Investigation of the Role of Low-Flow Events in the Ovens River

"February 2007".

Project Number: Investigation of the role of Low Flow Events in the Ovens - M/BUS/100.

MDFRC item.

The Ovens River, located in North East Victoria, extends from its source in the Victorian Alps near Mount Hotham, to the Murray River near Lake Mulwala. The Upper Ovens River (upstream of Myrtleford) plays an important role in North East Victoria’s economy, providing resources for agriculture, domestic use, tourism and recreational fishing. Such demands conflict with the significant environmental values of the river, especially in times of low-flow and drought. The challenge of the Upper Ovens River Stream Flow Management Plan is to continue to meet the above water requirements while simultaneously maintaining the environmental values and integrity of the river. This study aimed to assess the impact of periods of low-flow in the Upper Ovens River and its tributaries on components of the ecosystem, in particular, two-spined blackfish (Gadopsis bispinosus) and the macroinvertebrate communities on which they feed. To achieve this we examined the habitat preferences, abundance, distribution, and age structure of two-spined blackfish populations; and the abundance and distribution patterns of invertebrate communities, from nine separate streams of the upper catchment. Streams were classified as either large permanent (will not dry), medium permanent (unlikely to dry), or small discontinuous (likely to dry), with three streams in each category. Two-spined blackfish of the upper Ovens River catchment were found in slow flowing runs and pools with boulder and cobble substrates which were utilized along with instream debris to provide cover in water that was often less than 21cm deep (although they may be displaced into shallow water through the presence of trout). Two-spined blackfish were found to avoid small discontinuous streams, although there was evidence that they will briefly utilise them when conditions are suitable.

Length-age analysis indicated that two-spined blackfish are successfully breeding and recruiting in medium and large permanent streams, with spawning appearing to run from at least September to late January. Two-spined blackfish are a relatively sedentary fish, however, some movement was noted amongst all age classes. In particular, adults were observed to emigrate out of streams that became critically low (but remained flowing), causing an aggregation of fish into larger streams such as Morses Creek. This concentration of fish forced an increased level of intra and inter specific competition, with trout being the likely benefactor from such interaction. In light of this it appears that abundances of two-spined blackfish are likely to increase in permanent systems as smaller streams contract. Further contraction of permanent systems through water extraction during periods of low-flow is likely to be detrimental to the persistence of two-spined blackfish. In reference to the minimum environmental flow of 94 ML d-1 at Myrtleford as set by the Draft Upper Ovens River Stream Flow Management Plan. We found that as discharge approached this level at Myrtleford, changes in two-spined blackfish distribution and abundance had already commenced in the upper catchment, a process that intensified as discharge fell below the minimum environmental flow requirement.The current study has helped refine our understanding of the habitat requirements of two-spined blackfish and the potential for the effects of flow modification to interact with the effects of an introduced species (trout). The increased knowledge of the relationship between low flows and two-spined blackfish will help refine the Stream Flow Management Plan to ensure it meets its target of maintenance of the small native fish community in the Upper Ovens catchment. Once the Stream Flow Management Plan is implemented this knowledge will also help interpret environmental changes, thereby facilitating adaptive changes to flow management.Benthic macroinvertebrate sampling consisted of monthly surveys using a modified Boulton sampler between December and April. Samples were sorted and identified to the taxonomic resolution of family.

Macroinvertebrate communities from small discontinuous streams were found to be significantly distinct from those of medium and large permanent streams. Over half of the collected taxa were generalist. Macroinvertebrate community composition did not significantly change post cease to flow conditions or as flows contracted. This is most likely because there was no decline in water quality, no critical flow threshold was reached, and/or that sampling did not continue long enough for impacts on the community to be seen.Once again the current project has advanced our understanding of the role that low flow periods play in determining the macroinvertebrate community structure that will facilitate the development of the Stream Flow Management Plan. The new knowledge also reveals some knowledge gaps that will need to be filled through a monitoring program if we are to ensure that the SFMP meets its objective of maintaining the diverse macroinvertbrate community.The project reveals that for both the macroinvertebrate community and small bodied native fish that a catchment approach to monitoring is required that examines the distribution of taxa within the catchment rather than focussing solely on taxonomic composition or abundances at sites. The project also suggest that some of the impacts of altered flows may take quite some time to become manifest. Increasing the magnitude or duration of low flows may not have significant effects immediately but may have longer term consequences. Finally the study will provide some background information on habitat preferences that will help identify potential indicators of significant hydrological change (e.g. shrimp and hydroptilid)Results from this work indicate that native components of the ecosystem are well adapted to periods of low-flow in the Upper Ovens catchment, however, if such periods are exacerbated by water extractions, the system is seen to be less durable. Two-spined blackfish and flow dependant invertebrate taxa will decline in permanent waters. It is known that some invertebrates will recover slowly once flow recommences, however for two-spined blackfish such knowledge is currently unavailable. This current drought offers an ideal opportunity to assess and document the recovery ecology of the two-spined blackfish, and to a lesser extent the flow dependent maroinvertebrates of the Upper Ovens catchment.