Fish communities from four catchments in the Murray-Darling river system were analysed in relation to climate, hydrology and river regulation. Using the annual proportional flow deviation as a measure of river regulation, the Paroo River catchment was assessed as unregulated, the Darling River catchment as mildly regulated and the Murrumbidgee River and River Murray catchments as highly regulated. A total of 11 010 fish, representing nine native and three alien species, was caught during high and low flow seasons in the four catchments. Native species, such as golden perch Macquuriu ambigua (Percichthyidae), bony herring Nematalosa erebi (Clupeidae) and spangled perch Leiopotherapon unicolor (Teraponidae), dominated fish communities in the Paroo and Darling catchments, but alien species, mostly carp, Cyprinus carpio (Cyprinidae), were also abundant. Both native and alien species were more abundant in these catchments after flooding, but there was little change in species composition between high and low flow seasons at the catchment level. Carp dominated communities in the Murray and Murrumbidgee catchments. There was a significant trend for reduced species diversity in increasingly regulated catchments. River regulation may alter the relative abundance of native and alien fish by desynchronizing environmental cycles and the reproductive cycles of native species. Ordination of species abundances showed discrete fish communities that reflect the geographical separation between catchments. Differences between communities are related to opportunities for dispersal, the environmental tolerances of dominant species and the modifying effects of river regulation. Fish communities in lakes exhibited less seasonal variation than riverine communities within the same catchment, indicating the greater seasonal stability of lakes compared with regulated and unregulated river reaches. Management of fish resources needs to include catchment-specific strategies within current State and basin-wide management programmes.

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This study was undertaken as part of a long-term investigation of the ability of high-level fishways to rehabilitate fish communities upstream of high dams. Effects of Tallowa Dam on fish of the Shoalhaven River system were studied by comparing species abundances, population size-structures and the structure of fish communities above and below the dam. Fish were sampled twice yearly for two years at 12 sites throughout the catchment. Species richness was greater downstream of the dam, with 21 species, compared to 16 species upstream of the dam. Ten diadromous species are believed to be extinct above the dam because of obstructed fish passage. Another four migratory species capable of climbing the wall have reduced abundances upstream. Accumulations of fish, particularly juveniles, directly below the dam were evident for nine species. Fish communities upstream and downstream of the dam differed significantly, identifying the dam as a significant discontinuity in the available fish habitats within the system. Historical evidence suggests that before the dam was built, fish communities from the tidal limit to at least 130 m elevation were largely continuous. This study has demonstrated that Tallowa Dam is a major barrier to fish migration and has had adverse effects on the biodiversity of the system. The creation of Lake Yarrunga by Tallowa Dam has resulted in distinctive fish communities in riverine and lacustrine habitats. Populations of five species that occur both upstream and downstream of the dam have developed differences in their size structures. The fish community downstream of the dam also differs from its historical condition because of the virtual disappearance of Australian grayling (Prototroctes maraena) and the establishment of non-native species.A high-level fishway is now being designed for the dam to restore fish passage. Data from this study will serve as a baseline against which to assess the effectiveness of the fishway in rehabilitating fish communities of the river system.

A total of 436 logs were used to create 20 engineered log jams (ELJs) in a 1.1 km reach of the Williams River, NSW, Australia, a gravel-bed river that has been desnagged and had most of its riparian vegetation removed over the last 200 years. The experiment was designed to test the effectiveness of reintroducing woody debris (WD) as a means of improving channel stability and recreating habitat diversity. The study assessed geomorphic and ecological responses to introducing woody habitat by comparing paired test and control reaches. Channel characteristics (e.g. bedforms, bars, texture) within test and control reaches were assessed before and after wood placement to quantify the morphological variability induced by the ELJs in the test reach. Since construction in September 2000, the ELJs have been subjected to five overtopping flows, three of which were larger than the mean annual flood. A high-resolution three-dimensional survey of both reaches was completed after major bed-mobilizing flows. Cumulative changes induced by consecutive floods were also assessed. After 12 months, the major geomorphologic changes in the test reach included an increase in pool and riffle area and pool depth; the addition of a pool-riffle sequence; an increase by 0.5-1 m in pool-riffle amplitude; a net gain of 40 m 3 of sediment storage per 1000 m 2 of channel area (while the control reach experienced a net loss of 15 m 3 /1000 m 2 over the same period); and a substantial increase in the spatial complexity of bed-material distribution. Fish assemblages in the test reach showed an increase in species richness and abundance, and reduced temporal variability compared to the reference reach, suggesting that the changes in physical habitat were beneficial to fish at the reach scale.

Fish assemblages in six reaches of the Hawkesbury–Nepean River were studied to identify the effects of two types of riparian vegetation; well‐vegetated banks supporting complex flora dominated by trees and shrubs, and grassed banks, that have been colonised only by grasses after historical deforestation. The fish assemblages showed both spatial and temporal differences and habitats adjacent to grassed banks supported more individuals and more fish species than well‐vegetated banks. Three small species of fish, firetail gudgeon, Hypseleotris galii (Ogilby), flathead gudgeon, Philypnodon grandiceps (Krefft), and empire gudgeon, Hypseleotris compressa (Krefft), occurred in greater abundances adjacent to grassed banks, but freshwater mullet, Myxus petardi (Castelnau), were less abundant near grassed banks than beside well‐vegetated banks. Differences were also shown in the size frequencies of the four larger fish species between riparian vegetation types. The observed differences in the distributions of fish species appeared to be related to the greater abundance aquatic macrophytes near grassed banks, probably an effect of shading of macrophytes near well‐vegetated banks.