AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS

Aquatic Conserv: Mar. Freshw. Ecosyst. 20: 18–30 (2010)

Published online 3 December 2009 in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/aqc.1080

The impact of catastrophic channel change on freshwater mussels inthe Hunter River system, Australia: a conservation assessment

HUGH A. JONESa,b,� and MARIA BYRNEa

aDepartment of Anatomy and Histology, Building F13, University of Sydney, NSW 2006, AustraliabNew South Wales Department of Environment, Climate Change and Water, PO Box 3720, Parramatta, NSW 2124, Australia

ABSTRACT

1. Australia has a distinct suite of endemic freshwater mussel species, several of which are restricted to south-eastern Australia, an intensively modified region supporting much of the nation’s population and where pressureson freshwater ecosystems are increasing.2. Surveys were made of 78 sites in the Hunter River system to determine the distribution and abundance of the

six mussel species occurring in the region, to identify threatening processes and to locate populations of highconservation value.3. Mussel populations were mainly distributed in the hydrologically stable southern Barrington rivers, where

those in the Williams River have the highest conservation value. Strongholds for Hyridella drapeta were found inWollombi Brook.4. Mussels were not detected at 40% of the sites, some of which supported mussels in the past. These were

mainly reaches that have undergone river metamorphosis.5. Where found, most mussel populations had low densities and were highly fragmented. Major threats to these

remnant populations are degradation of riparian and instream condition from agricultural activities, extremeclimatic events (flood and drought) and the introduced macrophyte, Salvinia molesta.6. While threat mitigation can be achieved by habitat protection and strategies to reconnect mussel

populations, managers are largely unaware of this invertebrate group. Formal recognition of regionallythreatened mussel populations would do much to focus efforts on conservation.7. The proposed construction of a large dam on the Williams River is a potential threat to the most important

mussel populations in the Hunter River system. Copyright r 2009 John Wiley & Sons, Ltd.

Received 3 September 2008; Revised 27 August 2009; Accepted 29 August 2009

KEYWORDS: freshwater mussel species; freshwater ecosystems; distribution and abundance; disturbance; Hunter River system;

regionally threatened mussel populations; Unionoida

INTRODUCTION

Freshwater mussels are major contributors to the biomass oflowland rivers, and through their filter-feeding activity play an

important role in the functioning of river ecosystems (Strayeret al., 1994; Vaughn and Hakenkamp, 2001). In Australia,mussel populations have undergone declines due to river

regulation and intensification of catchment land use(Brainwood et al., 2006, 2008a; Playford and Walker, 2008).This worldwide trend is associated with extinctions, as seen inthe extensive loss of endemic mussel species in North America

(Bogan, 1993). The disappearance of mussels from rivers in

Australia is of concern from ecological and conservation

perspectives (Byrne, 1998; Walker et al., 2001; Brainwoodet al., 2006).

The degeneration and fragmentation of freshwater habitats

following urbanization and agricultural development ofcatchments has been identified as the major factor contributingto the global decline in freshwater biodiversity (Allan and Flecker,

1993). Intensification of catchment land use and river regulation(e.g. dams and river engineering) have profound impacts on riverhydrology and geomorphology with consequent effects on theecological integrity of these ecosystems (Poff et al., 1997, 2007;

Gregory, 2006). Altered hydraulic conditions and reduced bed

*Correspondence to: Hugh A. Jones, New South Wales Department of Environment, Climate Change and Water, PO Box 3720, Parramatta, NSW2124, Australia. E-mail: hugh.jones@environment.nsw.gov.au

Copyright r 2009 John Wiley & Sons, Ltd.

stability at bankfull flows adversely affect mussel populations(Gangloff and Feminella, 2007). Siltation and eutrophicationassociated with shifts in land use have been devastating to

freshwater mussels (Young et al., 2001; Poole and Downing,2004). Dams and weirs have disrupted their life cycles with directimpacts on population integrity, fragmentation of populations

and movement of their fish hosts (Layzer et al., 1993; Watters,1996; Vaughn and Taylor, 1999; Brainwood et al., 2008b).

Periods of rapid agricultural expansion within catchments

in south-eastern Australia and elsewhere around the globehave coincided with accelerated rates of erosion andsedimentation of river systems (Brooks and Brierley, 1997;Brierley and Stankoviansky, 2003; Knox, 2006), resulting in

substantial alterations in the channel morphology of coastalstreams (Rutherfurd, 2000), a process termed river meta-morphosis (Schumm, 1969). In south-eastern Australian rivers,

present-day sediment loads are estimated to be 10 to 50 timesgreater than those in pre-European times (Prosser et al., 2001).Catastrophic channel widening has occurred in association

with large floods in the latter half of the twentieth century(Rutherfurd, 2000). This occurred in the Hunter River system,the region in focus in this study, following severe floods

(1949–1956) with most of the sediment liberated from bankerosion retained in the channel as sand slugs (Erskine and Bell,1982; Erskine, 1994, 1996; Thomas and Druery, 1996).

The Hunter River system supports a high diversity of

mussel species with six species recorded, including species withrestricted distributions and species that occur at the margins oftheir range (Australian Museum records; McMichael and

Hiscock, 1958). Though this drainage is among the mostaltered of the south-eastern Australian coastal catchments, italso includes relatively intact streams that support diverse

populations of freshwater mussels (DLWC, 1999). Theconservation status of freshwater mussels in the region,however, has not been determined and this is of concern as

additional river regulation and dam construction are beingconsidered (Hunter Water, 2008).

