In lowland river systems, in-channel, slow-flowing or still-water areas (still-water patches, SWPs) are considered to be important habitats for many organisms, particularly the early-life stages of fish and shrimp. However, the distribution of the early life-stages of fish and shrimp among these habitats appears to be very patchy and studies suggest that the quality and diversity of microhabitat conditions within SWPs and the accessibility of SWPs to spawning adults and dispersing young may be important determinants of their suitability as nursery habitat. The aims of this thesis were to examine the utilisation of still-water patches by fish and shrimp in a lowland river in relation to habitat suitability and accessibility, with particular emphasis on early-life stages. To determine the factors influencing habitat selection among SWPs, the environmental variability in SWP habitat, and both the distribution and the movement patterns of fish and shrimp, were examined in the Broken River, a lowland river in south-eastern Australia. SWP habitat was found to be highly spatially and temporally variable in the Broken River. SWPs differed in relation to permanence, accessibility and microhabitat variables, and all life-stages of fish and shrimp were found to be significantly spatially aggregated among SWPs. This suggests that individual SWPs may differ in their suitability as habitat, and/or in their accessibility to dispersing organisms and indicates either differential rates of retention, movement into SWPs, spawning effort or survival among SWPs for these organisms. Significant associations were found for all species and life-stages in relation to the microhabitat characteristics of SWPs. The two introduced species, carp and gambusia, were found to have fewer associations, which suggests that these species are habitat generalists. Cover and SWP morphology variables were shown to be important for all native species. Significant, positive associations were found for most species and life-stages with large, deep, SWPs containing instream cover, however, the extent of cover preferred was variable. It was hypothesised that large, deep SWPs that contain instream cover are more environmentally stable and provide better foraging efficiency and reduced competition for space, whilst also providing refuge from predators and, that they may be easier to locate than smaller patches. Specific associations with microhabitat variables differed among all species and life-stages, and this was attributed to differences in diet and predation rates. Consequently, generalised microhabitat relationships for particular life-stages or species could not be identified and the results from this thesis suggest that a diversity of microhabitat conditions are required to meet the differing requirements of various life-stages and species. Significant associations were also found for most groups in relation to the accessibility characteristics of SWPs, indicating that the ability of individuals to access SWPs is an important factor in determining their distribution among SWPs. This further suggests that movement is an important factor in the distribution pattern of fish and shrimp among SWPs. Significant associations were found for most groups in relation to patch isolation, adjacent hydraulic habitat and entrance conditions, indicating that landscape composition and configuration as well as boundary conditions may be important determinants of organisms being able to locate suitable patches. Associations with accessibility variables differed among species and life-stages, and may be attributable to differences in movement capabilities. Field manipulations of instream cover and entrance depth were conducted to further examine the habitat associations found. The results confirmed a positive relationship between instream cover and fish and shrimp abundances. No species, however, responded consistently to the manipulation of entrance depths, and this was attributed to water level rises throughout the experiment and/or the correlation of entrance depth with SWP depth. However, the results from the field manipulations suggested that deeper habitats are able to be exploited by small-bodied adults and larvae when significant levels of instream cover are also available as refuge from predation. In order to confirm the importance of movement in the selection of SWP habitat by fish and shrimp, the movement patterns of fish and shrimp into and out of SWPs were investigated. Whilst the results from this aspect of the study were inconclusive for fish, the results for shrimp confirmed that adults and larvae moved routinely into and out of SWPs. However, for all shrimp species, movement appeared to be limited to a certain period of larval development, indicating that SWP quality and stability may be more important at particular stages of development than others. The results of this thesis have demonstrated the importance of SWP quality and stability for fish and shrimp in the Broken River and have shown that habitat preferences vary among individual species and life-stages. Consequently, in order to manage for multiple species and life-stages, consideration must be given not only to the availability of SWPs, but also to their stability over time and to the availability of a diverse range of microhabitats. In addition, consideration must also be given to the accessibility of SWPs and this will require a greater knowledge of the specific spawning and dispersal requirements of the organisms which utilise these patches, in combination with a greater understanding of the impacts of flow modification on riverine landscape composition and configuration.
|Date of Award||2007|
|Supervisor||Martin THOMS (Supervisor), Paul Humphries (Supervisor) & B Gawne (Supervisor)|