Fish movement and passage in a fragmented and freshwater-deprived estuary: implications for the management of estuarine-associated fishes

Abstract

Worldwide, freshwater deprivation and habitat fragmentation threaten estuarine ecosystems and their fishes, and these impacts are likely to increase in severity with increasing human population and climate change. Mitigation is reliant on knowledge of the diverse life histories of estuarine associated fishes, including the dependence of key population processes (movement, spawning and recruitment) on freshwater flow and connectivity. The Coorong Estuary situated at the terminus of Australia’s longest river system, the Murray-Darling, is an ideal location to investigate the influence of freshwater flow and fragmentation on estuarine fish movement and approaches to remediation. In this thesis, I present chapters that: 1) describe and categorise the fishes of the Lower Lakes and Coorong; 2) investigate the influence of connectivity and freshwater discharge on the movement and recruitment of diadromous (congolli, Pseudaphritis urvillii) and (3) estuarine (black bream, Acanthopagrus butcherii) fish species; 3) investigate the efficacy of fishways in promoting fish passage past a tidal barrier; and 4) integrate these findings with those from other published studies to provide a global review on the impacts of tidal barriers on fishes and approaches to remediation to guide future research and management. To provide biological context for future studies, I provide a review on the life history, population dynamics and contemporary management of fishes of the Lower Lakes and Coorong. Using contemporary monitoring data, and commercial fishery and museum records I provide an updated inventory of species recorded in the Lower Lakes and Coorong and formally classify species using the estuarine use functional guilds approach (Chapter 2). This ensures consistency with species classification in other estuaries globally and aiding comparisons of assemblages across geographic regions. The movement of adult female congolli and annual recruitment to young-of-the-year (YOY) was investigated across two years of varying freshwater flow and connectivity using acoustic telemetry and fishway trapping (Chapter 3). Individuals tagged in freshwater habitats undertook rapid downstream spawning migrations in association with day-of-year (austral winter) and moon phase. In the first year of the study, downstream migrations were obstructed by closed tidal barriers (The Murray Barrages) and tagged females failed to reach marine spawning grounds, while in the second year, connectivity was provided through a navigation lock and open barrage gates, and tagged females subsequently transited through the Coorong estuary and into the ocean. Annual recruitment of YOY was significantly greater following the year of successful migration to the ocean than following obstructed migration. This study demonstrated the potential impact of tidal barriers on the migrations and population dynamics of catadromous fish and the applicability of navigation locks to provide downstream passage. The movement of the estuarine black bream was investigated, using acoustic telemetry, across two 18-month periods characterised by no freshwater discharge and high salinity, and high freshwater discharge and low salinity, respectively (Chapter 4). Most tagged fish exhibited fidelity to the Coorong, but a proportion of individuals (~22%) left the estuary and did not return or returned after absences of up to 5 months. One individual that did not return was detected in an adjacent estuary. This frequency of estuarine-marine transitions was greater than reported in other regions of the species’ range and may have important implications for fisheries management. Within the estuary, linear movement extent was greater during high freshwater flow (77 km) than the period of no flow (41 km), and spatial patterns of intra-estuarine occupancy were associated with salinity. The delivery of environmental flows to promote favourable salinity regimes will support estuarine associated fishes in the Coorong and other similarly freshwater-deprived lagoonal estuaries. Fishways are commonly used to promote fish movement past instream barriers, yet application on tidal barriers has lagged riverine application. In chapter 5, I assessed and compare fish passage at a pair of hydraulically distinct vertical-slot fishways on a tidal barrier (Tauwitchere Barrage) between the Lower Lakes and Coorong.Collectively, the fishways facilitated the passage of a diverse migratory fish assemblage across the range of head differential experienced, and were complementary in function. Similar application of paired vertical-slot fishways, with differing yet complementary hydraulic characteristics, is likely a viable approach at other low-level tidal barriers. In my final core chapter (6), I present a global review on the impacts of tidal barriers on estuarine connectivity, hydrodynamics and estuarine-associated fishes, and approaches to remediation, using case studies at catchment (the Lower Lakes and Coorong, Murray-Darling Basin, Australia), regional (the southeastern United States) and national (the Netherlands) scales. The study identifies the application of fish passage solutions, including reinstatement of tidal flux and delivery of environmental flows as key management actions to rehabilitate estuarine-associated fishes. Suggestions for future research and management of tidal barriers are provided in the context of future challenges associated with increasing abstraction of freshwater and climate change (e.g. sea level rise). Estuaries and their fishes have long provided humans with immense ecosystem services, yet human disturbance threatens their persistence. In many instances, the abstraction of freshwater from catchments and construction of tidal barriers are necessary, allowing agricultural production and protection of lives and assets, yet, there is a need to do so in a more ecologically sustainable way. Ultimately, management of estuaries that is cognizant of aquatic connectivity and promotes natural estuarine ecosystem function will best support estuarine-associated fish populations into the future.