Study: Small fish don't like dug-out river bottom

Fewer of them live in Allegheny where aggregate is gone

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Darters are small, minnow-like fish that like to live among the rocks and sand on the Allegheny River bottom, but don't much like it when that bottom aggregate gets dug out by commercial dredging operations.

Several endangered and threatened darter species -- the Tippecanoe, bluebreast, gilt and longhead -- like those deeper, mud-bottomed stretches of river even less.

Those are the main findings of a Penn State University study, recently published in the journal Freshwater Biology, showing commercial sand and gravel dredging so alters the aquatic habitat that fewer small fish and fewer species of fish are found in dredged areas of the Allegheny River. The small fish are an important link in the food chain for larger sport fish like bass and walleye.

"We attempted to document the long-term effects of dredging and what we found was those deeper areas are not ideal habitat for lots of fish species, even though our focus was on the darters. Dredging is definitely having an effect," said Jonathan Freedman, who received a doctorate in wildlife and fisheries science in 2010 and now is a researcher at the Illinois Natural History Survey.

The river study, done from 2007 through 2009, compared dredged and undredged areas in Allegheny River navigation pools 7 and 8 near Kittanning and Templeton, between 45 and 62 miles upriver from Pittsburgh. Dredging is no longer done in those pools.

The study found that dredged sites were on average more than twice as deep as undredged sites, and that total fish catch, species richness and diversity were all significantly reduced in the deeper dredged sections of the river.

The river research was done using what was then an innovative electrified benthic trawl device designed to improve and make more accurate the fish sampling research done in deeper rivers. The combination of the bottom-dragged net and an electrical component that temporarily stuns the fish, allowed them to collect more than twice as many fish as were captured by previously used sampling methods, Mr. Freedman said.

"If you want to preserve a lot of darter species, some of which are state listed, you have to preserve more natural river bottom habitat," he said. "There needs to be strong environmental considerations given to preserve those species."

According to the study, fish habitat is lost as gravel and rocks are removed by dredging, resulting in deeper water and more turbidity that reduces light penetration and algae and plant food sources for the small fish. Sedimentation in the deeper dredged holes can also make those areas unsuitable for species reproduction. As a result, many of the native, "flowing water species" are displaced by still water invasives.

John Arway, executive director of the state Fish and Boat Commission and an adviser on the Freedman study when he was a commission fish biologist, said its findings are consistent with earlier studies and provide more support for the need to do full fish sampling surveys before new commercial dredging permit areas are approved.

He added that the electrified trawl device used by the Freedman study is now the preferred sampling method in the region's rivers.

"The study showed," Mr. Arway said, "that our sensitive aquatic resources are associated with some of the same high-quality sand and gravel areas the dredgers have depended on."

region - environment

Don Hopey: dhopey@post-gazette.com or 412-263-1983.


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