Extensive surveys of the Hunter River system wereundertaken to determine the distribution and abundance of

mussels, identify habitat features essential for theirconservation and identify major threatening processes. In alandscape-scale study of this system the present distribution of

mussels in the Hunter River was assessed with respect tophysical habitat characteristics at the reach scale and pastdistribution. This work complements similar research on

mussel populations in the adjoining Hawkesbury–NepeanRiver system (Brainwood et al., 2006, 2008a,b). Several strong-holds of mussel diversity and abundance were located inthe Hunter River system in relatively intact reaches. Mussel

populations are under threat in most of the system and thesestrongholds, particularly in the Williams River where popu-lations are large and stable, have high conservation value.

STUDY AREA

The Hunter Valley (321 300 S, 1511 000 E) is the largest coastalvalley in New South Wales with a drainage area of 22 000 km2

(Figure 1). Differences in geology and strong altitudinal andlongitudinal gradients in rainfall result in rivers that varywidely in physical form and hydrologic regime. Rainfall

increases with elevation and decreases away from the coast,

with the highest rainfall occurring on the Barrington Topsalong the north-eastern catchment boundary. The three majorrivers that drain the southern slopes of the Barrington Tops —

the Williams, Allyn and Paterson Rivers — contribute ca40% of total annual discharge from the catchment andform the Southern Barrington streams, a distinct group of

hydrologically stable rivers with high annual runoff and lowflood variability (Erskine and Livingstone, 1999). Furtherwest, but still with their headwaters in the Barrington Tops,

are Glennies Creek, Rouchel Brook and Davis Creek, whichhave lower annual runoff and higher flood variability than theSouthern Barrington streams. These Western Barringtonstreams are distinguished by their low-flow characteristics,

maintaining flow volumes during periods of low rainfall(McMahon, 1968). The streams in the southern and westernsections of the catchment cut through Triassic sandstone and

receive much lower rainfall than those further north.The Goulburn River drains almost 40% of the westerncatchment but contributes only 13% of total catchment

runoff. Streams in the Wollombi–Goulburn hydrologic areahave a low mean annual runoff combined with highly variableflows and are prone to catastrophic floods (Erskine and

Livingstone, 1999).Flow is regulated in the Hunter River by Glenbawn Dam

and smaller dams regulate Glennies Creek and the PatersonRiver (Figure 1). The Williams River is mostly unregulated

except for a dam on its major tributary, the Chichester River.The Hunter floodplain has been highly modified by intensiveagriculture and urbanization resulting in the loss of almost all

of the original floodplain and riparian vegetation (Peake,2000). Human disturbance and large floods have precipitatedgeomorphic channel metamorphosis of the Hunter River, the

Goulburn River and sections of Wollombi Brook (Table 1).

METHODS

Mussel surveys

Surveys were undertaken at 78 sites in 2006, selected to includethe major subcatchments and ensure broad spatial coverageand representation of the climatic and geological conditions

found in the Hunter Valley (Figure 1). At the time, the valleywas in a severe drought and many minor streams were dry orreduced to strings of pools. Certain geomorphic categories of

stream, such as small, temporarily flowing, sand-bed streams,were not surveyed because freshwater mussels do not occur inthese stream types. This eliminated all of the streams in the

south-western portion of the Hunter Valley, as well as manylow-order streams and most headwater reaches.

Timed searches were used to determine the presence of

mussels, to document species richness and to obtain a measureof abundance (Strayer and Smith, 2003). At each site a 100mlength of stream was searched for one person-hour by twopeople, either by wading or snorkelling. Additional searches

over 200–300m were conducted at some locations to confirmthe absence of mussels. The edges of the stream were searchedfirst, feeling beneath undercut banks and along the interface of

logs and boulders with the stream bed, followed by searchingtowards the centre of the stream. Counts were standardized ascatch-per-unit-effort (CPUE). Mussels were identified to

species following McMichael and Hiscock (1958) and the

IMPACT OF CATASTROPHIC CHANNEL CHANGE ON FRESHWATER MUSSELS 19

Copyright r 2009 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 20: 18–30 (2010)

DOI: 10.1002/aqc

presence of young mussels, less than 3 years of age (Jones,1983) was noted as evidence of recent recruitment.

Catchment-level indicators

Annual runoff and the flash flood magnitude index (FFMI)were adopted as respective indicators of hydrological stabilityand variability for stream sections. These statistics were

calculated from daily flow volumes obtained fromrepresentative hydrographic gauging stations for the period1940–2006. The FFMI is computed as the standard deviation

of the logarithms (base 10) of the annual flood peaks

(Baker, 1977). Streams with a large FFMI have a highprobability of experiencing catastrophic floods.

A catchment disturbance index (CDI) and a geomorphicriver condition index (GCI) were determined for each site. TheCDI integrates human disturbances at a catchment scale viaa drainage analysis based on a digital elevation model,

incorporating data on land use, infrastructure and humansettlement (Stein et al., 2002). The GCI provides an assessmentof the intactness of a river’s geomorphic processes, in relation to

a natural reference condition for a geomorphically similar typeof river (Fryirs, 2003). Stream reaches are rated on a 3-pointscale such that a stream reach in ‘good’ condition is in a

Figure 1. Diagram of the Hunter River catchment showing the major rivers and the locations of the surveyed sites. Dashed lines designate rivers thathave undergone major geomorphic adjustment (Table 1).

