Catfish impact on crab population to be studied

Blue catfish are getting renewed attention and unprecedented interagency cooperation by scientists in charge of monitoring it.

The Virginia Department of Game and Inland Fisheries (VDGIF) introduced the Mississippi native into the three primary rivers of the Commonwealth back in the early 80's as a sport fishing delight. The state record for one caught since then is 95 pounds.

Scientists admit that it has proliferated wildly and that it will eat virtually anything, including mud.

"It is extremely abundant," said Bob Greenley, VDGIF marine scientist. "They will eat anything. Anybody that works the tidal rivers and sees the abundance of catfish has concerns."

Greenley was one of the principal scientists, along with Rob Latour and Rom Lipcius of the Virginia Institute of Marine Science (VIMS) and at what Greenley called "the first formal discussion" of the catfish's impact on the crab population on May 20 at the Virginia Department of Marine Resources (VMRC) in Newport News.

Also present at the meeting was Warsaw waterman and president of the Twin Rivers Waterman's Association, Doug Jenkins. He has long been proclaiming the rampant predation of the catfish on the crab population in what scientists call the overlap areas.

Those are areas of the three Virginia rivers, the James, Rappahannock and York, and the Potomac River where the freshwater catfish population, which can tolerate salinity levels of up to about 12 parts per million, overlaps with the saltwater crab population.

Crabs migrate farther up the rivers in drier weather, which increases the salinity farther up river, and the catfish migrate farther down river in wetter weather, which has the reverse effect.

"The science that has been done up to this point shows that the overlap area is growing," said Greenley. "Crabs are moving up the rivers."

Jenkins has lived and worked more than 50 years in an overlap area around Warsaw. He works both the Potomac and the Rappahannock Rivers at their tidal flow delineation, or where the saltwater meets the freshwater.

He said that he has watched carefully as catfish have increased dramatically in number and is certain they play a role in the declining crab population. The overall decline as documented by VMRC surveys has led to many new restrictions on crabbers this past spring. In Jenkins mind the scientists have not acknowledged the impact of the catfish until now.

"They have been in denial of the catfish", said Jenkins referring to area fish scientists.

That is changing according to Greenley, Latour and VMRC spokesperson John Bull. Latour is an associate professor of marine science and he conducted a study of the eating habits of the catfish from 2004-06.

While he said that there were "some holes in the study" because it was a relatively limited study that only examined smaller catfish, the basic conclusion was that blue crabs were less than 10 percent of the diet by weight of the 700 sample catfish. Contents of the stomachs of catfish from all three rivers were examined.

Latour did say that if there was a huge increase in the amount of catfish, it could be a significant impact on crabs.

"Ten percent could turn into a large amount of crabs," said Latour. "It is a concern. The abundance of blue catfish has certainly increased substantially. To really understand the impact of the catfish on the crabs we need to do a comprehensive population level predation impact study."

He explained that such a study would last two to three years and collect and analyze a wide range of data about both species.

"We are beginning to discuss formally analyzing and structuring the blue catfish data," said Latour.

He was referring to data already collected in his study and numbers collected by Lipcius and other VIMS researchers in a monthly trawl survey that has been conducted regularly for five decades. The survey, which drags a net across the bottoms of sections of all three rivers, estimates the blue catfish population to be in the tens of millions

Bull confirmed that VIMS personnel are reviewing their information and that more research needs to be done. And for the first time VDGIF and VIMS are going to share data, according to Latour and Greenley

"We want to know if there is a problem out there," said Bull. "We do not know the extent of the blue catfish's proliferation and need VIMS to take a good close look at its impact on the blue crab population. They are looking at the information they already have with a fresh eye. This is an issue that does need to be examined and I salute Doug Jenkins for bringing it to our attention."

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Catfish impact on crab population to be studied