Page 19 - The Freshwater Mussels of Oklahoma

process itself may destroy mussel beds and once it is done, it alters the flow patterns and
sedimentation processes.
Watercraft: The passage of boats, barges and ships along rivers creates wave
action along the banks that is an unnatural phenomenon for flowing systems. This wave
action increases bank erosion, alters substrate composition and prevents establishment of
stabilizing vegetation. All of these limit the mussel composition to only a few relatively
adaptable species.
Headcuts: An interesting phenomenon occurs when the substrate in a river or
stream is disturbed. Disturbance is commonly caused by removal of sand, gravel or
cobble. Once substrate is removed, the current overflowing the upstream edge of the
disturbance begins to undercut the site and the undercutting process begins to move
upstream. As the process moves upstream, it can disturb and possibly bury mussel beds
and it also tends to spread laterally. Repeated headcut events can gradually alter a deep,
well-channelized stream or river into a wide, shallow, unstable system that is unfavorable
for all but a few mussel species.
Scouring: Any factor that increases flow in a stream or river can have either one
of two seemingly opposite results. If increased runoff increases the input of sediments
into the system such as in disturbed agricultural lands it can result in a “choking” of the
stream, thus burying mussels and creating a high sediment-load system. On the other
hand, if increased runoff is not associated with the inflow of sediments, then the result is
to move sediment substrates already in the river downstream, often resulting in exposure
of cobble, boulder or bedrock unsuitable for mussel habitat.
Predators: Mussels provide an important food source for a variety of animals
including muskrats, raccoons and otters. Under normal circumstances these are not
expected to have a significant impact on mussel populations. However, when populations
are reduced or imperiled then predation must be considered a possible detriment.
Commercial Harvest: The massive harvest of mussels can obviously have a
great impact on the species of commercial value. Because most of the commercial
species are common and widespread, their populations tend to recover with time.
However, the association of rare, uncommon or endemic species in the mussel beds
associated with the more common and abundant commercial species, they are subject to
the disturbance of the mussel bed substrate. Also, misidentification and harvest of rare or
protected species by harvesters is also a distinct possibility.
Competition: Since about 1970, Oklahoma has seen the invasion of two exotic
species, the Asian clam (Corbicula fluminea) and the zebra mussel (Dreissena
polymorpha). Both species can attain huge populations that because they are also filer
feeders, can greatly reduce the nutrients available to native species. Reduced nutrients
obviously can affect native species’ ability to survive, grow and, possibly most
importantly, to reproduce. Furthermore, the zebra mussel attaches to any hard substrate
in the water, often in tremendously dense numbers. This includes the exposed shells of
living mussels. This colonization of the living mussel shells has a heavy impact on native
mussels.
Isolation: In recent years it has become clear that fragmentation of ecosystems
results in loss of diversity. Fragmentation creates isolated “islands” of habitat that quickly

18