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Barriers to fish migration in drying climates: Contributions from south-western Australia

Allen, Mark (2016) Barriers to fish migration in drying climates: Contributions from south-western Australia. PhD thesis, Murdoch University.

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Threats to the depauperate and highly endemic freshwater fish fauna of south-western Australia have escalated in recent decades, leading to the listing of five of the eleven (~45%) teleost species as threatened or endangered under various Acts of State and Federal legislation. The most profound impacts on these fishes have resulted from salinisation of waterways, riparian habitat degradation, proliferation of instream barriers, and the introduction of exotic fish species. In addition, annual rainfall has declined considerably in south-western Australia since the 1970s, resulting in a dramatic reduction in annual surface flow and groundwater levels. This trend is projected to continue and is likely to exacerbate other stressors and directly impact upon the region’s unique freshwater fishes. Using south-western Australia as a model region, this study aimed to investigate how a drying climate may influence the impacts of instream barriers on freshwater fish migration, and importantly, how it may influence management actions designed to mitigate these impacts. In achieving this aim, the study also addressed key knowledge gaps on distributional range, seasonal movements and life history traits of two of the region’s most endangered fishes: the Trout Minnow (Galaxias truttaceus) and the recently described Little Pygmy Perch (Nannoperca pygmaea).

A global review of instream barrier impacts revealed a wide range of effects on aquatic ecosystems such as loss of habitat, alteration of natural flow regimes, and disruption of longitudinal river connectivity. Attempts to mitigate their effects on migratory fishes have most commonly involved fishway construction; however, complete physical removal of redundant barriers is increasing globally (particularly in the United States), mainly due to safety concerns and prohibitive costs of maintaining aging structures. The review revealed a number of processes used in different jurisdictions for assessing and prioritising barriers for remediation or removal; however, the influence of climate change was not specifically considered in any process.

Physico-chemical shifts in river systems, reductions in surface discharge and alterations to seasonal flow regimes driven by climate change are likely to exacerbate the impacts of instream barriers on freshwater fishes; however, the manner in which such changes will influence the mitigation of impacts of instream barriers has received limited attention. In south-western Australia, climate change will exacerbate instream barrier impacts by reducing habitat connectivity, compromising the efficacy of fishways, facilitating the introduction of climatically mismatched species, and adding pressure on environmental water allocations. Paradoxically, climate change may also enhance the conservation value of some barriers and their associated impoundments as ecological refuges. The trade-offs between the positive and negative ecological impacts of barrier decommissioning are likely to become key considerations in the management of river infrastructure in non-perennial systems in drying climatic regions.

Given the complex impacts of instream barriers, both positive and negative, sound socioeconomic and ecological information should underpin decisions surrounding barrier decommissioning. The current study proposes specific criteria pertaining to fishes that merit consideration in barrier removal decisions in drying climatic regions with nonperennial rivers, and addresses the implications of climate change for these criteria. These include: ecological benefits of retaining barriers (e.g. providing artificial refuges); the spatial distribution of other barriers and key habitats (e.g. natural refuges) within watersheds; the presence of existing fish passage infrastructure; and biological characteristics of native fishes (e.g. resilience to climate change).

To this end, a process for identifying, assessing and prioritising instream barriers for consideration of removal or remediation that incorporated these criteria, was developed and trialled in six catchments in south-western Australia. A total of 64 potentially significant barriers to fish migration were identified and prioritised according to their potential impact on fish migration. The highest ranked barriers tended to be located in the lower reaches of catchments, impacted higher proportions of resident fish assemblages, blocked access to higher proportions of available habitat (including refuges), and were less frequently drowned-out and passable to fish. Low-altitude aerial surveys (using helicopter) proved highly effective for validating information on artificial instream barriers gathered during a desktop review. The study also demonstrated that aerial surveys are especially useful in mapping permanent refuge pools and natural barriers, which are crucial to understanding distributions and movements of fishes in intermittent systems, but for which data are largely non-existent in this region. Gathering these data would have proved extremely difficult without the use of helicopter in these remote and largely inaccessible (by ground) catchments.

