White catfish, the original catfish of the Chesapeake Bay
White catfish Ameiurus catus were once the dominant catfish species found in the brackish waters of the Bay. Native to many coastal rivers from New York to Florida, these small catfish are easily identified by their proportionally huge head, which is similar to many native bullhead species, though white catfish can be distinguished by their slightly forked tail. Most are fairly small, averaging around 10-15” in length, though some individuals can grow to 24” and weigh up to 6 lbs. They are excellent to eat and once comprised a significant portion of the commercial catfish harvest in the state of Virginia. Commercial harvesters in Virginia are not required to report catfish landings by species, but biologists with Virginia Department of Game and Inland Fisheries say that catfish harvest was primarily channel catfish and white catfish until the late 1990s, after which blue catfish began to dominate Virginia's tidal rivers.
White catfish were once abundant in the tidal stretches of the James, York, and Rappahannock rivers, but are now less common. They are still abundant in some smaller tidal creeks, but larger blue catfish appear to have displaced them from the main river. White catfish prefer brackish waters, and we frequently encounter them in salinities ranging from 3 ppt to 8 ppt.
Not a great deal is known about the biology or life history of white catfish, and few studies exist within the scientific literature. We know that capture rates of white catfish have declined precipitously since the introduction of blue catfish in the Bay (Tuckey and Fabrizio 2010), but we do not know if this is due to predation, competition for food, competition for spawning habitat, or reasons not related to blue catfish at all.
While a great deal of concern has revolved around blue catfish predation of blue crab, American shad, and river herring, little attention has been directed toward their potential negative impact on native white catfish. This is not entirely unfounded as American shad, blue crab, and river herring supported, at one time or another, very lucrative fisheries within the Chesapeake Bay, while white catfish were always of lesser economic importance.
One way to examine negative interactions between blue and white catfish is to explore potential competition between the two species. To do so, we most first locate areas where the two species co-exist/overlap in reasonably high densities. We've found that these species commonly overlap in smaller oligohaline and mesohaline tributaries of the James, York, and Rappahannock rivers, yet, as previously stated, we rarely encounter white catfish in the main channels of these rivers. Unfortunately, we do not have great information on the distribution of white catfish prior to the blue catfish introduction, so it will be difficult to determine if things have changed through the years, e.g. have blue catfish driven white catfish out of the rivers, or have white catfish always preferred smaller tributaries?
A large male white catfish with bite marks on his head.
One way to explore competition between the two species is to examine their food habits, and determine the degree of dietary overlap between the species. That is, are the two species relying on the same food sources? This is explored using diet overlap indices, (Levins’, Horn’s, Schoener’s, and/or Morisita’s overlap indices are all commonly used) that quantitatively describe the degree to which two species utilize the same prey resources. If they are relying on the same food sources, the next question is, are those shared resources limited within the system? If they are, blue catfish could simply be outcompeting white catfish, which may explain observed declines in the white catfish population.
We have extracted diet contents from hundreds of white catfish and blue catfish in brackish tributaries of the York and Rappahannock rivers, and are in the process of constructing diet overlap indices for fish captured simultaneously within these areas. Diet overlap between the two species appears to be high, as smaller individuals of both species regularly consume the same aquatic invertebrates, crustaceans, and vegetation, while larger individuals feed on blue crab and menhaden. Are these resources limited? Vegetation, invertebrates, and mud crabs are likely not limited, while blue crabs and menhaden may be limited, at least at small spatial scales (e.g. within these small creeks). Estuaries are some of the most productive systems in the world, so it is quite possible that none of these resources are limited, which would mean something else is driving white catfish declines.
Observed declines in white catfish may also be due to competition for spawning habitat. While little is known about the reproductive behavior of either species, we know that North American catfish are typically “cavity spawners”, meaning they utilize undercut banks, hollowed out logs, and other similar structures that provide cover and protection from egg predators. Once the eggs are laid, male catfish actively defend the nest from invaders. It is quite possible that larger, stronger blue catfish are forcing white catfish out of prime spawning habitat, which could have negative impacts on white catfish reproductive output. We have seen large bite marks on numerous white catfish during the spawn, and it is possible that blue catfish attacked them to force them out of premium spawning sites. Unfortunately, this would be hard to determine without directly observing these interactions, which would be nearly impossible in these muddy rivers.
Notice how nearly every white catfish pictured here has large bite marks on the head- these bite marks appear to be from larger blue catfish; likely either a territorial dispute or a fight over spawning habitat.
Ultimately, blue catfish are here to stay, while the fate of white catfish is “to be determined”. Will white catfish continue to persist in limited numbers, largely confined to brackish creeks? Or will populations rebound to historic levels? Only time will tell.
Another male white catfish with bite marks on his head