Table 1. Documented cases of channel metamorphosis for the Hunter River system

River and reach Channel changes Reference

Upper Hunter River (GlenbawnDam–Goulburn River)

Channel widening contributed23 million m3 sediment to channel;reduction in sinuosity

(Reddoch, 1957; Erskine and Bell, 1982)

Lower Hunter River (downstreamWollombi Brook; Maitland)

Channel widening; several cutoffs, morethan halving of sinuosity; levee construction;sand slugs

(Hunter River Flood Commission, 1870;Holmes and Loughran, 1976; Erskine et al., 1992;Raine and Gardiner, 1995; Thomas and Druery, 1996)

Lower Pages River Channel widening and reduction in sinuosity (Reddoch, 1957; Erskine and Bell, 1982)Goulburn River Channel widening of 500 % in the lower

reaches generating a series of bedload waves;less extensive widening in the middle reaches

(Erskine, 1994)

Lower Wollombi Brook Channel widening of 100% over 83 km anddeposition of up to 4m sand

(Erskine, 1996)

Lower Congewai Creek Degradation of channel followed by wideningdownsteam of Millfield

(Erskine, 1996)

Upper Wollombi Brook 7 km of incision followed by channel wideningbetween Fernances Ck and Hungry Ck;pools and channel smothered by sand betweenFernances Ck and Watagan Creek.

(Erskine and Melville, 2008)

See Figure 1 for sites.

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Copyright r 2009 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 20: 18–30 (2010)

DOI: 10.1002/aqc

near-natural state, streams in ‘moderate’ condition generallyhave poor catchment and riparian vegetation cover and locallydisrupted fluvial processes, and streams in ‘poor’ condition

exhibit accelerated processes (e.g. stream bank erosion andaggradation), highly altered channel morphology and poorstreamside vegetation cover.

Assessment of biophysical structure

A visual assessment of the instream and surrounding

topographic and vegetative features considered important instructuring physical habitat quality for a broad range ofaquatic biota was conducted at each site using the US

Environmental Protection Agency’s Rapid BioassessmentProtocols (RBP) (Barbour et al., 1999). Ten metricsrepresenting instream habitat, channel geomorphology andriparian vegetation were assessed for each site (Table 2). Scores

for each metric range from 0–20 and are implicitly scaledagainst a reference condition, with a high score indicatinggood condition. Metric 1—benthic habitat—was fine-tuned

with respect to mussel habitat by placing a positive emphasison flow refugia, low overhanging vegetation and protective,bed-stabilizing structures (e.g. logs, boulders) and indicators of

bed stability. The scores of the 10 RBP metrics were summedto produce an index of physical habitat quality. This index hasbeen shown to correspond with fish and macroinvertebrate

diversity in North America (Barbour et al., 1999) but it hasyet to be demonstrated that this index is correlated withAustralian mussel habitats. Hence, this index will be referredto here as the index of biophysical condition (IBC). The RBP

metrics were supplemented by visual estimates of substratecomposition, stream shade, large woody debris loads andexotic riparian and aquatic vegetation.

Strong inter-correlation existed among several RBP metrics,implying a certain degree of redundancy among variables, and sofactor analysis was used to determine whether spatial variation

in habitat quality could be explained by a smaller number offactors. These factors were used to examine relationshipsbetween mussel distribution and habitat, avoiding the

problems caused by multicollinearity among predictors. Factoranalysis was fitted by the method of maximum likelihoodfollowed by varimax rotation of the factors (Rencher, 1995). The

number of factors and soundness of the model were determinedas recommended by Rencher (1995).

Statistical modelling of mussel distribution and abundance

The relationship between mussel abundance and environ-mental covariates was examined by fitting generalized linearmodels (McCullagh and Nelder, 1989). The presence ofan excessive proportion of zero counts invalidated the

application of standard models of count data, and so hurdleregression models (Heilbron, 1994) were used to investigatethe relationships between mussel distributions and physical

habitat indices. Hurdle models are two-part models wherebythe presence–absence component is modelled by a GLM with aBernoulli error distribution (i.e. a logistic regression model)

and the count component is modelled, conditional on musselsbeing present at a site, by a GLM with a truncated discretedistribution—usually a truncated Poisson or truncated

negative binomial distribution (Heilbron, 1994). The set ofcovariates included in each part of the model need not bethe same and each model component can be interpretedindependently of the other. A forward selection strategy was

used to fit covariates, with care being taken to omit highlycorrelated variables. Selection of covariates for each modelcomponent was done by assessing the magnitude of changes in

Table 2. Indicators of catchment disturbance and rapid assessment indices of IBC

Variable Description Score range

Substratum variablesSand Percentage sand cover over stream bed 0–100Boulders Presence (exceeding 5% of channel area)

or absence of boulder cover0/1

Packing index Bed armouring score. Ranges from bedrock,tightly packed cobble and gravel, throughto highly mobile, loose sand

0–5

CPOM Score quantifying the amount of leaf litter andsmall brancheso10 cm diameter coveringthe stream bed