This study undertook the first field application of Passive Inducer Transponder (PIT) technology to study the movement patterns of freshwater fishes in Western Australia. Mature G. truttaceus were tagged and their movement patterns through a vertical slot fishway elucidated during a 5 month period that encompassed the annual breeding season. A total of 25 of the 144 (~16%) PIT-tagged individuals were detected at least once during the study. Sixteen individuals successfully passed through the fishway at least once, with some individuals ascending and descending on multiple occasions. Successful passage through the fishway tended to coincide with flow pulses, although no passage events were recorded during a significant flood pulse that inundated the weir and fishway. The lag period between consecutive passage events for individual fish ranged from 1 to 116 d. Mean duration of fishway ascent (57.44 ± 11.50 min) was longer than the mean duration of descent (22.70 ± 3.04 min) and fish were detected in the fishway more frequently during the night (74.6% of total detections) compared to the day (25.4%).

The number of PIT-tagged fish ascending the fishway was unexpectedly low, suggesting that spawning sites exist downstream of the weir, thus precluding the need for mature fish to undertake an upstream potamodromous migration via the fishway. A proportion of the nine fish that were detected at the fishway but did not pass through, visited the fishway entrance regularly over a period of days to weeks. It is unlikely that these fish were physically incapable of passaging the fishway, as the structure was specifically designed for this purpose; rather, this observation may represent the use of the fishway as habitat and could be interpreted as evidence of home-ranging behaviour in this species. Possible reasons for these observations are discussed.

Widespread sampling undertaken during the study extended the known range of one of the region’s rarest teleosts, N. pygmaea, a species that was previously known only from a small section of the Hay River. The study revealed that it occupies parts of the Denmark and Kent rivers, and an outlying population was also discovered in Lake Smith ~200 km west of the nearest known population in the Kent. Its extent of occurrence is 3,420 km2 and area of occupancy is only 10 km2.

This study elucidated aspects of the biology of N. pygmaea in the Hay River system with the aim of determining key threats to its persistence. The population was found to undertake a short potamodromous migration into the seasonally flowing Mitchell River to spawn in August and September before retreating to permanent refuge pools in the Hay River mainstem as flows abated in spring. It is a serial spawner and reached maturity at the end of its second year of life. No spawning females were captured in the salinised Hay River main channel despite extensive sampling, suggesting that access to non-salinised habitat is vital for reproduction in this population. The biology was compared with two sympatric percichthyids and discussed in reference to the considerable disparity in population size that was estimated in a baseflow refuge pool in the Hay River using mark-release-recapture methods. This was the first time the abundances of native freshwater fishes had been determined in a natural river system in south-western Australia.

The highly restricted area of occupancy of this species was interpreted as being indicative of a recent historical decline within the rivers that it inhabits, probably due to a limited salinity tolerance. The species is considered susceptible to catastrophic losses from potential perturbations such as prolonged drought, drying of critical baseflow refuges, increasing salinisation, and introductions of alien species. A number of refuge habitats for this species were identified throughout its restricted range, including two artificial refuges (fire-management ponds) in the Denmark catchment. There is potential to mitigate the threat of population declines or losses of this species by providing additional artificial refuge habitats, and further research into the efficacy of such a conservation strategy should be prioritised.

The information gathered during the study on the distribution, life history, threats and population size for N. pygmaea were used to assess its conservation status. It qualified as Endangered under IUCN Red List assessment criteria, and was recently added to Schedule 1 of Western Australia’s Wildlife Conservation Act 1950.

The current study is the first to develop a barrier prioritisation process that specifically accounts for the influence of climate change on the impacts of instream barriers on fish migrations, which should enhance the robustness of decisions surrounding their removal or remediation in order to enhance fish migration. Climate change will become increasingly important in management decisions due to its direct and indirect impacts on fishes, particularly the effect it will have in exacerbating the impacts of instream barriers and water abstraction, and in reducing the quality and quantity of refuge habitats. The knowledge gained here on the life history and stressors impacting two of the continent’s rarest and most endangered fishes was incorporated into the barrier prioritisation process and will additionally assist environmental managers with the task of conserving the region’s aquatic biodiversity in the face of escalating threats.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Beatty, Stephen and Morgan, David
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