Salinity, Season, and Size are Drivers in Virginia Blue Catfish Diets

When a new species is introduced to an area, scientists are typically interested in the species' feeding behavior to determine how it will fit into its new ecosystem. Blue Catfish were introduced to Virginia tidal rivers beginning in the 1970s and have since exploded in abundance and expanded their range to new Chesapeake Bay tributaries. One concern is the impact to native species through predation, especially for depleted species and species with commercial value. Blue Catfish food habit studies support that the species functions as an “opportunistic omnivore” in the Chesapeake Bay region. Opportunistic indicates Blue Catfish eat what’s available without preference for particular foods, whereas omnivores are creatures that eat both plants and animals. Given the opportunistic nature of their feeding tendencies, a simple table listing the foods found in Blue Catfish stomachs likely leaves out some details on their feeding behavior. Prey species may be more or less prevalent during certain times of the year or in certain areas, which may lead to differences in diets.

Virginia Tech scientists present a 90 lb. Blue Catfish from the Rappahannock River during an electrofishing survey in 2015. Photo: Donald J. Orth.

Scientists from Virginia Tech, Virginia Department of Game and Inland Fisheries, and Clemson University recently published a study examining the influence of different factors on Blue Catfish feeding habits in the James, Mattaponi, Pamunkey, and Rappahannock rivers from 2013-2016. Based on stomach contents of 7,302 Blue Catfish (14,488 sampled in total, ~50% of stomachs were empty), the research team explored season, salinity, and Blue Catfish size on the presence or absence of various prey groupings, including fish, mollusks (e.g., Asian clams), crustaceans (e.g., blue crab, amphipods), other invertebrates, vegetation, and other (included rare food items such as birds, mammals, and trash) in Blue Catfish stomachs. A second modeling effort examined the influence of salinity, season, and size specifically on species of concern in the Bay tributaries, including American Eel, American Shad/river herring combined (referred to as "alosines"), and blue crab. Previous work showed Blue Catfish diets differed by Blue Catfish size and all the species of concern here are known to make upstream and/or downstream movements during the year. Further, aquatic communities in these systems are organized based on salinity and species' tolerances for salty water. The analysis only included Blue Catfish collected between from April to October due to sampling limitations.

Map of sampling locations (denoted by black dots) from which diets where examined for influences of salinity, season, Blue Catfish size. Figure 1, Schmitt et al. 2019. Fishery Bulletin 117(4):277-290.

The modeling results provided statistical support that Blue Catfish diets varied by river, season, salinity, and Blue Catfish size. Blue Catfish were more likely to eat crustaceans and other invertebrates in the spring. Plant material was most common in Blue Catfish diets during the spring and summer, a portion of the year when plant and algae production is highest. Blue Catfish ate more invertebrates in lower salinity waters, but ate more fish at higher salinity. Size was important as fish were more likely to be found in the diets of large Blue Catfish. There were also differences among rivers, for instance, Blue Catfish consumption of plant material was most common in summer in all rivers except the Rappahannock where it was more common in the spring.

The modeling effort examining species of concern specifically also supported that salinity, season, and size were important factors. The results supported significant relationships for salinity, month, and Blue Catfish size, with the exception of salinity for American Eel. Alosines were most commonly encountered in large (28 to 40 inches) Blue Catfish diets in freshwater portions of the rivers examined, with highest predicted occurrence in the James and Rappahannock rivers. Models predicted highest predation on blue crab in higher salinity waters. The model predicted blue crabs to be present in nearly 30% of stomachs in the James River when salinity exceeded 8 psu, but was predicted to be less than 5% in other rivers. Larger Blue Catfish were predicted to consume more crabs and the model output supported fish 24-35 inches were most likely to eat blue crab. Further, the model predicts greatest blue crab predation from August to October. American Eel was not a commonly encountered prey in the diets, but was correlated with month and Blue Catfish size. Eels was most common in diets of Blue Catfish from 20-35 inches and during April and October.

Predictions of blue crab occurrence in Blue Catfish stomachs based on salinity (measured as psu) for four Virginia tidal rivers. A predicted probability of occurrence of 0.10 indicates we would expect to find blue crab in 10% of the stomachs with food in them at a given salinity value. Modified from Figure 4, Schmitt et al. 2019. Fishery Bulletin 117(4):277-290.

So what does all this mean? Blue Catfish seem to eat whatever is around. The time periods in which species of concern were most prevalent in the diet corresponds with migration or periods when salinity is higher and blue crab are more common in upstream portions of these rivers. Large fish are more likely to consume at-risk species. In most cases, the James had the highest predicted probability of occurrence, but the James has the highest proportion of large fish. Blue Catfish may not be the most important part of the declines in American Eel, American Shad, and river herring but are an additional source of deaths due to predation. The study supported that some areas, time periods, and fish sizes may present opportunities to focus harvests to reduce predation on species of concern. However, Blue Catfish management in Chesapeake Bay is always more complicated than it appears. For instance, increasing harvest of large individuals may expose consumers to higher levels of environmental contaminants. Further, many recreational anglers target trophy-size fish. Blue Catfish remains a contentious issue due to competing objectives among stakeholder groups.

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