Aquatic life flourishes in Cahaba with dam gone

Aquatic life flourishes in Cahaba with dam gone
Tuesday, June 24, 2008
KATHERINE BOUMA
News staff writer
Alabama's first dam removal to save aquatic life has succeeded at restoring populations of various species, scientists say.
Once a year, biologists sit on the shoals of the Cahaba River 40 miles south of Birmingham, peering through masks or plastic-bottomed buckets to count every snail or mussel within an assigned plot. A concrete dam once used as a bridge by trucks was removed in October 2004.
Scientists saw changes immediately, but this year's survey, which will conclude Thursday, has been especially eventful.
"Where there were virtually no snails, now there are thousands," said Paul Freeman, a freshwater ecologist for The Nature Conservancy of Alabama who is coordinating the survey effort.
The Cahaba is one of the richest rivers on the continent for snails, mussels and fish.
Scientists say the rivers of Alabama are rivaled only by the Amazon basin or Asia's Mekong Delta in the number and diversity of animals living in them.
But dams constructed up and down most of the state's large rivers in the last century killed millions of snails and mussels.
In the 1960s, a coal company built a six-foot-high concrete dam across the Cahaba near the Bibb-Shelby county line. Fish could not move upstream and a deep upstream pool made the area unsuitable for mussels and snails.
The dam separated the lower 100 miles of the Cahaba from the 40-mile section that flows south from Birmingham.
"The dam itself, the bridge itself, was certainly an impediment to fish movement and migration," Freeman said. "It disconnected the river and impeded or prevented the fish from getting to their historic spawning grounds, feeding grounds, upstream."
Many of the moving fish carry with them young mussels, which spend the first weeks of their life as parasites on fish, much like ticks on a dog.
The mussels put out lures designed to look like the target fish's favorite food. When the fish bites on the lure, eggs erupt and cling to the fish. They drop off later, allowing the mussel to distribute itself around the river system.
"All the native freshwater mussels in the river are dependent on fish to complete their life cycle," Freeman said.
But if the mussels are trapped in a reservoir too deep for the fish to see them or if the fish cannot get to them, the chain is broken.
Scientists surveying the former dam area, including state, federal and other nonprofit scientists, are not counting fish. But they have found a huge jump in mussels of all sorts.
They set an orange metal square in the river and then count every live mollusk inside. When the count ends this week, they will know how many of every type of snail and mussel now inhabits the former dam area.
Since the dam removal, the biologists have found seven federally protected species of snails as well as more common species.
That is important, because it means the void isn't simply being filled by opportunistic or invasive species. Instead, animals seem to be filling in at rates that would be normal in other healthy sections of the river.
Scientists in this year's count found as many as 2,000 snails in a square meter, in areas where there had been none at worst or 100 at best.
"We restored the river to its natural depth and velocity," Freeman said, "which is prime habitat for many of the rarest animals in the Cahaba."
More than 131 species of fish and more than 75 species of freshwater mussels and snails have been observed in the Cahaba, including five fish and 11 mollusk species protected under the federal Endangered Species Act.
E-mail: kbouma@bhamnews.com

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