0–5

US EPA Habitat metrics Adapted from (Barbour et al., 1999). 0–20Metric 1 Benthic habitat (flow refugia, bed stability,

slackwater, bank complexity)Metric 2 Embeddedness/pool substrate characterizationMetric 3 Diversity of velocity–depth regimesMetric 4 Sediment depositionMetric 5 Channel alterationMetric 6 Channel flow statusMetric 7 Pool-riffle ratio/channel sinuosityMetric 8 Bank stabilityMetric 9 Bank vegetation coverMetric 10 Riparian vegetation extentIBC Sum of 10 component metrics 0–200Additional habitat indicesLWD Index of large woody debris based on the size,

density, stability and geomorphic effectivenessof wood in the channel

0–20

Exotic riparian vegetation None; occasional; moderate; dominant 0–3

IMPACT OF CATASTROPHIC CHANNEL CHANGE ON FRESHWATER MUSSELS 21

Copyright r 2009 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 20: 18–30 (2010)

DOI: 10.1002/aqc

deviance via the likelihood ratio statistic (LR test) and byexamining the regression coefficients in each part of the modelusing partial Wald tests. Standard criteria for assessing model

fit and validating GLMs were applied to the hurdle models.Statistical analyses were undertaken using the statistical

package R, Version 2.7.1 (R Development Core Team, 2007)

and the functions contained in the ‘pscl’ package (Jackman,2007).

RESULTS

Stream habitat quality

IBC scores (i.e. sum of metrics 1–10) varied widely among sitesfrom 26 to 173. The scores were negatively correlated with thecatchment disturbance index (r5�0.48) and differed

significantly among the categories of river geomorphiccondition (ANOVA: F2,75 5 6.44, Po0.01) and amongsubcatchments (ANOVA: F7,70 5 9.10, Po0.001). Tukey’sHSD test confirmed that the IBC was significantly lower for

the Hunter and Goulburn subcatchments than for all othersubcatchments. IBC scores for the Williams River were higherthan for other subcatchments, although the difference was not

significant for most pairwise comparisons. The poor scores forthe Hunter and Goulburn subcatchments were due to lowscores in the metrics for channel alteration, pool-riffle ratio,

sediment deposition, extent of riparian vegetation and benthichabitat quality.

Spatial trends in the IBC were evident within some

subcatchments. The IBC improved significantly withdownstream distance in the Williams River (Spearman’s rankcorrelation: rs 5 0.79, Po0.05) but not for the Paterson(rs 5�0.29, P40.05) and Allyn Rivers (rs 5 0.09, P40.05).

There was wide variation in the IBC among stream reacheswithin the Wollombi subcatchment. Sites on Watagan Creek,Wollombi Brook between Watagan and Congewai Creeks,

and the upper reaches of Congewai Creek and Wollombi

Brook were of higher biophysical condition (mean 138.2,s.e.5 4.3) than the lower reaches of Wollombi Brook (mean98.3, s.e.5 5.8).

Many of the component metrics of the visual habitatassessment score were highly correlated, suggestingredundancy between some of the metrics. A factor analysis

showed that nearly 70% of the variance among sites could beexplained by three underlying habitat factors (likelihood ratiotest of the hypothesis that three factors were sufficient:

w218¼ 24:97, P5 0.13). The first factor separated sites along agradient of increasing geomorphic complexity, distinguishingsites that have undergone river metamorphosis in the Hunterand Wollombi subcatchments from those in the rest of the

catchment (Figure 2). The second factor represented a gradientin riparian condition, whereas the third factor represented agradient from streams with severely aggraded channels

towards those unaffected by siltation or sediment deposition(Table 3). The scores of the three habitat factors showed astrong subcatchment-related pattern (Figure 3).

Figure 2. Biplots of site and IBC metric scores from factor analysis: (a) factors 1 and 2; (b) factors 1 and 3. Arrows indicate the direction of mostrapid increase in the value of a metric. The biplots indicate a tendency for sites to cluster according to their streams. Sites on reaches that haveundergone river metamorphosis (Table 1) (bold font) are separated from other sites by high levels of channel alteration and sediment deposition inthe channel. Stream codes: A, Allyn; C, Congewai; F, Glennies; G, Goulburn; H, Hunter; P, Paterson; R, Rouchel; T, Watagan Creek; W, Williams;O, Wollombi Brook. IBC metric codes: chn, channel alteration; emb, embeddedness; flw, flow stability; hab, instream habitat; prf, pool:riffle ratio/

sinuosity; rip, riparian extent; sed, sediment deposition; stb, bank stability; veg, bank vegetation cover; vdd, velocity-depth diversity.

Table 3. Factor loadings from a factor analysis of the componentmetrics of the visual habitat assessment index

Factor loadings

Factor 1 Factor 2 Factor 3 SpecificVariance explained (%): 29 26 13 variances

Benthic habitat 0.65 0.41 0.29 0.33Embeddedness –0.36 –0.62 0.48Velocity-depth diversity 0.60 0.37 0.39 0.35Sediment deposition –0.53 –0.70 0.21Channel flow status 0.51 0.25 0.67Channel alteration –0.91 –0.35 0.05Pool-riffle ratio 0.79 0.33Bank stability 0.50 0.59 0.26 0.34Bank vegetation cover 0.89 0.23 0.14Riparian vegetation

width0.27 0.77 0.33

Loadings with absolute values less than 0.20 are omitted and loadingsexceeding 0.5 are in bold type.

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DOI: 10.1002/aqc

Spatial patterns of mussel distribution

Five mussel species were encountered in the surveys, Hyridelladrapeta, Cucumerunio novaehollandiae, Alathyria profuga,H. australis and H. depressa in decreasing order of abundance

(Table 4). Hyridella depressa was scarce, making up less than0.1% of all mussels. Spatial patterns of abundance were highlycontagious for all species as indicated by the dispersion index

(Table 4). Mussels were not found at 40% of surveyed sites(Figure 4) and only a quarter of sites had counts exceeding100mussels h�1. Species richness varied widely among streamsand only the Williams River supported all five species (Table 5).

