Science / Uncategorized

Clean Water Superstar #2: Threats to native mussels

Every month, the Clean Water Initiative Program will be highlighting a Clean Water Superstar. These are species that help keep our state’s water resources swimmable, drinkable, and fishable. This is a follow-up to the first blog post about native mussels, January’s Clean Water Superstars. Throughout the month, there will be information, resources, pictures, and videos about these superstar animals posted on the Clean Water Vermont Facebook page.

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An eastern pearlshell in its typical coarse gravel habitat. Photo credit: Steve Fiske, Department of Environmental Conservation

Freshwater mussels are some of the longest-lived invertebrates in the world. The eastern pearlshell – a freshwater mussel native to Vermont – tops the longevity list, frequently living to the ripe old age of 75 and sometimes reaching 150. Eastern pearlshells live in rivers and streams, as do most rare native Vermont mussels, although the eastern pearlshell requires cooler waters. Despite the thousands of miles of cold streams and rivers in Vermont, the eastern pearlshell is listed as threatened and its populations are quite limited. But they need more than just cold water to thrive. Mussel populations are tied to the health of the river system in which they live, and their presence can indicate a pristine watershed. So what is a healthy river? There are as many answers to this question – unpolluted, stable, resilient, unobstructed by culverts or dams – as there are threats to these qualities that are critical to the eastern pearlshell’s survival.

As discussed in the previous blog about native mussels, unpolluted water is necessary for all native mussels, including the eastern pearlshell. Nutrient pollution often comes from agriculture or stormwater runoff. Sediment pollution can be caused by erosion and instability, especially in Vermont, where tall, steep stream banks of exposed dirt are common. These steeply cut banks are exacerbated when the river cannot access its floodplains or is artificially confined to a certain path without an outlet. As the river erodes its channel, sediment is washed away in some places and deposited in others, leaving eastern pearlshells on unstable footing. Mussels have limited mobility and are at risk of being washed away or choked by sediment when the ability of the river to be naturally resilient has been hampered and the impacts of flooding are worsened.

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An eastern pearlshell mussel, with its foot extended, struggling in newly deposited sand after flooding near Marshfield, VT in 2011. Photo credit: Steve Fiske, Department of Environmental Conservation

Obstruction by dams further limits eastern pearlshell populations. All freshwater mussels need specific species of fish to host their glochydia – immature mussels – which attach to the gills of host fish. Atlantic salmon and trout are the hosts of choice for the eastern pearlshell. Where the river is obstructed by a dam or culvert that is impassable to fish, mussels will be absent from the blocked river system. Additionally, if a host fish is endangered, the mussel population will be as well, so mussels are dependent on healthy river conditions for their host species as much as for their own health.

As Mike Kline and Kim Greenwood wrote in the June 2013 Vermont Natural Resources Council publication Reading Vermont’s Rivers¸ “Rivers and their movement become easier to understand once we realize that the shape of the river is the result of a dynamic yet delicate balance between erosion and deposition; between volume and power of water and the size and quantity of sediment resisting the water’s power. If one element of the system changes, then others change in response.” The eastern pearlshell is just one of 13 mussel species listed as threatened or endangered in Vermont. Hope for these rare Clean Water Superstars lies in cultivating natural river processes to keep rivers unpolluted and resilient.

For more information, visit the Clean Water Vermont Facebook page, and stay tuned for the next blog post about aerobic wastewater bacteria.