Is the decline of freshwater mussel populations in a regulated coastal river in south-eastern Australia linked with human modification of habitat?

1. The Hawkesbury-Nepean River provides potable water for 5 million people living in the Sydney basin, and water for agricultural and horticultural production that meets most of Sydney's daily needs for fresh food. Anecdotal evidence indicated that numbers of freshwater mussels have seriously declined in much of the river over recent decades. 2. A field survey revealed the presence of populations of three species of mussels, Hyridella depressa, Hyridella australis and Velesunio ambiguus in the river. Higher density mussel populations were most common in catchment areas with little human modification to the channel bed or adjacent vegetation communities. 3. Levels of disturbance of riparian vegetation and, to a lesser degree, land use, were identified as being strongly associated with the absence of mussels from some reaches. 4. Catchment geomorphology was also shown to be relevant to the abundance, population structure and suite of mussel species present in different geomorphic reaches of the river. Absence of mussels was noted from areas where they had been recorded in previous studies. These results demonstrate that mussel species are under threat in the catchment. 5. As filter feeders with the ability to remove excess nutrients and bioaccumulate toxic substances, freshwater mussels play an important role in natural remediation processes in freshwater systems. 6. The decline in mussel populations in the Hawkesbury-Nepean River highlights concerns for the overall health of the river system, and supports the need to identify the subsidiary impacts of physical habitat modification in developing both riverine and riparian management strategies.