The other four species were found together at the same sitesonly in the Paterson and Allyn Rivers.

Alathyria profuga, although not nearly as abundant as

H. drapeta, was the most widely distributed species and wasthe only species found in all eight subcatchments (Figure 5).Cucumerunio novaehollandiae is at the southern limit of its

range in the Hunter Valley and was abundant in the middlereaches of the Paterson and Williams Rivers and at two siteson the Allyn River (Figure 4). The only other location for thisspecies was a small population in the central reaches of

Wollombi Brook (Figure 4). Hyridella australis was commonin the Southern Barrington streams, but a few individuals weredetected in the neighbouring Glennies Creek and in Wollombi

Brook (Figure 4). Hyridella depressa was restricted to a singlesite on the Williams River.

Mussel abundance was highest in streams with high annual

runoff (rs 5 0.85, Po0.01) and low flood variability(rs 5�0.82, Po0.01) (Figure 6). This included the threesouthern Barrington streams and the upper reaches of

Congewai Creek in the Wollombi subcatchment. Streamsegments with an FFMI exceeding 0.65 supported very fewmussels and invariably had low IBC scores (rs 5�0.72,Po0.01).

The Hunter River downstream of Glenbawn Dam wasvirtually devoid of mussels with the exception of a fewA. profuga. Of 12 sites surveyed along the Hunter River

between Maitland and Muswellbrook, mussels were foundonly at Jerrys Plains Weir and Keys Bridge, downstream ofMuswellbrook (Figure 4). The single site surveyed upstream of

Glenbawn Dam supported a small population of A. profuga.Low densities of A. profuga (o5 mussels per 100m) were

detected in the central reaches of the Goulburn River, wherethe river was confined by the sandstone escarpment withinundisturbed bushland.

Young mussels were most common at sites where adult catch

rates exceeded 100mussels h�1, in the Southern Barringtonstreams, upper Congewai Creek and Watagan Creek (Table 5).Several young A. profuga were found in the Rouchel Brook

subcatchment despite the low density of adults (Table 5).

Mussel distribution and indicators of physical habitat

condition

The probability of occurrence of all species significantlyincreased with the IBC score, although the influence of the

underlying habitat factors on species distributions variedamong species (Table 6). All species were negativelyassociated with the level of geomorphic alteration of streamreaches, although this relationship was barely significant for

H. australis and C. novaehollandiae. Hyridella drapetawas significantly associated with all three habitat factors,A. profuga was most strongly associated with geomorphically

unaltered reaches (factors 1 and 3), and H. australis andC. novaehollandiae were significantly associated with reaches ingood riparian condition. All species, except H. drapeta, were

negatively associated with sand-bed streams, and all species,except A. profuga, were positively associated with large woodydebris loads.

Hurdle models of abundance were different for each speciesbut all models showed that the numbers of all species, exceptH. australis, significantly increased with improving instreamhabitat quality, despite varying widely among sites (Figure 7).

The numbers of A. profuga were much higher in gravel-bedstreams than sand-bed streams (Figure 7(a)). The odds ofdetecting A. profuga and C. novaehollandiae significantly

increased with the presence of boulders at a site (Table 6)and the abundance of C. novaehollandiae was strongly relatedto the presence of boulders (Figure 7(c)). While the probability

of detecting H. australis increased with IBC, abundance of this

Figure 3. Spatial patterns of variation among the three habitat factors, geomorphic complexity, riparian condition and channel sediment deposition/siltation (low scores indicate excessive amounts of sediment stored in the channel).

IMPACT OF CATASTROPHIC CHANNEL CHANGE ON FRESHWATER MUSSELS 23

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Figure 4. Distributions of the four major mussel species found in the Hunter River system.

Table 5. Summary of freshwater mussel distributions by subcatchment

Subcatchment (no. sites) No. sites with mussels No. sites with young Number of species Median CPUE Maximum CPUE IQR

Goulburn (6) 2 1 1 0 4 0–2.3Hunter (13) 3 0 1 0 23 0Rouchel (4) 3 3 2 14.5 25 9–19Glennies (7) 5 1 3 5 169 1–15.5Paterson (7) 7 5 4 255 1323 86–1046Allyn (6) 4 3 4 32 201 7.75–78Williams (8) 8 8 5 382.5 810 148.3–525Wollombi (27) 15 5 4 2 600 0–29.5Upper Congewai (4) 4 3 2 170 549 125.5–296.3Watagan Creek (5) 4 1 1 24 600 4–33Upper Wollombi (5) 3 0 2 10 100 0–25Lower Congewai (6) 4 0 3 1.5 7 0–5.5Lower Wollombi (7) 0 0 0 0 0 0

CPUE5 catch per unit effort (as number per person-hour); IQR5 interquartile range.

Table 4. Catch statistics for freshwater mussels found in the Hunter Valley

Hyridella drapeta Cucumerunio novaehollandiae Alathyria profuga Hyridella australis Hyridella depressa

No. sites with mussels (N5 78) 35 11 37 19 1No. subcatch. with mussels (N5 8) 6 4 8 5 1Mean CPUE per site 80.5 20.0 13.7 6.5 0.06Max. CPUE per site 1300 446 250 203 5Total CPUE 6276 1558 1065 506 5Dispersion index 483.3 298.4 90.8 99.1 5.0

The dispersion index is the variance: mean ratio. All values were significantly greater than 1 indicating aggregated spatial distributions. CPUE: catchper unit effort.

