Otolith chemistry delineates the influence of natal origin, dispersal and flow on the population dynamics of golden perch (Macquaria ambigua) in a regulated river

The golden perch (Macquaria ambigua Richardson 1845) is an iconic freshwater fish native to Australia’s Murray-Darling Basin. Like many other native fishes, golden perch have suffered declines in abundance and range since European settlement as a result of overfishing, habitat destruction, and dams that impede migration and regulate flows of the Murray-Darling river system. For more than four decades it has been widely considered that flow pulses and floods are proximate stimuli for spawning, and that floods enhance recruitment to sustain golden perch populations. It has, however, been shown recently that spawning and recruitment can occur in the absence of these conditions, that strongest recruitment events can occur outside of flood periods, and that both spawning and recruitment can occur during periods of low and even zero flows – at least in the dryland rivers of the Basin’s arid zones. Despite observations of golden perch spawning and recruitment across a range of hydrological conditions and locations, much speculation exists within the literature regarding the role of flow pulses and floods in the species’ life history, as there remain few observations of spawning in the wild and even fewer ecological studies of the early life history stages. The aims of this thesis are: to examine major aspects of golden perch life history with emphasis on the role of flows as stimuli to initiate spawning; to examine the role of floodplain habitats in the species’ early life history; to refine a conceptual model of the species’ life history and key life history events; and to evaluate the utility of this model by predicting and recording the role of river regulation in disrupting key life history processes for golden perch. The reproductive ecology of golden perch was examined in the Darling River throughout 2004-2006. During this period, temporal patterns of oocyte maturity were examined to reveal that spawning could occur at almost any time of the year in the Darling River system. Distinct differences were also examined between the stages of oocyte maturity observed for mature-aged females at various locations along a broad spatial gradient between Wilcannia and Menindee (~300 km), to reveal a prevailing spatial pattern in the occurrence of ‘ripe’, ‘transitional’ and ‘resting’ golden perch females within this reach of the Darling River.

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.

Tributary and mainstem connections represent important links for the movement of fish and other biota throughout river networks. We investigated the timing, frequency and environmental conditions associated with movements by adult golden perch (Macquaria ambigua) between the mainstem of the mid-Murray River and a tributary, the Goulburn River, in south-eastern Australia, using acoustic telemetry over four years (2007–2011). Fish were tagged and released in autumn 2007–2009 in the mid-Murray (n = 42) and lower Goulburn (n = 37) rivers within 3–6 km of the mid-Murray-lower Goulburn junction. 38% of tagged fish undertook mainstem–tributary movements, characterised mostly by temporary occupation followed by return of fish to the original capture river. Approximately 10% of tagged fish exhibited longer-term shifts between the mainstem and tributary. Movement of fish from the tributary into the mainstem occurred primarily during the spawning season and in some years coincided with the presence of golden perch eggs/larvae in drift samples in the mainstem. Many of the tributary-to-mainstem movements occurred during or soon after changes in flow. The movements of fish from the mainstem into the tributary were irregular and did not appear to be associated with spawning. The findings show that golden perch moved freely across the mainstem–tributary interface. This demonstrates the need to consider the spatial, behavioural and demographic interdependencies of aquatic fauna across geographic management units such as rivers.

A 2 year (2000–2001) radio‐tagging study was undertaken to investigate the movements of 51 golden perch Macquaria ambigua in the Murray River at Nyah in north‐western Victoria, Australia. During the winter of both years, golden perch did not undertake movements >5 km and displayed strong home range fidelity. In the first year of the study there was an increase in the distance of golden perch movement during late spring which coincided with increasing water temperature and river discharge. Nineteen golden perch were tracked during this period, of which 10 travelled downstream between 11 and 290 km. Seven of these fish moved to an area below the junction of the Murray and Wakool Rivers. Five golden perch travelled upstream between 13 and 35 km, four of which travelled to an area around the junction of the Murray River and Speewa Creek. The remaining four golden perch undertook localized movements of <5 km. Many of the long distance movements undertaken in spring 2000 were rapid and 53% of these golden perch returned to within 3 km of their release locations, indicating homing behaviour. Given that the rapid movements of golden perch in spring coincided with the known spawning season of this species, these long distance movements may be associated with reproductive strategy.

Pelagic spawning riverine fish (pelagophils) spawn in free‐flowing river habitats with downstream drift of eggs and larvae but the spatial scale is often unknown, and this constitutes a major ecological knowledge gap. In the arid Darling River in south‐eastern Australia, the present objectives were: (i) to determine the potential downstream dispersal distance of young golden perch (Macquaria ambigua); and (ii) to evaluate whether provision of environmental water enhanced dispersal of young fish from Menindee Lakes to the lower Darling River (LDR) while also cueing further spawning in downstream lotic reaches. Golden perch spawned in unregulated lotic tributaries on a flood pulse and larvae drifted or dispersed >1,600 km downstream and entered large ephemeral productive floodplain lake nursery habitats as fully scaled fingerlings. Planned releases of environmental water cued golden perch spawning in the LDR and enabled juvenile fish to disperse downstream from the Menindee Lakes nursery into receiving populations in the LDR, Great Darling Anabranch, and southern Murray River, with some fish potentially completing an active migration of >2,100 km by age 1 year. The Darling River case study highlights the need for a system‐scale approach to the conservation management of pelagophilic fish, along with multi‐year perennial flow strategies to improve ecosystem integrity in large rivers globally.

