Response of fishes to anthropogenic fragmentation of the Barwon-Darling River

Abstract

<p>Dryland rivers are ecologically important freshwater habitats in otherwise dry environments. They drain a substantial proportion of the world's freshwater, yet they have been understudied relative to rivers in temperate climates. Rivers are threatened by water demands for human needs and in arid environments by an increasing scarcity of surface water due to a changing climate. The Barwon-Darling River in the upper Murray-Darling Basin in Australia is a dryland river characterised by extreme hydrological variability. It has experienced changes in hydrology and geomorphology post-European settlement, and reduced abundances of native species. This thesis aims to better understand the impact of water resource development and increased fragmentation on food web structure and genetic connectivity of fishes in the Barwon-Darling River.</p><p> A systematic literature review was undertaken to identify the body of literature available to understand the impact of fragmentation on fish in all river types globally. The literature was then used to test specific hypotheses related to food web and genetic responses of fishes to fragmentation. There was strong evidence that the creation of lentic habitat alters food webs, but inconsistent evidence about the effect of hydrological modification on food web structure. There was evidence that some species-specific traits influence the effect of fragmentation on genetic differentiation, but inconsistent evidence that barriers lead to genetic differentiation in general. The literature review highlighted the need for more long-term, interdisciplinary research which includes a range of causes of fragmentation. More comprehensive studies which account for a range of relevant factors influencing the response of fish to fragmentation are needed to help understand the mechanistic link between anthropogenic fragmentation and the biotic response.</p><p> The use of stable isotopes in food web studies often relies on invasive or lethal sample collection methods. This has the potential to alter community structure and is undesirable particularly in areas with an already sparse population. This study found a strong isotopic relationship between fin and muscle tissue in three species common to Australian dryland rivers: <i>Macquaria ambigua, Cyprinus carpio, and Nematalosa erebi</i>. The results showed that non-lethally collected fin tissue can be used as a proxy for muscle tissue in isotopic trophic studies. Tissue conversion equations were developed to predict muscle δ¹³C and δ¹⁵N isotope ratios from fin tissue values. There were significant but consistent differences between fin and muscle δ¹³C values for all species, and fin tissue was a good predictor of muscle tissue δ¹³C (<i>r</i>² = 0.77 for all species). The relationship between δ¹⁵N values was less consistent, with a significant difference found in only one species, fin tissue was still a good predictor (<i>r</i>² = 0.72 for all species). Developing species-specific tissue conversion models resulted in the least amount of error, but regional models result in similar error and are more accurate than general global models. These results are consistent with findings from prior studies of different species, enabling the use of non-lethally collected fin tissue in stable isotope food web studies.</p><p> Food web structure was investigated in six sites in the main channel of the Barwon-Darling River to determine if varying levels of hydrological isolation influenced trophic interactions. Samples were collected in March 2019 during a period of no flows when the river had receded into a series of disconnected waterholes. Analysis was restricted to four species that were caught in high enough abundance at all sites. Stable isotopes (δ¹³C and δ¹⁵N) were used to determine consumer trophic position and community metrics. There were high levels of omnivory at all sites and indications of spatial variation with some sites showing more trophic overlap. However, no consistent pattern was found between food web structure and site differences with respect to connectivity. Widespread omnivory and short trophic links are expected in variable environments. Explaining spatial variation among sites likely requires a more thorough assessment of food web structure and site variables than this study allowed.</p><p> Functional connectivity within the Barwon-Darling River was assessed based on genetic structure of bony bream, <i>Nematalosa erebi</i>, a common and widespread species. Samples were collected from eight sites along the length of the main river channel and two remote sites, one from a tributary river and one from near the mouth of the Murray River. Single nucleotide polymorphism (SNP) markers were used to assess if genetic structure is related to anthropogenic fragmentation. There was differentiation among sites, but there was no positive correlation between genetic differentiation and geographic or resistance distance between sites. Anthropogenic fragmentation of the river began about 150 years ago with substantial modification beginning in the 1960s and detecting any genetic response likely requires several more generations. Similar to other species in extinction prone environments, <i>N. erebi</i> may rely on metapopulation dynamics to persist in this highly variable river.</p><p> The dynamic nature of dryland rivers complicates efforts to understand the response of the biological community to anthropogenic fragmentation. Species that have persisted in these variable environments are adapted to extended periods of natural fragmentation, making any response to altered fragmentation difficult to detect. Overall, there was spatial variation in food web structure and genetic differentiation among sites within the river. However, there were no consistent patterns that would help us understand the impact of fragmentation. This study has helped illustrate that assessing the response of the fish community to water resource development in dryland rivers is complicated and finer-scale assessment may be necessary to detect impacts.</p>