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Figure 5. Mean freshwater mussel abundance (as CPUE) among subcatchments of the Hunter Valley. Bars represent 1 standard error of the mean.

Figure 6. Subcatchment variation in freshwater mussel abundance (log 10 CPUE) with two measures of hydrological stability: (a) annual runof;(b) flash flood magnitude.

Table 6. Catchment and site-related features most closely associated with the probabilities of occurrence for each mussel species as determined fromlogistic regression modelling

Variable Hyridella drapeta Alathyria profuga Cucumerunio novaehollandiae Hyridella australis

LRT (w2) Odds ratio LRT (w2) Odds ratio LRT (w2) Odds ratio LRT (w2) Odds ratio

Catchment Disturbance Index 24.7��� 0.17 0.9ns 1.5ns 1.5ns

River Geomorphic Condition 12.4�� 9.25a 1.1ns 1.5ns 1.5ns

Index of Biophysical Condition 55.5��� 21.89 11.9��� 2.41 13.0��� 4.76 18.3��� 4.33channel form complexity (factor 1) 26.1��� 4.55 8.3��� 2.06 5.6� 2.54 4.4� 1.86riparian condition (factor 2) 14.4��� 3.03 0.4ns 8.4��� 3.63 9.1��� 2.80non-aggraded reaches (factor 3) 7.8�� 2.20 5.8� 1.94 0.1ns 2.6ns

Sand cover 1.1ns 23.6��� 0.29 5.3� 0.41 10.5��� 0.37Boulder presence 4.4� 2.83 7.8�� 4.13 9.7��� 8.50 1.5ns

Large woody debris 37.2��� 6.88 2.7ns 10.3��� 2.83 13.5��� 2.74

Statistical significance was tested by likelihood ratio tests against a null model. The odds ratio is the ratio of the estimated probability of musselsbeing present versus the probability of mussels being absent per unit change in the explanatory variable. The explanatory variables were normalizedbefore analysis so that odds ratios could be compared among variables. Significance codes: �Po0.05; ��Po0.01; ���Po0.001. ns5not significant.aOdds ratio of mussels being present in reaches of ‘moderate’ versus ‘poor’ river geomorphic condition.

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species was unrelated to the IBC index at sites where it wasfound (Figure 7(d)). Abundance of H. australis wassignificantly associated with the proportions of overhanging

riparian vegetation and undercut stream banks.In the Barrington rivers where most species co-occurred,

their microhabitats appeared to be partitioned with regard to

strength of the current. Cucumerunio novaehollandiae andA. profuga often occupied strong current zones, whereasH. drapeta was generally restricted to slackwater habitats close

to stream banks, protected from the main stream flow.Hyridella australis was always found beneath stream banksin still water, often embedded in soft sediment.

Populations of H. drapeta were very patchily distributed

within streams of the Wollombi subcatchment and musselpresence was strongly and inversely associated with theseverity of channel alteration and aggradation (LR test:

w21 ¼ 22:9, Po0.001). No mussels were detected in the lowerreaches of this system where geomorphic alteration was mostsevere. The impact of geomorphic channel alteration on mussel

abundance is shown in Figure 8 with high densities ofH. drapeta only in undisturbed reaches. Several counts werelower than expected and substantial numbers of empty valves,

indicating recent mortality, were observed at some sites inthe Wollombi subcatchment. At one location on WollombiBrook, a bloom of the introduced macrophyte, Salviniamolesta, was an obvious cause of mortality as indicated by

the abundance of empty shells in a deep layer of organic oozeformed by plant decay.

Stream levels were extremely low on Watagan Creek, and at

one location a large number of recently dead mussels strandedby falling stream levels was observed. Deep accumulations ofleaf litter dominated the channel at most sites on this stream

but living mussels were found only in clean sand. Emersionand death of mussels due to low stream levels were also evidentat Rouchel Brook and Davis Creek (Figure 1).

DISCUSSION

Historical changes and their causes

Although few historical data are available to reconstruct the

distribution of mussels in the Hunter River before European

Figure 7. Plots of modelled relationships between numbers of mussels and physical habitat variables: (a) Alathyria profuga and benthic habitatquality; solid circles indicate sand-dominated sites (460% of the bed); (b) Hyridella drapeta, solid circles indicate sites with marginal or poor scoresfor channel alteration or sediment deposition; (c) Cucumerunio novaehollandiae, solid circles indicate sites with boulders; and (d)H. australis and IBC.

Figure 8. Relationship between Hyridella drapeta and the channelalteration index for sites in the Wollombi subcatchment where thisspecies was present. The filled circles represent mussel abundance atsites infested by Salvinia molesta or with extremely low stream levelsdue to drought. The solid line is the predicted abundances fromthe count component of a hurdle regression model of H. drapetaabundance versus the channel alteration index. Broken lines indicate

the 95% confidence interval.

