Every spring, the rivers of the Chesapeake Bay come to life as thousands of hickory shad, American shad, blueback herring, and alewife swim upriver to spawn.
American shad are referred to as "America's founding fish", due to their importance during the early history of the United States. Colonial settlers relied on shad runs in the spring, and they also provided valuable nutrients to bears, osprey, eagles, and other native fauna; much like salmon on the west coast. Read more about the history of shad in the Chesapeake Bay here.
The scientific name for American shad is Alosa sapidissima which translates as "delicious". Early settlers trapped shad during their upstream migrations using weirs and nets, and they often smoked them slowly on wooden planks next to the fire, known as "shad planking".
American shad were once the most commercially valuable species in the Chesapeake Bay, but populations crashed in the 1970s. Degraded habitat, poor water quality, obstructions to migration (dams), over fishing, and predation have all been cited as contributing to these declines.
Concurrently, populations of closely-related river herring also crashed. River herring are two very similar species, blueback herring and alewife, that are collectively managed. These two species are smaller than American shad, but have similar life histories and are also good to eat. Like American shad, they also once supported lucrative fisheries along the Atlantic seaboard.
Harvest moratoriums for American shad began in the 1980s, but moratoriums for river herring were just initiated a few years ago.
Neither shad nor herring have recovered to historic levels, and fisheries managers have wondered whether or not predation by large, non-native catfish may be hindering restoration efforts.
To address this, Virginia Tech collected stomachs from catfish during March, April, and May, during which American shad and river herring ascend our rivers to spawn.
Our efforts were focused on the James River, as it supports dense populations of blue and flathead catfish, and stomachs were collected from Bosher Dam all the way downriver to the confluence with the Chickahominy.
Stomach contents were extracted from thousands of fish (2,495), many of them large. Catfish were collected in five locations: non-tidal freshwater (Belle Isle/Bosher Dam), tidal freshwater (14th St. Bridge downriver to Henricus Park), Herring Creek, Wards Creek, and Gordon Creek.
We encountered more blue catfish than flathead catfish; however, most of the blue catfish were small (less 20" long or 500 mm), and incapable of preying upon American shad or river herring.
Flathead catfish fed exclusively on other fish, while blue catfish ate a variety of things including vegetation, detritus, insects, birds, small rodents, turtles, clams, frogs, and some really disgusting human waste that will be the topic of another blog post.
As you can see from the figure above, small blue catfish (less than 500 mm or 20") fed heavily on vegetation, mollusks, crustaceans, and various invertebrates. Only bigger fish (>20") began to include fish in their diets. Fish prey were often highly digested, so we used DNA barcoding to identify fish to species (read more here).
We found "Alosa" species (American shad, hickory shad, blueback herring, and alewife) in nearly 17% of flathead catfish stomachs.
(%O is the percent of catfish that had a prey item present in their stomachs. If ten fish out of 100 had eaten American shad, then %O of American shad would equal 10%)
Alosa species were found in less than 5% of blue catfish stomachs.
Flathead catfish primarily ate blueback herring (river herring), while blue catfish primarily ate hickory shad.
Hickory shad are in the same family as American shad and river herring, but have not experienced drastic population declines, and are still fairly abundant in the Bay. There is no harvest moratorium for this species in Virginia.
In terms of relative importance (%PSIRI; see below), Alosa species were far more important to flathead catfish than to blue catfish, which further indicates the different feeding patterns exhibited by these catfish species. Alosa species had a relative importance of approximately 13% for flathead catfish and less than 1% for blue catfish. Again, flathead catfish appear to be relying heavily on blueback herring in the spring.
%PSIRI is the "prey specific index of relative importance", and basically looks at the number of stomachs a prey type occurred in, its contribution in terms of weight, and its contribution in terms of numbers. From this you can calculate the overall importance of a prey type. As you can see, shad and herring were more important to flathead catfish than to blue catfish.
We also noticed differences in predation of Alosa species by spatial location. Predation of Alosa species by both blue and flathead catfish increased further upriver, and there was significantly higher predation of shad and herring by blue catfish at the non-tidal freshwater sites above the fall line (Bosher Dam, Manchester Bridge). The James River above the fall line is full of rapids, boulders, and remnants of old bridges and dams. This complex structure provides numerous ambush points for catfish, and may explain higher predation rates in these areas.
Percentage of stomachs with Alosa species (American shad, river herring, hickory shad) by location:
The differences in feeding habits between blue and flathead catfish are striking. The majority of the blue catfish population is comprised of small fish that prey on invertebrates and vegetation. Only blue catfish greater than 20" in length begin to prey heavily on other fish. Flathead catfish, on the other hand, begin to eat other fish at much smaller sizes (by about 8" length).
Flathead catfish rely heavily on Alosa species in the spring, namely blueback herring, though they also prey on American shad. Blue catfish occasionally prey on Alosa species, but they mostly eat hickory shad, which are not a species of management concern.
Given this information, flathead catfish are likely to have a greater per capita impact on American shad and river herring. This is not really surprising, as other studies have already shown that flathead catfish are dangerous invasive species that are capable of damaging native fish populations (read more here).
While blue catfish rarely prey on American shad or river herring, they may still have greater population-level impacts than flathead catfish, as they are more abundant and their populations extend further downriver into brackish areas. We won't be able to estimate impacts on American shad and river herring without estimates of population sizes for both of these catfish species.
Interestingly, American shad stocks are showing signs of recovery in the Rappahannock and Potomac Rivers, which both have dense populations of blue catfish (read more here). It appears that American shad are capable of recovering in the presence of these big catfish, which is good news.
Shad and herring still face several obstacles to recovery including unintentional harvest by offshore fisheries, obstacles to fish migration, degraded spawning habitat, pollution, and predation. These catfish are here to stay, but there is some hope that American shad can recover in the presence of the whiskered beasts.
Virginia's tidal rivers come to life during the spring, as river herring and shad move upriver to spawn. Big striped bass come upriver too, and are often caught in the rapids near Richmond, VA.