Wuenschel et al.: Habitat and diet overlap of 4 piscivorous fishes 
355 
moved immediately after capture and preserved in for- 
malin for laboratory analysis. In some months, tagging 
and releasing Striped Bass was a higher priority than 
determining stomach contents; therefore, all fishes cap- 
tured were not available for diet analysis. 
To account for size-related changes in habitat use 
(Able and Fahay, 2010), diet composition within species 
(Garrison and Link, 2000b), and interactions across 
species (Buckel and McKown, 2002), species were split 
into multiple size classes when data permitted: small 
(Summer Flounder: 200-300 mm total length [TL] ; 
Weakfish: 80-200 mm TL; Bluefish: 55-300 mm fork 
length [FL]), medium (Summer Flounder: 301-400 mm 
TL; Weakfish: 201-350 mm TL), and large (Summer 
Flounder: 401-670 mm TL; Weakfish: 351-565 mm TL; 
Bluefish: 301-732 mm FL). For Striped Bass, a single 
size class was used because of limited sample sizes, the 
absence of prior evidence for ontogenetic shifts beyond 
the YOY stage (Walter et al., 2003), and the relatively 
large sizes of our specimens (422-920 mm FL). 
Diet analysis 
In the laboratory, preserved stomachs were carefully 
opened and the contents transferred to a solution of 
rose bengal stain and 95% ethyl alcohol. Prey items 
were identified to the lowest practical taxonomic level 
by using available keys and guides for the Mid-Atlantic 
region (Weiss, 1995; Able and Fahay, 1998) and enu- 
merated. For each stomach, abundant or large prey 
types were sorted and placed on preweighed filter pa- 
pers or aluminum weighing pans and dried to a con- 
stant weight (+0.0001 g) in a drying oven (70°C). Dry 
weights were chosen because they are more representa- 
tive of nutritional value and have less weighing error 
than wet weights (Hyslop, 1980), especially for small 
or partial prey (Carr and Adams, 1972). For small and, 
therefore, hard-to-separate prey items (e.g., copepods 
and mysids), an aggregate sample was dried and the 
percent contribution by volume of different prey types 
was recorded and later converted to weights. Through 
the use of this protocol, prey-specific dry weights were 
obtained directly for larger prey or estimated from ag- 
gregate samples of smaller, mixed prey items for each 
stomach analyzed. 
Trawl collections yielded “clusters” of individuals 
within species and size classes per location; therefore, 
the percent contribution by weight of prey items was 
calculated with the following cluster sampling estima- 
tor (Buckel et al., 1999a; 1999b; Gartland et al., 2006). 
For a given size class of a predator, the percent contri- 
bution by weight of each prey type k (%W^) to the diet 
was calculated with the following equation: 
£ M Mik 
%w k =^ 100, (1) 
I Mi 
i=l 
where q., = — — ; 
lk w; 
n = the number of trawls; 
M x = the number of species size class sampled 
per tow i; 
W{ = the total dry weight of all prey in stomachs 
for that species size class in tow /; and 
Wfe = the total dry weight of prey type k 
in all stomachs for that species size class 
collected in tow i. 
To facilitate analysis, prey items were grouped into 
the following general categories: squids (predominantly 
Loligo spp.), decapod crustaceans (including swimming 
crabs, sand crabs, rock crabs [Cancer borealis and C. ir- 
roratus], spider crabs, hermit crabs, decapod zoea, and 
shrimps [predominately Crangon septemspinosa and 
Palaemonetes sp.]), nondecapod crustaceans (including 
amphipods, isopods, cumaceans, mysids, and mantis 
shrimp), bivalves (clams and periwinkles), fishes (44 
species identified), worms and wormlike organisms 
(nematodes, polychaetes, annelids, and leeches), and 
other unidentified (UID) items (inorganic matter, or- 
ganic matter, eggs, and insects). In addition, prey items 
(species or higher taxa) that contributed on average 
>5% by weight to the overall mean diet of a species 
size class were included as additional prey categories. 
Therefore, if Bay Anchovy (. Anchoa mitchilli) composed 
>5% of the diet for a given species size class in any 
month, it was included as a prey category and the cat- 
egory “fishes” represented all remaining fish prey that 
contributed <5% to the diet of that species size. 
The cumulative trophic diversity was calculated for 
each species size class to determine whether the sample 
sizes that were analyzed were sufficient to describe the 
diet of a given species size class in each month (Ferry 
and Cailliet, 1996; Cortes, 1997; Braccini et al., 2005; 
Belleggia et al., 2008). The Shannon-Wiener index ( H '), 
which describes entropy on the basis of information 
theory, was calculated as each stomach that contained 
prey was added to the analysis for 100 randomizations 
of the data for each species size class: 
H' = -'L(p i )-(Xog c p i ), (2) 
i=l 
where S - the number of prey categories; and 
Pi = the proportion of the cumulative (total) sam- 
ple (gut contents) represented by the ith 
prey category. 
Following Jost (2006), H' was converted to effective 
number of species (exp(iL')), a true diversity. Only 
groups (monthly species size classes) with mean tro- 
phic diversity curves that appeared asymptotic or with 
>40 sampled guts were included in the similarity anal- 
ysis (described in the next paragraph). 
To evaluate the degree of similarity in diets between 
species and size classes, nonmetric multidimensional 
scaling (nMDS) and hierarchical clustering were used 
