rapid and substantial (Chapter 3). This response superficially supports the flood-pulse model, where flooding promotes high abundances of biota due primarily to recruitment driven by floodplain derived energy. Nevertheless, growth in the golden perch population was promoted by increased abundances of age 0+ and 1+ fish, the product of spawning and recruitment in the flood year and the year prior, respectively. Recruitment of a g e 0 + fish was substantial, demonstrating the capacity of fishes with periodic life histories to respond to episodic events that may promote high survival of early life stages. In addition, however, approximately 50% of the population sampled post-flooding was age 1+ fish, that were not detected in the population as age 0+ the year prior, and were assumed to have migrated from elsewhere in the system. Consequently, immigration of juvenile fish was considered a substantial driver of population growth. In order to understand the spatial arrangement of recruitment sources, and the influence of movement on population structure, I used otolith chemistry to retrospectively determine the provenance and movement history of individuals from specific age cohorts (Chapter 4), and radio telemetry to investigate the movements of adult fish (Chapter 5). Water and otolith chemistry, specifically ⁸⁷Sr/⁸⁶Sr, was used to delineate the provenance and movement of golden perch. Water ⁸⁷Sr/⁸⁶Sr was distinct among the Darling River and lower and mid-River Murray. In turn, otolith chemistry revealed that golden perch collected in the lower River Murray were the progeny of spawning in either the River Murray or Darling River, during years characterised by within-channel rises in flow, or in both rivers in a year characterised by extensive overbank flooding. Movement of fish from the Darling River was a substantial driver of population structure in the lower River Murray, with fish dispersing from natal habitats in the Darling River either in the year of birth, as eggs and larvae, or at age 1+ in association with flooding. Importantly, the Darling River constituted a recruitment source for golden perch when environmental conditions were unsuitable for spawning and recruitment in the River Murray. In regulated river systems worldwide, the ecological importance of tributaries and tributary-mainstem junctions is increasingly recognised. To investigate the habitat use and movement of adult golden perch in relation to flow, season and water temperature, I used a combined radio-telemetry and passive integrated transponder (PIT) tag approach. Site fidelity was common, with 36% of fish remaining at the site of capture throughout the study period (~2 years). Over the same period, however, 29% of fish migrated long distances upstream (up to 270 km), coinciding with steady, rising and falling flows. These movements were correlated with seasonal variation in water temperature and to a lesser extent, flow variability. Whilst environmental factors, such as flow, may constitute an impetus for movement, movement may also be driven by endogenous cues such as sexual maturity and age. The role of these factors in promoting movement and interactions with flow, warrant further investigation. Golden perch in the lower River Murray appeared to exhibit partial migration, whereby some fish in a population migrate and some do not. This combination of retentive and dispersive behaviours minimises risks associated with habitat and environmental heterogeneity. For example, in large river basins, where climate variability and river regulation lead to regionally diverse flow patterns, within-population variability in migratory movements and destinations increases the chance of at least some fish being exposed to environmental conditions conducive to spawning and recruitment. For golden perch, this mechanism may contribute to the basin-wide persistence of this species. In this thesis, I have addressed concepts relating to the autecology, population structure and movement of golden perch and provided new insights regarding the spatial structuring of populations. Globally, these factors are considered key contemporary knowledge requirements for understanding the impacts of anthropogenic disturbances on riverine fish populations. Despite increasing recognition of the need to manage freshwater fishes, and indeed ecosystem function, at the river-scale, research and management are often undertaken in a spatially disaggregated manner. Ultimately, conservation and rehabilitation of riverine fishes requires management at a spatial scale concordant with life history and population processes. Such approaches also need to integrate recruitment source, life history and migratory diversity, and the hydrological and hydraulic characteristics of rivers that support critical life history processes.Thesis (Ph.D.) -- University of Adelaide, School of Biological Sciences, 201

Size, growth and mortality of riverine golden perch (Macquaria ambigua) across a latitudinal gradient

