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Wednesday, 29 August 2007 00:00

The perilous world of mussels

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Decades ago, fresh water mussels littered the river bottoms of Western North Carolina.

River otter and muskrats ate them, Cherokee harvested them and early settlers apparently cussed them.

“Most of the old-timers will talk about the river bottoms being paved with mussels,” said John Fridell, a mussel guru with the U.S. Fish and Wildlife Service. The mussels were nicknamed “heel-splitters” for their sharp shells and the risk they posed to barefoot waders.

Today, several species have been wiped out completely — the exact number isn’t known. Others are barely hanging on. One of those is the Appalachian elktoe mussel, an endangered species holed up in just a few remaining enclaves.

“They are fragments of what used to be large interconnected populations,” Fridell said. “You just find them in little pockets.”

Of the seven rivers where these pockets exist, three are right here at home: the Tuckasegee, the Little Tennessee and the Pigeon River.

Mussels aren’t exactly a hardy bunch. The biggest threat today is sediment. Fine silt in the water easily clogs their sieve-like gills. They are constantly filtering water through their bodies, straining out edible morsels, extracting oxygen, and during mating season, absorbing water-borne sperm. Any silt in the water also gets sucked in through their gills, and over time, smothers the mussels, Fridell said.

Anything that’s in the water for that matter passes through the mussels’ body. They are extremely sensitive to pollution, making them an “indicator species” of something gone wrong with the water. Fertilizer from golf courses, pesticides from cornfields and oil from parking lots are also bad news. For decades, the paper plant in Jackson County dumped chemicals into the Tuckasegee River, and could be a major factor behind the crash in mussel species on that river, Fridell said.

While silt can slowly asphyxiate them and chemical pollution slowly poison them, there’s a quicker and deadlier enemy on the horizon for mussels: large drifts of sediment. Mussels — not very mobile in the first place — are even less so when partially burrowed into the gravel river bottom. Half-buried, they crack their shell, revealing a small siphon used to suck water into their gills. Just a quarter inch of sediment settling on the bottom can choke their siphons.

It’s enough to make Fridell and fellow mussel fans cringe during heavy rains when mud pours off construction sites, turning the creeks and rivers red with sediment plumes.

When sediment accumulates in drifts on the river bottoms, it poses a different type of risk to baby mussels. Baby mussels hitch-hike on fish, falling off when they reach a certain size.

“if they land in silt, they sink into it and suffocate,” Fridell said.

A final peril for the mussels are dams. Dams block the path of the mussels and cut off their ability to migrate up and downstream. It seems odd, since mussels themselves are far from mobile, moving literally at a snail’s pace. But mussels do get around as infants. The eggs latch onto fish, developing into larvae in the fish’s gills before the fish shakes them off. Wherever the baby mussels get shook off becomes their new home. (see article on mussel life cycle)

When dams block fish from moving up and downstream, they in turn block the distribution of the mussels. Mussels can end up stuck in between two dams, unable to co-mingle and breed with neighboring colonies, Fridell said. That, in turn, weakens their gene pool.

“Fragmenting the populations, isolating them from one another, made them more susceptible to environmental factors,” Fridell said.

There’s one more negative side-effect humans have imposed on mussels — we’ve cut down their food supply. While mussels are river bound, they rely on trees and bushes on land as their primary food source. Leaves fall into the rivers and decompose. Mussels eat this decomposing organic matter in the water. Large stretches of WNC’s river valleys have been denuded of trees, both for farming and development, leading to less food for the mussels.

“A big problem is the loss of woodlands around the river,” Fridell said. “There’s a lot of animals in the stream that eat that breaking down plant matter.”

— Becky Johnson

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