Understanding the spatial and temporal scales over which key population processes occur is fundamental to effective fisheries management, especially when informing recovery actions following extreme events. In 2018–19, hypoxia-induced fish kills occurred in the lower Darling River, south-eastern Australia. We collected carcasses of three potamodromous species that perished during these events to reconstruct their lifetime movements and identify potential recovery mechanisms. Golden perch Macquaria ambigua, Murray cod Maccullochella peelii and silver perch Bidyanus bidyanus otolith 87Sr/86Sr profiles were compared with water 87Sr/86Sr ratios to better understand natal provenance and movement history, and to identify the scale at which migration influences population processes. Golden perch were predominantly locally spawned (Darling River), although we found some evidence of emigration into the nearby Murray River early in life and return movements into the Darling River. Murray cod were mainly locally spawned and thereafter lifelong residents, with some evidence of stocking supplementing populations. Silver perch were mostly immigrants, with the Murray River (>500 km away) the principal source of fish. For recovery of native fish populations to be effective in the Darling River, recovery actions are required that incorporate knowledge on the relevant spatial and temporal scales over which life history processes occur.

We analysed otoliths from excavations along the Lower Murray River (n=24), dating from the mid‐ to late‐Holocene period. We identified the species, and estimated the size and age of fish. The potential habitat that fish used throughout their life was estimated from chemical information in the otoliths. The majority of the fish (identified as Maccullochella peelii n=22 and Macquaria ambigua n=2) were caught in freshwater environments during the warm season, and had grown to an age and size indicative of sexual maturity. These observations accord with Ngarrindjeri oral tradition concerning sustainable management strategies. Data indicate that M. peelii grew to a significantly larger size than present fish; historical data suggests this size reduction may be the result of European fishing practices, introduced species and habitat degradation. The study demonstrates the unique nature of otoliths and their potential for investigating Indigenous subsistence strategies.

Summary Many studies of the movements of riverine fish have found that most individuals are sedentary and occupy very restricted home ranges. Recently, this ‘Restricted Movement Paradigm’ has been challenged and there is currently a need for tests of the home range concept as a theoretical basis for describing the movements of riverine fish. In this paper, I describe a radio‐tracking study of golden perch (Macquaria ambigua) and carp (Cyprinus carpio) in the Broken River, Australia that aims to assess the home range concept as a means of describing the movements of these species. A random movement analysis and a translocation experiment were conducted to test for site fidelity and home range occupation. Both golden perch and carp exhibited strong site fidelity and occupied restricted home ranges. Carp had larger total home ranges than golden perch, and both species had areas of concentrated use (core areas) within the home range. Several golden perch and carp exhibited shifts in the locations of their home ranges during the study. To incorporate such shifts into a theoretical framework, a ‘home range shift’ conceptual model is proposed and the need to consider the temporal stability of site fidelity when describing home range movements is discussed.

Understanding the scale and characteristics of animal movements allows for effective species management. These movement characteristics can vary widely within a species, so there is a need to understand and account for the spectrum of movement behaviours within a population. Movement ecology studies of freshwater fish predominantly aim to uncover environmental correlates of population-wide movement behaviours. However, few studies consider the specific movement types that may exist within a species or population. This is somewhat surprising considering the importance of fish movement and migration in determining population characteristics and species persistence over time. In this study, we used acoustic telemetry to investigate the role and extent of intrapopulation variation in the movement behaviour of golden perch (Macquaria ambigua, Percichthyidae) in the Condamine-Balonne River system, Australia. After tracking 132 individuals between 2018 and 2025, we employed a novel approach to characterise annual fish movement behaviour into distinct movement groups using a suite of movement data metrics. These groups were then compared to morphological and environmental variations, assessing group membership. We found strong variation in the movement behaviour of golden perch and identified five distinct movement groups. The proportion of individuals in each movement group varied with hydrological alteration and demonstrated behavioural plasticity within the population, possibly in response to the increased density within weir pool habitats. Fish that exhibited strong home-site residency and low mobility (under 5km linear range) were most prevalent, and only a small number of fish (n = 18) moved more than 50km per year. Higher mobility was associated with larger body size (length and weight) and increased age. Future management actions for freshwater fish may benefit from recognising the diverse movement syndromes present within a population. Integrating these behavioural groups, and their differing requirements, into environmental flow management would better preserve key ecological functions and strengthen population resilience.