Effective fisheries management requires fish size, growth and mortality information representative of the population and location of interest. Golden perch Macquaria ambigua is long lived, potamodromous and widespread in the Murray–Darling Basin (MDB), Australia. Using a sample spanning 13 river systems and 10° of latitude, we examined whether the maximum size of golden perch differed by latitude and whether growth and mortality varied between northern and southern MDB regions. The length, weight and age ranges of golden perch sampled (n = 873) were 52–559 mm, 2–3201 g and 0+ to 26+ years respectively, and maximum length and weight were unaffected by latitude. Length and age–length distributions represented by age–length keys varied by region, with greater variability in age-at-length and a larger proportion of smaller individuals in northern MDB rivers, which generally exhibit greater variability in discharge. Growth and mortality rates were similar between regions, and an MDB-wide von Bertalanffy growth model (L∞ = 447, k = 0.32 and t0 = –0.51) and instantaneous mortality rate (Z = 0.20) best described the data. An MDB-wide length–weight equation also provided the best fit (W = 6.76 × 10–6 L3.12). Our data suggest that the MDB can be treated as one management unit in terms of golden perch maximum size, growth and mortality parameters

A compendium of ecological knowledge for restoration of freshwater fishes in Australia’s Murray–Darling Basin

Many freshwater fishes are imperilled globally, and there is a need for easily accessible, contemporary ecological knowledge to guide management. This compendium contains knowledge collated from over 600 publications and 27 expert workshops to support the restoration of 9 priority native freshwater fish species, representative of the range of life-history strategies and values in south-eastern Australia’s Murray–Darling Basin. To help prioritise future research investment and restoration actions, ecological knowledge and threats were assessed for each species and life stage. There is considerable new knowledge (80% of publications used were from the past 20 years), but this varied among species and life stages, with most known about adults, then egg, juvenile and larval stages (in that order). The biggest knowledge gaps concerned early life stage requirements, survival, recruitment, growth rates, condition and movements. Key threats include reduced longitudinal and lateral connectivity, altered flows, loss of refugia, reductions in both flowing (lotic) and slackwater riverine habitats, degradation of wetland habitats, alien species interactions and loss of aquatic vegetation. Examples and case studies illustrating the application of this knowledge to underpin effective restoration management are provided. This extensive ecological evidence base for multiple species is presented in a tabular format to assist a range of readers

Flows for native fish in the Murray-Darling Basin: lessons and considerations for future management

Increased regulation and extraction of water from rivers has contributed to the decline of fishes, and the use of environmental water allocations (EWAs) is now a key rehabilitation measure. Major reform of water policy in the Murray-Darling Basin (MDB), Australia, has recently provided significant EWAs to improve ecological outcomes. Conflict over water buybacks, the value of the water and the need to maximise environmental benefits and minimise risks of unwanted outcomes has increased the expectation for science to underpin and justify such actions. Recent research has focussed attention on the need to understand fish–flow relationships. The Native Fish Strategy for the Murray-Darling Basin 2003–2013 (NFS), while not specifically targeted at water policy reform or water delivery, has provided fish ecology research and flow restoration experimentation and contributed considerable new scientific knowledge to support flow management. It has contributed to a substantial and positive change in environmental watering for fish, with native fish targets now regularly incorporated into watering objectives. This study documents changes to water management in the MDB, summarises current knowledge of flow-related fish ecology in the MDB, highlights the benefits and risks of some water management practises and provides recommendations for future management and research. A major recommendation is the need for a coordinated, cross-jurisdictional approach to flow restoration for native fish, ensuring that the best available science is being used in all watering allocations. We caution on the use of environmental works such as regulators to artificially inundate floodplains and suggest that such approaches should be viewed as large-scale experiments with the significant risks posed to fish needing to be recognised, adequately monitored and adaptively managed

Demonstration reaches: Looking back whilst moving forward with river rehabilitation under the Native Fish Strategy

‘Demonstration reaches’ are sections of river where multiple threats to native fish are addressed through river rehabilitation and strong community participation. They are an important way of promoting the key driving actions of the Murray-Darling Basin Authority's Native Fish Strategy (NFS) by using on-ground community-driven rehabilitation. Measuring rehabilitation success against well-defined targets and using this information to adaptively mange activities is fundamental to the demonstration reach philosophy. Seven years on from the establishment of the first demonstration reach, there are now seven throughout the Murray-Darling Basin (MDB), all in differing states of maturation and but all applying a standardised framework for monitoring native fish outcomes. In this study, we reflect on the role that demonstration reaches have played within the NFS, synthesise some key findings from 32 monitoring and evaluation outputs, and highlight some of the successes and barriers to success. We make recommendations as to how to strengthen the demonstration reach model to ensure it remains a relevant approach for fish habitat rehabilitation beyond the NFS and MDB

