The Chesapeake Bay region may be home to roughly 100 million hungry blue catfish — with plenty of room for their numbers to expand, according to a recent estimate.
If the estimate is correct — and there are plenty of caveats — that would mean the Bay has roughly one blue catfish for every three blue crabs, though the average catfish is many times larger than the average crab.
Only the James, Rappahannock, Potomac, Pamunkey, Mattaponi and Piankatank rivers have high densities, according to the analysis prepared by biologists from the Virginia Commonwealth University Rice Rivers Center. Others, including all Maryland tributaries, had only medium or low densities of the invasive fish.
The estimate was prepared for the Bay Program’s Sustainable Fisheries Goal Implementation Team, and drew on years of data collected by VCU biologists about blue catfish population densities at several locations within the James River.
It also drew on information about blue catfish distribution collected by state agencies and other sources, and used that information to classify rivers as having high, medium or low densities. The analysis used GIS maps to determine the potential habitat for blue catfish. Then it applied adjusted density estimates to those habitats to come up with an overall population estimate.
The result was a range of 94 million to 111 million blue catfish living in the fresh and brackish areas of tidal Bay rivers. Blue catfish avoid salinities of more than about 17 parts per thousand.
“There are all kinds of opportunity for error to sneak into this,” cautioned Greg Garman, director of the VCU Center for Environmental Studies, who led the effort that he acknowledged relied heavily on the use of best professional judgment.
Nonetheless, he added, “we think it is a reasonable, coarse estimate of the total number of fish.”
Blue catfish are a native of the Mississippi River basin and were introduced into Virginia tributaries in the 1970s. In the last two decades, their numbers and range have expanded greatly within the Bay’s tidal tributaries.
Their abundance has sparked concern among fishery managers because they are a top predator and could disrupt the Bay’s food web, outcompeting native fish, and increasing predation on species such as blue crabs, shad, river herring and others.
Garman said he was surprised by the estimate of 100 million fish. “I wasn’t expecting something quite that big,” he said.
But he said the figure could underestimate the number of blue catfish because the mapping done by his team to estimate habitat excluded many smaller tidal creeks where catfish can thrive.
“They certainly go up into some of these smaller tidal creeks, and there are a lot of them,” Garman said.
He and his colleagues hope to refine their estimate over time.
It is difficult to make apples-to-apples comparisons between the blue catfish estimate and the population estimates for other fish. Population estimates for many species don’t exist, and those that do use a variety of different methodologies to arrive at their figures.
The Chesapeake Bay Stock Assessment Committee’s most recent annual blue crab winter dredge survey estimated the total number of blue crabs in the Bay, both juveniles and adults, at 308 million. Blue catfish feed on small blue crabs when their habitats overlap.
The most recent stock assessment by the Atlantic States Marine Fisheries Commission put the entire East Coast striped bass population at 215 million — a figure that includes all fish age 1 or older.