Nonnative catfish not primary driver of American shad declines in James River

Nonnative species that are introduced into ecosystems are a growing concern for scientists and conservationists because they can potentially suppress vital native species, alter food-web dynamics, and threaten biodiversity. Researchers from the Department of Fish and Wildlife Conservation, along with Aaron Bunch of the Virginia Department of Game and Inland Fisheries, explored whether nonnative catfish are to blame for declining native fish populations in Virginia’s rivers.

Doctoral student Joseph Schmitt and other investigators conducted a study on predation of native fish species, including American shad, blueback herring, and alewife, in the James River by nonnative catfish. The results were published in the journal Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science.

To determine whether predation by nonnative blue catfish and flathead catfish is a major cause in the decline of native species, researchers used a technique called low-frequency electrofishing. “The electricity stuns them, and they rise to the surface and begin swimming erratically for a minute or two,” Schmitt explained, adding that the method has been approved as safe. Once a fish was netted, a team member shot a stream of pressurized water into its stomach, making it regurgitate whatever it had eaten to determine which species the catfish were preying on.

In spring 2014 and 2015, Schmitt and his team caught approximately 2,500 catfish and found that American shad, blueback herring, and alewife were present in the stomachs of only 4.46 percent of blue catfish and 17 percent of flathead catfish tested. They concluded that catfish predation could not be considered the main cause of their decline. “Other factors besides predation are also at play, including habitat degradation, bycatch from offshore fisheries, migration barriers like dams, and predation by other fish,” Schmitt said. Their research, which is part of a larger Virginia Department of Game and Inland Fisheries study, will help influence the agency’s future management practices and restoration efforts.

Read the full press release.