Pittsburgh Post-Gazette
23 August 2006
By Deborah Weisberg
They're not the jumbo kind listed on restaurant menus, but a smaller cousin known as grass shrimp. They were found by scientists trawling the bottom of a 40-mile stretch of the Mon from Fayette City to the West Virginia line in recent weeks.
"We're really scratching our heads about it," said David Argent, a California University of Pennsylvania biology professor who captured the tiny, translucent crustaceans at Point Marion and at the mouth of Little Redstone Creek in Fayette City. "But wow, it's really neat ... the last thing we expected to find."
The discovery is significant because the species, known as Palaemonetes kadiakensis, is native to the Mississippi River basin -- of which the Mon is a part -- and an indication of good water quality, Dr. Argent said. More than half of the 11 shrimp he caught carried eggs.
Benthic or bottom trawling has never been performed before on the Mon, so how long the shrimp have been there is anyone's guess, but Dr. Argent said capturing such a small number suggests they are relative newcomers, and only beginning to expand their range in the Mon's improving water. He said he has ruled out that the shrimp were planted, like the tilapia that surfaced in Braddock this month.
"I showed them to my colleague Bill Kimmel, and we think they're the real deal," Dr. Argent said. "They've probably never been seen this far north before, although they've been in the drainage for a hundred million years."
Something similar occurred in Lake Erie, said Ed Masteller, a biologist specializing in benthic communities and a professor emeritus at Penn State University's Behrend College.
"I found a few grass shrimp in Erie when I came here in 1967, then I didn't see them for years until they started showing up again in large numbers, kind of like mayflies, when the habitat improved enough for them to reproduce," said Dr. Masteller, who had even considered trying to raise grass shrimp in Erie, as an academic experiment, several years ago. "It was exciting to me, because they seem to be indicators."
Dr. Argent found the Mon River shrimp in eelgrass, a plant with one-inch blades seen undulating in the water, and native to the Mon but seldom found in the Allegheny and Ohio. "We also got a lot of little bluegills, rock bass, smallmouths and spotted bass in the same eelgrass, so it appears it's a kind of nursery area," he said.
Tucked up underneath their legs, six of the shrimp had eggs, which on their two-inch long, nearly transparent bodies appear as tiny green dots.
Dr. Argent said he's trying to figure out how the shrimp got to Pennsylvania. They could have "hitchhiked" in the ballast water of a boat that had been to the Gulf of Mexico or some other coastal area where shrimp are more at home, or they might have alternately swum and drifted up on their own. The Mon flows north through West Virginia. Shrimp power themselves with their tails and have big eyes and long antennae that help them forage, mostly for plankton and other microscopic matter. Their translucence helps them elude predators.
Although grass shrimp are more common to brackish or estuarial water, Dr. Argent said "the temperatures and habitat on the Mon are suitable enough for them. They like a silty habitat, which the Mon certainly has. You can stand on Mount Washington and see sediment plumes where the Mon mixes with the Ohio."
As bottom dwelling filter feeders, grass shrimp would survive over the long term only by being able to withstand the heavy metals and other toxins embedded from the region's industrial past. "The only way to really know how much pollution they're absorbing is to mash them up and study their tissue, and I have no interest in doing that," said Dr. Argent, who has the shrimp, pickled, in a jar in his lab. "They wouldn't be attempting to breed if conditions in the river were that poor, chemically or otherwise."
Though they are edible, it would take a thousand of the tiny shrimp to make a meal, he said.
Equally encouraging are the other species Dr. Argent's trawling operation yielded, including a big bounty of Johnny darters and a significant number of channel darters and silver chubs, which are both state-threatened species.
"Out of the 3,000 fish we caught, 1,000 were Johnny darters, and 454 were channel darters, which are indicative of good water quality," Dr. Argent said. "We wouldn't have expected to get many Johnny darters 20 feet down in the river, because they're more of a riffle or shallow pool species, but then again, we've never explored with benthic trawling before."
The only disappointment was the absence of larger fish, although Dr. Argent said trawling at night might have produced more. "It could be that the bigger ones saw the trawl coming and got out of the way," he said.
He expects more surprises when he bottom-trawls again this fall, enlisting some of his students in the process. Other benthic studies are slated for the Allegheny and Ohio rivers, since the Pennsylvania Fish and Boat Commission wants to gauge the impact of sand and gravel dredging on those rivers. Dredging is a 100-year-old practice that has come under closer environmental scrutiny in recent years.
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