H. A. JONES AND M. BYRNE26

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DOI: 10.1002/aqc

settlement, museum records confirm that A. profuga had awidespread distribution throughout the system and that allspecies were abundant in the Southern Barrington streams

(Australian Museum, 2008). Mussels, most likely A. profuga,were also present in large numbers in the Hunter River nearMuswellbrook and downstream from Singleton (Grant,

1803; Wood, 1972), reaches where they are now rare orabsent (this study). The scarcity of museum records forHyridella spp., other than for the Southern Barrington and

Wollombi streams, implies that these species may neverhave been abundant outside of these subcatchments at thetime of European colonization. Though not detected by thissurvey, a sixth species, Velesunio ambiguus, has been collected

in the Hunter River at Muswellbrook on three previousoccasions, most recently in 2003 (Australian Museum, 2008).This species may still occur in the middle reaches of the

Hunter River.A strong regional pattern in mussel distributions

corresponding to geographic variation in geology and

hydrology was observed in this study, reflecting the influenceof these factors on river morphology and behaviour (Brierleyand Fryirs, 2005). In the hydrologically stable Southern

Barrington rivers, mussel diversity and abundance was high,in concordance with the trend observed in several NorthAmerican catchments (Strayer, 1983; Arbuckle and Downing,2002; McRae et al., 2004). In contrast, other parts of the

system have high flood variability by both Australian andworld standards (Erskine and Saynor, 1996; Erskine andLivingstone, 1999) and had few mussels. This is especially true

in the Wollombi and Goulburn subcatchments wherecatastrophic erosion caused by major floods (Erskine, 1994,1996) had disastrous consequences for mussel populations. On

the lower reaches of Wollombi Brook, large floods(1949–1955) destroyed a shaded, narrow, sinuous channelreplacing it with one twice as wide and filled with sediment.

Fifty years later, these reaches have not been recolonized bymussels although the channel has narrowed and deepened(Erskine, 1996). The present-day channel has poor instreamhabitat and highly mobile bed sediments.

Although large floods have the potential to devastatemussel populations, few studies have documented floodimpact. A major flood on the River Kerry, Scotland,

destroyed an estimated 4–8% of the Margaritiferamargaritifera population (Hastie et al., 2001). Mussel beds inboulder-dominated microhabitats were least affected by the

flood, as was also the case for M. falcata following large floodsin the Salmon River, Idaho (Vannote and Minshall, 1982).Similarly, C. novaehollandiae exhibits a strong affinitywith boulder and gravel-bed streams where they are

often tightly wedged in cobble and gravel between bouldersin fast-flowing sections of the stream. Boulders promotelocalized bed stability, facilitating retention of mussel

populations in flood conditions (Hastie et al., 2001; Vannoteand Minshall, 1982). Large woody debris is critical forproviding bed stability and habitat heterogeneity in

sand-bed streams (Brooks and Brierley, 2002; Erskine andWebb, 2003), thereby contributing important flow refugia formussels during floods. In this respect, large logs perform

a similar function to block-boulders, but in many riversthroughout south-eastern Australia logs have been removed bydesnagging programmes (Reinfelds et al., 1995; Erskine andWebb, 2003).

Characteristics of mussel habitats

The spatial pattern of mussels within sites in the Hunter Riveralso indicated that localized shear stresses, affecting themobility of bed sediments, determined mussel distribution atthis scale (Hardison and Layzer, 2001). Mussels were rarely

detected more than 1m from the toe of the stream bank unlessthey were associated with boulders and large logs. They werenever found on loose gravel bars. In many locations, mussels,

particularly the Hyridella species, were associated with low,overhanging riparian vegetation in slackwater habitats andbeneath undercut banks.

The microhabitats of the species in the Hunter River systemare similar to observations made in nearby river systems(Jones, 1983; Brainwood et al., 2008a). The affinities of the

Hyridella species for protected pockets along stream marginshave implications for the rehabilitation of instream habitats.Placement of engineered log jams along the bank can createslackwater habitat and stabilize bed sediments, while riparian

plantings of trees and shrubs that overhang low over thewater (e.g. Callistemon viminalis and Tristaniopsis laurina) willfurther enhance mussel habitat. Large boulders placed in the

stream, especially in moderate–fast flowing water, shouldprovide habitat for C. novaehollandiae and A. profuga.

Current status and threats

There is a strong link between declines in mussel communities

and the intensity of catchment disturbance in the HunterRiver, as seen in the nearby Hawkesbury–Nepean River andriver systems in the northern hemisphere (Young et al., 2001;Poole and Downing, 2004; Brainwood et al., 2006). In

particular, river metamorphosis and the construction ofdams and weirs have fragmented mussel habitats throughoutmuch of the Hunter system. Some of the most severe habitat

degradation occurs along the main stem of the Hunter Riverand the lower reaches of its tributary streams, isolatingresident mussel populations. Dams are a major factor

contributing to the demise and extinction of freshwatermussels and have overwhelmingly negative impacts onmussel communities (Bogan, 1993; Vaughn and Taylor, 1999;Watters, 2000). Dams modify downstream flow regimes and

sediment processes resulting in channel adjustments thatdisrupt freshwater biota (Graf, 2006). In the Hunter River,construction of the Glennies Creek Dam has caused reductions

in the magnitude of stream flows and a contraction in the sizeof the channel immediately below the dam. Deposition of finesediment (ca 0.5m depth) and growth of a dense reed bed

across the channel has all but extirpated the mussel populationin the reach below the dam (H. Jones, pers. obs.).

