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Scientists Use Genetic Testing to Analyze Diet of Monster Blue Catfish

In a laboratory at the Smithsonian Environmental Research Center in Edgewater, Maryland, zoologist Rob Aguilar examines bottles containing preserved specimens of an astonishing array of different varieties of aquatic life.

“We have speckled swimming crabs, long finned squid, jackknife clam, ponderous arc,” said Aguilar, scrutinizing a thick mussel with a serrated shell.  “This is a fish-gill isopod.  And this is a big marine leach that prefers to be on skates and rays.”

Aguilar is engaged in a project to study the genetic codes of numerous species in the Chesapeake Bay. He and colleagues record them in public databases called GenBank and the Barcode of Life Database, so that researchers around the world can use the information to identify fish and other critters.

  “Just like, you know, your item in the store has a barcode and you scan it and each one is unique for that item?” Aguilar asked. “Each genetic sequence is unique to that species.”

Why is this genetic barcoding useful?  Well, for one thing, it helps researchers like Aguilar identify what different fish are eating – and therefore, what impact they are having on the balance of life around them.  Examining the contents of the stomachs of fish is important to biologists. But often the remains there are just mush. That makes the genetic analysis of the material critical.

Recently, Aguilar and colleagues have been using this technique to try to discover the ecological impact of an invasive species in the bay and its tributaries: blue catfish.  Blue cats are the largest catfish species in North America, growing up to five feet long and 150 pounds.  They are native to the Mississippi River basin, and were introduced intentionally by the state of Virginia in the 1970s and 1980s as a game fish for sportsmen in the James, York and Rappahannock rivers.

The problem is, blue cats are voracious predators that are several times larger than native catfish.   They have spread to several other rivers, including the Potomac in the  1990s – and are multiplying rapidly.   Some biologists worry that blue cats might gobble up and outcompete threatened native fish species that they are working to restore, such as American shad.

So what Aguilar and his fellow researchers did for a recent scientific journal article titled, “Gutsy Genetics,” published in Environmental Biology of Fish, was examine the contents of the stomachs of 600 catfish that were caught and dissected.  Aguilar found that while the remains of American shad were rare, other important native species of fish were much more common – including white perch and menhaden. This suggests the blue cats could be having a negative impact on the bay ecosystem.

Also interesting was the fact the bellies of blue catfish were often filled with baby catfish. These included channel catfish, another exotic species introduced from the Great Lakes to Maryland rivers as a sportfish in 1889.  And the blue cats were also eating a lot of their own young.

“A fair number of species of fish and other invertebrates are cannibalistic,” Aguilar said.  “IIt happens in nature. And that’s why a fair number of these species have as many young as they do, to sort of counter-act the effects of cannibalism and predation in general.”

Wildlife managers can’t rely on cannibalism to these invaders in check, simply because blue cats have so many offspring.   But another kind of predation – by humans -- could keep their numbers from overwhelming native species. 

The market for blue catfish in local restaurants and seafood markets is growing, as these monsters are not only voracious but – fortunately—delicious.  

Tom Pelton, a national award-winning environmental journalist, has hosted "The Environment in Focus" since 2007. He also works as director of communications for the Environmental Integrity Project, a non-profit organization dedicated to holding polluters and governments accountable to protect public health. From 1997 until 2008, he was a journalist for The Baltimore Sun, where he was twice named one of the best environmental reporters in America by the Society of Environmental Journalists.