Integrated Monitoring of Environmental Flows: Freshwater Shrimp in Regulated Rivers

"April 2010".

Project Number: Integrated Monitoring Of Environmental Flows: Freshwater Shrimp in Regulated Rivers - M/BUS/278.

MDFRC item.

The current study was undertaken as part of the NSW Integrated Monitoring of Environmental Flows Program which aims to provide an understanding of the flow responses of river and wetland ecosystems, and to evaluate the environmental performance of environmental flow rules that have been developed for regulated rivers in the state. The study aimed to examine the relationships between river flows and shrimp populations in two regulated rivers in the northern part of the Murray-Darling Basin focussing on the effect of hydrology on the different life-history stages. The key findings were: The hydrology of the two rivers differed throughout the study period. The Namoi River, which had previously been found to be less impacted by modified hydrology than the nearby Gwydir River, was characterised by lower median and minimum flows, and higher numbers and duration of flows below the 90th percentile; All three species of freshwater shrimp (Paratya australiensis, Caridina mccullochi and Macrobrachium australiense) were collected in higher abundances in the Namoi River and most species/life-stage groups were positively related to low minimum and/or median flows and to other low-flow variables; There were consistent differences in the hydrology among study years in the Namoi River but not in the Gwydir River. The inter-annual differences in the Namoi River were associated with differences in the number and mean duration of low flow spells and in the variability of flows; Temperature did not differ significantly between the two rivers, however, significant differences were found among years. In both rivers, average water temperatures were significantly higher in the first sampling year than in the second and third sampling years; Significant differences were found among sampling years for almost all of the species/life-stage groups. In both rivers, the highest abundance of all life-stages of M. australiense and C. mccullochi were collected in the first year of sampling. This was also the case for all life-stages of P. australiensis except stage I larvae in the Gwydir River. This suggests that temperature may be a key driver of shrimp distribution patterns. The findings of this study support those of studies undertaken in more southern areas of the Murray-Darling Basin for two of the three species, C. mccullochi and M. australiense. These studies have found that both species occur in higher abundances in less regulated environments and have hypothesized that low flows during the breeding season are important v for both species. However, this was not the case for the third species, P. australiensis, which has been found to be tolerant of modified flow regimes in southern parts of the Murray-Darling Basin. The results from this study suggest that relationships between P. australiensis and hydrology may be complex and that distribution patterns may be related to the availability of hydraulic habitat rather than discharge per se. Further research into the distribution of P. australiensis in fast-flowing habitats and into the relative abundance of different hydraulic habitats in the Namoi and Gwydir Rivers would be required to confirm this hypothesis and to elucidate flow-ecology relationships for P. australiensis. This study has provided insight into the relationship between freshwater shrimp populations and river hydrology in the Murray-Darling Basin and the results suggest that water temperature and the provision of low flows, particularly for M. australiense and C. mccullochi during the breeding season, are important drivers of shrimp population dynamics. However, given that i) the relationships found in the present study are relatively weak; ii) the results, particularly for P. australiensis, suggest that the relationship between shrimp populations and flow are likely to be complex; and iii) that correlative studies of this nature are unlikely to elucidate causal mechanisms, it would not be appropriate to attempt to develop specific flow recommendations based on the results of this study alone. Rather, this study provides a powerful baseline and has developed a number of hypotheses that if tested will more clearly elucidate the relationship between flow conditions and freshwater shrimp populations. It is recommended that manipulation experiments (for example, managed flow events) be undertaken as part of the adaptive management framework to generate the data necessary to develop specific flow recommendations for freshwater shrimp in regulated rivers.