What do blue catfish eat in Virginia's tidal rivers?
Over the past several years, we have spent hundreds of hours on the water collecting blue catfish diet information. We’ve collected thousands of stomachs from the James, Pamunkey, Mattaponi, and Rappahannock Rivers. We’ve sampled during the spring, summer, fall, and winter months, and along the salinity gradient from freshwater to brackish water. With over 16,000 blue catfish stomachs in our database, we’ve just finished the largest, most comprehensive diet description for this species. So what do these catfish eat?
Blue catfish are opportunistic generalists that will eat just about anything. We have found vegetation, various clams, mud crabs, birds, snakes, frogs, muskrats, fish, jellyfish, aquatic insects, mulberries, and even corn in catfish stomachs. We’ve also encountered some disgusting items in stomachs, including raw sewage, surgical gloves, condoms, and feminine products, all from stomachs collected in downtown Richmond near the 14th St Bridge.
Surprisingly, most blue catfish have a plant-based diet during the spring, summer, and fall. Large fish are rare, and most of the catfish in these rivers are under 15” in length. As these catfish grow to larger sizes, they begin to incorporate more fish in their diet. Nonetheless, imperiled species like American shad, river herring, and American eel were found in less than 1% of stomachs overall. Big catfish seem to prefer eating gizzard shad, which is a common food for blue catfish in their native range (see Edds et al. 2002; Eggleton and Schramm 2004). Gizzard shad are incredibly abundant in the James River, and this abundance of food may explain the trophy fishery this river supports.
We constructed statistical models to explore patterns in blue catfish predation of “species of concern”. These species include American shad, American eel, and river herring, all of which have declined along the Atlantic coast. Blue crab are also a species of concern, since they support valuable commercial fisheries in the Chesapeake Bay. In general, large catfish are more likely to eat all species of concern, though predation rates decline for trophy-sized catfish (42"; see Gabelhouse 1984). American eel, river herring, and American shad were rare in stomachs, and blue crab were much more common.
Predation of American shad and river herring peaks in the April in freshwater areas, which makes sense since this is when these fish migrate upriver to spawn. Predation of shad and herring was also detected during the fall, which is when juveniles migrate downriver into the estuary. In all circumstances, the probability of finding an American shad or river herring in a blue catfish stomach is less than 8%.
American eels are primarily consumed during the spring and fall. This makes sense since eels are known to migrate during these seasons. Salinity does not significantly affect American eel predation rates, which also makes sense since eels are a euryhaline species that thrive in a fresh and brackish waters. In all circumstances, the probability of finding an American eel in a blue catfish stomach is less than 4%.
Predation of blue crab increases with increasing salinity, which is intuitive since blue crab populations are more dense in brackish areas (King et al. 2005). Additionally, predation of blue crab increases during the autumn months. This may be related to limited rainfall during the late summer/early fall, which results in the upriver advancement of saline waters from the Bay. This increases spatial overlap between blue catfish and blue crabs. In Richmond, it is not unusual to see blue crabs around the 14th St. Bridge in the autumn, especially during dry years. In brackish portions of the James River, the probability of finding a blue crab in a blue catfish stomach can be as high as 30%, which is concerning.
I have purposely avoided going into great detail here, but I still want to summarize a few key points from our diet work:
1) Blue catfish are opportunistic generalists. They will eat just about anything, and selectivity analysis during the spring revealed that they eat whatever is locally abundant (Schmitt et al. 2017).
2) Because so many blue catfish are herbivorous, the "top carnivore" narrative is mostly false (e.g. scientists and the media have called them "apex predators" and "Bengal tigers of the Chesapeake"). Over 50% of the stomachs we collected contained vegetation. Trophic level calculations demonstrate that most of the population occupies a similar trophic position as common carp. Not surprisingly, blue catfish occupy lower trophic positions than true carnivores like flathead catfish and striped bass.
3) Blue catfish predation of species that are in decline (American shad, river herring, and American eel) is uncommon. Each of these species was found in less than 1% of stomachs overall.
4) Blue crabs were commonly consumed, and were found in up to 30% of stomachs from brackish portions of the James River. This will be a concern moving forward.
5) While predation of imperiled species (river herring, American shad, American eel) is rare, high population densities of blue catfish could still mean that catfish are having negative impacts on these species. A recent study (Fabrizio et al. 2017) estimated there to be nearly 220 blue catfish per acre within the oligohaline portions of the James River. That’s a lot of catfish.
6) Our generalized additive models show that blue catfish in the 20”-40” range inflict the most damage on species of concern. The smallest fish (<20”) are vegetarians or eat small invertebrates, while the largest fish (>40”) are often cannibalistic or eat gizzard shad.
If you are interested learning more, our final report it is located here. Ultimately, blue catfish are a mixed bag. While they are not “Bengal tigers of the Chesapeake”, they do have the potential to impact important native species. Efforts to reduce population densities, particularly in brackish areas, may be warranted.
Edds, D. R., Matthews, W. J., and F.P. Gelwick. 2002. Resource use by large catfishes in a reservoir: is there evidence for interactive segregation and innate differences?. Journal of Fish Biology, 60(3):739-750.
Eggleton, M. A., and H.L. Schramm. 2004. Feeding ecology and energetic relationships with habitat of blue catfish, Ictalurus furcatus, and flathead catfish, Pylodictis olivaris, in the lower Mississippi River, USA. Environmental Biology of Fishes 70(2):107-121.
Fabrizio, M. C., Tuckey, T. D., Latour, R. J., White, G. C., and A.J. Norris. 2017. Tidal habitats support large numbers of invasive Blue Catfish in a Chesapeake Bay subestuary. Estuaries and Coasts 1-14.
Gabelhouse Jr., D.W. 1984. A length-categorization system to assess fish stocks. North American Journal of Fisheries Management, 4(3):273-285.
King, R. S., Hines, A. H., Craige, F. D., and S. Grap. 2005. Regional, watershed and local correlates of blue crab and bivalve abundances in subestuaries of Chesapeake Bay, USA. Journal of Experimental Marine Biology and Ecology 319(1-2):101-116.
Schmitt, J. D., Hallerman, E. M., Bunch, A., Moran, Z., Emmel, J. A., and D.J. Orth. 2017. Predation and prey selectivity by nonnative catfish on migrating alosines in an Atlantic slope estuary. Marine and Coastal Fisheries, 9(1):108-125.