Schmitt et al.: Modeling the predation dynamics of invasive Ictalurus furcatus in Chesapeake Bay 
285 
specimens captured during spring were mature, spawn¬ 
ing adults. The model for American shad and river her¬ 
ring also revealed a small increase in predation during 
September and October. All alosines consumed during 
this period were small, and autumn is associated with the 
outriver migration of juvenile alosines in Atlantic estuar¬ 
ies (Loesch and Lund, 1977; Hoffman et al., 2008; Palko- 
vacs et al., 2014). Therefore, this migratory behavior may 
make juvenile alosines more susceptible to predation by 
blue catfish at this time. It is important to note that small, 
juvenile fish are digested more rapidly than adults (Brom¬ 
ley, 1994) and that, as a result, we may have underesti¬ 
mated predation on juvenile alosines during autumn. 
Predation on American shad and river herring was high¬ 
est for blue catfish ranging in size from 600 to 900 mm 
TL, although predation probabilities decrease for trophy¬ 
sized catfish (>1072 mm TL; Gabelhouse, 1984). These 
feeding patterns may be driven by individual diet special¬ 
ization, where trophy-sized blue catfish are cannibalistic 
or specialize on gizzard shad (Dorosoma cepedianum ) as 
observed by Schmitt et al. (2019). The predicted percent 
occurrence of alosines in the diet of blue catfish was rel¬ 
atively low in all circumstances (<8% occurrence). It is 
also important to note that some Alosa species can expe¬ 
rience high post-spawning mortality because of energetic 
demands (Glebe and Leggett, 1981), and the presence of 
alosines in blue catfish stomachs could be, in part, due to 
scavenging (Schmitt et al., 2019). 
Along the Atlantic coast, declines in populations of river 
herring and American shad began well before the prolif¬ 
eration of blue catfish and must have been initiated by 
other mechanisms (e.g., declines in populations of Amer¬ 
ican shad began during the 1800s, and river herring pop¬ 
ulations declined precipitously in the 1960s; Limburg and 
Waldman, 2009). Alosines face many challenges, including 
habitat loss, overharvesting, poor water quality, climate 
change, and impediments that block migratory corridors 
(Limburg and Waldmen, 2009; Raabe and Hightower, 
2014). Moreover, alosines are frequently taken as bycatch 
in Atlantic herring (Clupea harengus ) fisheries (Bethoney 
et al., 2013; Hasselman et al., 2016). Nonetheless, preda¬ 
tion by invasive catfish could further destabilize these spe¬ 
cies, especially if functional response curves are such that 
predation rates increase at low prey densities (Dick et al., 
2014). Interestingly, some signs of recovery have been 
observed for populations of American shad in the Rappa¬ 
hannock and Potomac Rivers (Cummins 1 ; Hilton et al. 2 ). 
Both of these rivers support dense populations of blue cat¬ 
fish, possibly indicating that blue catfish play an insignif¬ 
icant role in the population dynamics of American shad. 
1 Cummins, J. 2016. The return of American shad to the Potomac 
River: 20 years of restoration. Final Report. Interstate Comm. 
Potomac River Basin, ICPRB Rep. ICP16-5, 23 p. [Available 
from website.] 
2 Hilton, E. J., R. Latour, P. E. McGrath, B. Watkins, and A. Magee. 
2016. Monitoring relative abundance of American shad and 
river herring in Virginia rivers 2015 annual report, 98 p. Va. 
Inst. Mar. Sci., Coll. William Mary, Gloucester, VA. [Available 
from website.] 
Blue catfish predation on blue crab increased with salin¬ 
ity, and this increase was likely driven by the relative den¬ 
sity and spatial dynamics of blue crab populations in the 
James, York, and Rappahannock Rivers. Previous research 
has indicated that blue crab abundance (measured as the 
number of blue crab caught per 24 h in fyke nets) was 
positively correlated with salinity in tidal tributaries of 
Chesapeake Bay (King et al., 2005). Moreover, low-salinity 
areas are typically dominated by adult male crab, which, 
because of their large size, are less susceptible to preda¬ 
tion, yet smaller juvenile and female crab become more 
abundant at higher salinities (Hines et al., 1987). Many 
of the blue crab we found in stomachs were immature, a 
finding that correlates well with the observed relationship 
between salinity and predation on blue crab. Rates of pre¬ 
dation on blue crab were highest for blue catfish around 
800 mm TL and declined in larger blue catfish. Maximal 
predation rates occurred during the autumn months in all 
rivers, although predation on blue crab increased during 
spring in the Mattaponi River. The autumn months are 
typically associated with reduced freshwater inflow, which 
often results in the upriver advancement of the salt wedge 
(Schubel and Pritchard, 1986). As the salt wedge advances 
upriver, we would expect there to be greater spatial over¬ 
lap between blue catfish and blue crab (King et al., 2005). 
We directly observed this phenomenon in the James River 
when we found blue crab at high densities along the lower 
edge of the fall line during August and September. This 
area is usually home to freshwater species, like the small- 
mouth bass (Micropterus dolomieu) and various sunfish 
species (Lepomis spp.), but the upriver advancement of the 
salt wedge enables blue crab to colonize this area during 
extended dry periods. 
Blue crab naturally co-occur with blue catfish in estuar¬ 
ies in Louisiana (Baltz and Jones, 2003) and are consumed 
at higher rates there (up to 50% of stomachs; Perry, 1969) 
than those observed in Virginia’s tidal rivers. In spite 
of high predation rates, Louisiana continues to sustain 
valuable blue crab fisheries, and annual harvests exceed 
those in both Virginia and Maryland (NMFS 3 ). This is not 
surprising because blue crab have complex life histories 
(Hines et al., 2010) and population dynamics appear to be 
strongly influenced by abiotic factors (Bauer and Miller, 
2010; Colton et al., 2014). Nonetheless, predation of blue 
crab by blue catfish should be considered in future popula¬ 
tion models; after all, the predicted percent occurrence of 
blue crab in stomachs of blue catfish can be quite high (up 
to 28% in brackish portions of the James River). 
For American eel, salinity did not significantly affect 
predation rates. This result is intuitive because eels read¬ 
ily colonize freshwater, estuarine, and marine habitats 
and move freely between them (Feunteun et al., 2003; 
Daverat et al., 2006). Size of blue catfish significantly 
affected predation rates on American eel, and maximal 
predation rates were observed for blue catfish between 
3 NMFS (National Marine Fisheries Service). 2017. Fisheries of 
the United States 2016. NOAA, Natl. Mar. Fish. Serv., Curr. 
Fish. Stat. 2016, 147 p. [Available from website.] 