Even without further habitat deterioration, marginal

populations of mussels, in the Hunter River and elsewhere,may continue to decline because of failure to complete theirlife cycle (Downing et al., 1993; Byrne, 1998) and lack ofrefugia during periods of drought and flood (Lake, 2003). The

protracted 2006 drought affected much of south-easternAustralia (BOM, 2007) and in the Hunter River system driedseveral streams resulting in mussel mortality (Jones pers. obs.).

While emersion was an obvious contributor to musselmortality, accumulation of particulate organic matter (POM)might also have influenced mussel survival due to build-up of

toxic leachates and anoxia caused by leaf decay (Lake, 2003).

IMPACT OF CATASTROPHIC CHANNEL CHANGE ON FRESHWATER MUSSELS 27

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DOI: 10.1002/aqc

In the present survey, living mussels did not occur in poolswith large amounts of POM. Climate models predict warmingand drying of south-eastern Australia accompanied by an

increase in droughts (CSIRO, 2007). Under this scenario,mussel populations would be at risk of extirpation across theregion, due to emersion and lack of refugia and shade to

ameliorate the effects of drought on water quality (Lake, 2003;Golladay et al., 2004). In addition to drought, the invasivemacrophyte, Salvinia molesta, caused significant mortality

throughout Wollombi Brook, probably due to anoxia andtoxicity caused by the high organic load. This new stressincreases the vulnerability of small, fragmented mussel popu-lations to further declines.

The Williams and the neighbouring Paterson River supportthe most extensive mussel populations in the region across allspecies, including virtually all the C. novaehollandiae present

in the Hunter drainage. This species is at its southern limitin these rivers. Coarse estimates indicate that 470% of theregional populations of the other species are also located in

these rivers. However, stream-bank erosion and sedimentationis severe along much of the Paterson River, threatening thelong-term viability of mussel populations. In contrast, the

riparian and instream habitat condition of the Williams Riveris the highest in the Hunter catchment, and threats to musselpopulations are less intense. Hence mussel populations in theSouthern Barrington streams, especially the Williams River,

are clearly of high conservation value according to IUCNcriteria (IUCN, 2003).

A potential threat to mussel populations in the Hunter

Valley is the plan for a large, 450 GL capacity, dam on theWilliams River and proposals to increase the capacity ofexisting dams (Hunter Water, 2008). The mussel communities

identified here as having the greatest conservation value aredownstream of the proposed dam site on the Williams Riverand this was the only reach where C. novaehollandiae was

found. Dam construction will undoubtedly affect thesurvivorship of these diverse mussel communities.

Conservation and stream restoration

Addressing the processes and stressors underlying the declinein mussel populations will benefit a wide range of resident

biota. Freshwater mussels have been shown to serve as usefulsurrogates of lotic biodiversity in the UK (Aldridge et al.,2007) and the positive association of mussel populations with

the IBC observed in this study lends support to this finding.The dependence of mussels on fish to complete their life cyclealso implies that healthy mussel populations indicate healthy

fish communities. Indeed, the Williams River, which supportsa diverse and abundant mussel community, is rated as a ‘highconservation value’ subcatchment in recognition of its high fish

and macroinvertebrate diversity (Chessman et al., 1995;DLWC, 1999).

Although measures to maintain and restore aquatichabitats have been implemented in an effort to improve local

biodiversity (HCRCMA, 2007), without additional attentionto ecological processes these actions may not be effective(Bond and Lake, 2003). This is especially relevant for

freshwater mussels, which have specialized life historyrequirements and restricted dispersal ability. Mussels maynot be able to recolonize restored sites if the dispersal distances

between source populations and these sites are too large.

Furthermore, landscape connectivity is partly determined bythe behaviour of a mussel’s fish-hosts and there is a dearth ofinformation regarding fish-host preferences and host

movement patterns for coastal mussel species.Despite limited empirical data on ecological processes,

especially in a landscape context, there is still much that can be

done to facilitate conservation of regional mussel populations.There are several places where mussels still occur in abundancein the Hunter River system and the presence of young mussels

indicates successful recruitment. It is imperative that theseremnant populations be managed to prevent further declinessince the time-scales required for mussels to recolonize restoredhabitats could be decades or even centuries given the highly

fragmented nature of these remnant populations. Long-termconservation goals will be advanced if site-based conservationactions are combined with strategies to repair riparian

vegetation and reconnect aquatic habitats at reach andcatchment scales (Lake et al., 2007). Most remnantpopulations are in streams draining small subcatchments

with low-impact agricultural land use (e.g. upper CongewaiCreek), which will increase the feasibility and likelihood ofsucccess of restoration efforts. In addition to countering

habitat fragmentation, large-scale restoration efforts shouldprovide long-term assistance in buffering mussel populationsfrom the extremes of climate as improved in-stream habitatshould provide drought and flood refugia, and riparian

vegetation will increase the boundary resistance of streamchannels, reducing the erosive impacts of large floods.Currently, no freshwater mussel species is afforded statutory

protection in NSW and river managers are largely unaware ofthis invertebrate group. Formal recognition of regionallythreatened populations of mussels would do much to focus

restoration and conservation.

ACKNOWLEDGEMENTS

Thanks to Tom Prowse, Sergio Barbosa and Sarah Mika for

assistance in the field. David Outhet, Nick Cooke and BruceChessman provided helpful advice and information. We areindebted to the many landowners who granted us access to

their properties or provided accommodation, especially Steveand Fran Crane, Warren Gorton, Max Smith, MargaretAbbott and Lyn Tunin. This research was funded by a NSW

Environmental Trust Grant No. 2004/RD/0077.

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