Stocking of native fishes is conducted to augment riverine fisheries in many parts of the world, yet most stocking activities are conducted without empirical information on their effectiveness or impacts. In the Murray–Darling Basin (MDB), Australia, stocking has been underway for several decades to maintain recreational fisheries. We stocked chemically tagged golden perch (Macquaria ambigua) fingerlings in three rivers to determine the proportions of stocked fish within populations of the species. Stocked sites were monitored for up to 5 years in the Murrumbidgee River, Edward River and Billabong Creek and non-stocked sites were monitored in the Murray River. Catch per unit effort of stocked year classes increased substantially in Billabong Creek, with stocked fish contributing 100 (2005), 79 (2006) and 92% (2007). Chemically tagged fish comprised 18–38% of the respective age classes in the Murrumbidgee and Edward rivers and there was little evidence of natural recruitment in the non-stocked Murray River. Tagged fish generally attained the legal minimum size within 4 years and had dispersed up to 60 km from the original release location. Our results demonstrate that artificial stocking has the potential to strongly influence the abundance and population structure of golden perch in rivers of the MDB.

Otolith chemistry to determine movements of diadromous and freshwater fish

The role of flow and floodplains for the recruitment of fishes in the Murray‐Darling Basin, Australia, remains a contentious issue. In this context, the reproductive ecology of golden perch Macquaria ambigua, a widespread, high‐profile angling species with a propensity to produce large numbers of semi‐pelagic eggs remains to be resolved. Back‐calculation of spawning dates from otoliths of juvenile golden perch (Macquaria ambigua) collected in August and November 1997 from the Menindee Lakes on the floodplain of the Darling River, in western New South Wales, Australia, advances understanding of spawning of this species in three ways: (1) golden perch spawned at lower temperatures (≤18.8°C) than has previously been reported from the Murray‐Darling Basin; (2) golden perch in the Menindee Lakes can be derived from spawning in all seasons of the year in contrast to what is apparent in more southerly locations of the Murray‐Darling Basin; and (3) spawning can occur in the absence of floods.

Accounting for false mortality in telemetry tag applications

Freshwater ecosystems are under extreme stress due to anthropogenic influences including changing climate, river regulation and water abstraction. Improving our understanding of the hydrological determinants of key life‐history processes of fish, as well as the spatial scales over which these processes occur, is fundamental to inform effective recovery actions. We monitored the spawning response of native fish to a drought‐breaking long‐distance flow pulse that was protected from extraction by a legal intervention order in Australia's northern Murray–Darling Basin. Sampling sites were distributed across >1600 km of the Barwon–Darling River and three of its major tributaries. Larvae of the pelagophilic golden perch (Macquaria ambigua) were captured at all sites, with the size and age distribution indicative of both mainstem and tributary spawning. A mismatch between estimated hatch dates and river discharge at some locations suggested substantial flow‐assisted dispersal from upstream spawning sites, although this was site‐specific and more prevalent at downstream locations. Early life growth rates were the highest at tributary sites compared with mainstem sites, and within mainstem sites, golden perch grew faster in upper reaches compared with lower reaches. The present study provides insight into the environmental benefit of a post‐drought protected flow event whereby connected lotic habitats promoted fish spawning and dispersal over a large spatial scale. Protection of future flow events should occur to support the conservation of golden perch and other pelagophil species, particularly following future drought periods which are forecast to become more intense and frequent.

Bacterial production is important in aquatic carbon cycles because it represents a key component whereby dissolved and particulate carbon can be recycled back into food webs. Despite its acknowledged importance, few studies have examined bacterial production in lowland rivers. Since studies have suggested bacterial production is closely related to some carbon pools, we anticipated this to be the case in the Murray River, but that the timing and type of carbon inputs in the Murray River may lead to bacterial dynamics that differ from studies from other sites. Bacterial abundance and production were measured at three contrasting sites of the lowland Murray River, south-eastern Australia, over an 18-month period. Bacterial abundance varied across the three sites on the Murray River and was correlated with chlorophyll a concentrations but not with temperature, nutrients, particulate organic carbon and dissolved organic carbon concentrations. Bacterial production also varied across the sites. Lowest production was at the site most immediately downstream of a large reservoir, with production generally ranging from 0.88 to 8.00 µg C L-1 h-1. Bacterial production in a reach within a large forest ranged from 4.00 to 17.38 µg C L-1 h-1. Production at the reach furthest downstream ranged from 1.04 to 23.50 µg C L-1 h-1. Bacterial production in the Murray River was generally greater than in the European River Spree, reaches of the Meuse and Rhine without immediate impacts from major urban centres and the Amazon River, but was similar to the concentration measured in the Mississippi and Hudson Rivers. Bacterial production was closely correlated with chlorophyll a concentration and total phosphorus, but not with temperature, dissolved organic carbon, particulate organic carbon or inorganic nitrogen. Despite the differences in production and respiration measured at different sites across the Murray River, bacterial growth efficiency was very similar at the three sites. Bacterial populations in the Murray River appear to be influenced by reach-specific conditions rather than broad-scale drivers such as temperature, carbon and nutrient concentrations.