Threats to freshwater mussels
Mussels have been affected by changes to the Australian landscape since European settlement. Their numbers have declined in some NSW coastal rivers, including sections of the Hawkesbury-Nepean and Hastings River systems. Mussels have disappeared from some streams in the Hunter Valley and the south coast of NSW. Our understanding of the status of mussel populations in many streams is limited because very few surveys have been done.
Mussel populations have become fragmented, and remnant populations are often small and isolated from one another, as in the Hunter Valley. Mussels depend on fish for dispersal between habitats, but in many streams fish movements are restricted by dams, weirs or long stretches of poor habitat. Small, isolated populations have a higher risk of extinction than unfragmented populations.
Habitat degradation is caused by:
dams and weirs, which are also barriers to fish movement
high rates of stream erosion and sedimentation
invasive aquatic weeds.
The Hunter River at Elderslie has little streambank vegetation remaining to provide shade and protect the streambanks from erosion. The channel is overwidened and the aquatic habitat has been overwhelmed by a massive slug of sand.
Photo: Hugh Jones, OEH
Habitat loss and fragmentation
Widespread removal of catchment and riparian vegetation has accelerated rates of stream erosion. Many rivers are now much wider and shallower than they were at the time of European settlement. Most of the eroded sediment has been stored in the middle to lower reaches of coastal streams simplifying aquatic habitats and producing shallow channels with highly mobile bed sediments. Removal of snags from rivers has compounded the problem.
Commercial sand and gravel extraction has also destabilised some streams by causing ‘head cutting’, a progressive upstream erosion of the stream bed which is later followed by streambank erosion and subsequent widening of the channel. For example, in the lower Wilson River, a tributary of the Hastings River on the mid-north coast, gravel extraction has degraded aquatic habitats for many kilometres upstream of the extraction point. Mussels have almost completely disappeared from the affected reaches.
Siltation of streams is associated with runoff from urban and agricultural lands. Roads, stock access tracks, cultivation and overgrazing contribute to siltation of the stream bed. Fine sediment can blanket the stream bed and suffocate adult mussels and interfere with their feeding. Juvenile mussels live buried in the streambed for the first year or two of life and are sensitive to changes in sediment conditions. Fine sediment clogs the spaces between streambed particles, creating an environment which is low in dissolved oxygen. Deep layers of silt accumulate in the impoundments formed behind dams and weirs creating an environment that is hostile to mussels. Large dams also reduce the frequency of spates and floods that would otherwise flush fine sediments and nutrients from the lower reaches of streams.
Chemical and organic pollution
Runoff from agricultural lands and urban areas are also responsible for high nutrient and organic matter loads in streams. The sediments in affected reaches are frequently anoxic (lacking oxygen) or may contain toxic compounds (e.g. ammonia and sulfides). Adult mussels often thrive in eutrophic (excessive nutrient) waters but juveniles struggle to survive in organic sediments.
The aquatic weed, Salvinia molesta, is a threat to remnant mussel populations in some coastal streams like Wollombi Brook.
Photo: Hugh Jones, OEH.
Invasive aquatic weeds are a serious threat to remnant mussel populations in some coastal streams. The main culprits are salvinia (Salvinia molesta) and water hyacinth (Eichhornia crassipes) which deposit large quantities of decaying organic matter on the stream bed. This creates anoxic conditions and the fine particulate matter clogs the gills of mussels and kills them.
Many of the mussel populations which are threatened by salvinia are already fragmented. Increased mortality caused by invasive species could accelerate the loss of mussels from river systems.
Barriers to fish movement
There are few fish hosts in degraded habitats or restricted fish movement between habitat patches in fragmented river systems. This could be detrimental to mussel populations.
Climate change is likely to interact with other pressures and magnify the impact of other threats on mussel communities. The impact of climate change is still poorly understood, but it is likely that there will be shifts in the hydrological and thermal regimes of rivers. Because stream habitats are fragmented, and mussels have poor ability to disperse, climate change is likely to hinder migration.
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Page last updated: 03 February 2014