Riverscape recruitment: a conceptual synthesis of drivers of fish recruitment in rivers

Most fish recruitment models consider only one or a few drivers in isolation, rarely include species’ traits, and have limited relevance to riverine environments. Despite their diversity, riverine fishes share sufficient characteristics that prediction of recruitment should be possible. Here we synthesize the essential components of fish recruitment hypotheses and the key features of rivers to develop a model that predicts relative recruitment strength, for all fishes, in rivers under all flow conditions. The model proposes that interactions between flow and physical complexity will create locations in rivers, at mesoscales, where energy and nutrients are enriched. The resultant production of small prey will be concentrated and prey and fish larvae located (through dispersal or retention) so that the larvae can feed, grow, and recruit. Our synthesis explains how flow and physical complexity affect fish recruitment and provides a conceptual basis to better conserve and manage riverine fishes globally.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Fish productivity in the lower lakes and Coorong, Australia, during severe drought

Anthropogenic modification of catchments and river flow can significantly alter estuarine habitats, hydrology and nutrient delivery with implications for fisheries productivity. The Coorong estuary at the terminus of Australia's River Murray supports an economically important fishery as well as being recognised internationally as a critical site for migratory birds. Salinity near the Murray Mouth varies between fresh and marine depending upon river flow, but the Coorong becomes increasingly saline along its 120km length. Freshwater flow to the Coorong is naturally variable but has significantly reduced by extraction for irrigated agriculture and domestic use upstream. Extreme drought from 2000 to 2010 and over-allocation of water resources resulted in the cessation of freshwater flow to the Coorong, significantly increasing salinity. During this period the diversity and abundance of organisms in the Coorong declined which reduced food web complexity. During lower flows the system generally becomes less productive as evidenced by: lower nutrient concentrations and loads, lower chlorophyll and primary productivity, a decrease in the abundance of fish-prey items (zooplankton, macroinvertebrates and small fish), a decrease in fish abundance, although this is not well reflected in fishery catch data because of the concentration of fishing in available habitat. The maintenance of flow is the only management strategy that stimulates recruitment, delivers nutrient resources to the estuary and ensures maintenance of habitable area by maintaining appropriate salinity

Stage-dependent effects of river flow and temperature regimes on the growth dynamics of an apex predator

In the world's rivers, alteration of flow is a major driver of biodiversity decline. Global warming is now affecting the thermal and hydrological regimes of rivers, compounding the threat and complicating conservation planning. To inform management under a non‐stationary climate, we must improve our understanding of how flow and thermal regimes interact to affect the population dynamics of riverine biota. We used long‐term growth biochronologies, spanning 34 years and 400,000 km2, to model the growth dynamics of a long‐lived, apex predator (Murray cod) as a function of factors extrinsic (river discharge; air temperature; sub‐catchment) and intrinsic (age; individual) to the population. Annual growth of Murray cod showed significant, curvilinear, life‐stage‐specific responses to an interaction between annual discharge and temperature. Growth of early juveniles (age 1+ and 2+ years) exhibited a unimodal relationship with annual discharge, peaking near median annual discharge. Growth of late juveniles (3+ to 5+) and adults (>5+) increased with annual discharge, with the rate of increase being particularly high in adults, whose growth peaked during years with flooding. Years with very low annual discharge, as experienced during drought and under high abstraction, suppress growth rates of all Murray cod life‐stages. Unimodal relationships between growth and annual temperature were evident across all life stages. Contrary to expectations of the Temperature Size Rule, the annual air temperature at which maximum growth occurred increased with age. The stage‐specific response of Murray cod to annual discharge indicates that no single magnitude of annual discharge is optimal for cod populations, adding further weight to the case for maintaining and/or restoring flow variability in riverine ecosystems. With respect to climate change impacts, on balance our results indicate that the primary mechanism by which climate change threatens Murray cod growth is through alteration of river flows, not through warming annual mean temperatures per se