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Fishery Bulletin 11 7(4) 
Predator total length (mm) 
Month 
Figure 5 
Predictions from the generalized additive model for pre¬ 
dation on American eel ( Anguilla rostrata) by blue catfish 
(Ictalurus furcatus ), calculated as probability of occur¬ 
rence in a catfish stomach, by salinity, predator total 
length, month, and river. All predictive factors were sig¬ 
nificant (P<0.05), with the exception of salinity (P=0.06). 
Blue catfish were collected in the James, Pamunkey, 
Mattaponi, and Rappahannock Rivers in eastern Virginia 
during 2013-2016. 
Table 3 
Whole-model and variable-wise statistical significance for 
each generalized additive model (GAM) used to explore 
trends in rates of predation by blue catfish ( Ictalurus 
furcatus) on American eel ( Anguilla rostrata)-, imper¬ 
iled alosines, including American shad ( Alosa sapidis- 
sima ), blueback herring ( Alosa aestivalis), and alewife 
(A. pseudoharengus); and blue crab (Callinectes sapidus) in 
Chesapeake Bay in Virginia during 2013-2016. An aster¬ 
isk indicates that the value was insignificant (a=0.05). 
Model performance was evaluated by using area under the 
receiver operating curve (ROC) and deviance explained. 
Predictor 
American 
eel 
Imperiled 
alosines 
Blue 
crab 
River 
P<0.001 
P=0.009 
P<0.001 
Month 
P<0.001 
P=0.019 
P=0.017 
Salinity 
P=0.063* 
P<0.001 
P<0.001 
Total length 
P=0.023 
PcO.001 
P<0.001 
Model fit 
Area under ROC 
0.859 
0.874 
0.843 
Null deviance 
651.305 
698.980 
2032.733 
Residual deviance 
127.875 
151.977 
440.578 
Deviance explained 
523.430 
547.003 
1592.155 
500 mm TL (James River) to 900 mm TL (Pamunkey 
River) and may be driven by the relative abundance of 
small fish prey in each river (Schmitt et al., 2019). 
Although large, piscivorous catfish compose a small frac¬ 
tion of each population, they could still have deleterious 
effects on important native taxa like American shad, river 
herring, and American eel. Ample evidence indicates that 
blue catfish feed on locally abundant prey, as other studies 
have reported that blue catfish are nonselective, opportu¬ 
nistic feeders (Eggleton and Schramm, 2004; Schmitt 
et al., 2017). These life history traits have been docu¬ 
mented in many other estuarine and marine fish species, 
for which diets are a reflection of the variability of avail¬ 
able prey (Beumer, 1978; Livingston, 1984; Ley et al., 1994; 
Jaworski and Ragnarsson, 2006). 
Our GAMs revealed the circumstances that lead to 
greater predation on species of concern by invasive blue 
catfish. For American shad and river herring, maximal 
predation occurred in freshwater areas, a finding that cor¬ 
responds with those of Schmitt et al. (2017), who reported 
that predation on depleted alosines peaked in both tidal 
and non-tidal freshwater segments of the James River. For 
simplicity, we pooled imperiled alosines for analysis; how¬ 
ever, river herring were consumed more frequently than 
American shad, which are larger and faster-swimming 
fish (Waldman, 2013). For more detailed, species-specific 
data, please see Schmitt et al. (2019). For all rivers, the 
highest predation rates occurred in April, a result that 
agrees with previous work from the James River (Schmitt 
et al., 2017). Although some of the alosine prey items 
were in the late stages of digestion, all of the discernible 
