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

Abstract

ABSTRACT1. 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.

The role of geomorphology in substratum patch selection by freshwater mussels in the Hawkesbury–Nepean River (New South Wales) Australia

“…Previous research identified mussel distribution patterns that showed a high level of correlation with geomorphic reach type (Brainwood et al, 2006;Brainwood, in press). To explore this relationship further, the second stage of analysis involved examining data separately within geomorphic reach types.…”

“…Density of mussels was not normally distributed, and density of mussel size classes within these sites showed a right skewed distribution characteristic of mussel populations examined in the Hawkesbury-Nepean River Table 1. Results of hypotheses testing using forward stepwise logistic regression for presence/absence of mussel assemblages in the Hawkesbury-Nepean River with substrate characteristics (Brainwood et al, 2006;Brainwood, in press). Data were characterized by high numbers of zeros recorded for mussel densities at sites sampled.…”

“…Mussels were allocated to size classes based on a range of shell characteristics, with classes summarized as: small=540 mm valve length; medium=40-60 mm; and large=>60 mm length (see Brainwood et al, 2006). When populations were described by density within size classes per species, this resulted in nine faunal groups, and enabled differentiation between ageing populations with mussels all at, or close to, maximum size, and recruiting populations with mussels encompassing a range of sizes.…”

“…Brainwood et al (2006) found that, for 26 sites in the lower catchment, presence of mussels was adversely affected by increasing levels of riparian disturbance, and that distribution of species was related to geomorphic reach type. In a further study of 64 sites at 32 weirs throughout the greater Hawkesbury-Nepean catchment, presence of mussels was linked with location above or below a major impoundment, and distribution of species and mussel abundance was related to geomorphic reach type (Brainwood et al, in press).…”

“…The consistently sympatric distribution of Velesunio ambiguus, Hyridella australis and Hyridella depressa raises questions about habitat partitioning, resource partitioning and synergistic relationships among these species that may enhance their survival in adverse conditions. Given the detrimental effects of human modification of habitat on mussel populations in the Hawkesbury-Nepean system (Brainwood et al, 2006), these mussels may simply be congregated in remnant patches of suitable habitat.…”