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

Abstract

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.