170 
Fishery Bulletin 107(2) 
H generalist 
<0 planktivore 
▲ carnivore 
G predator 
□ anadromous 
* □ 
O o 
°<§P 0° 0 cfD 
0° ° 
* * © 
A° A « % ffO 
«D 
AD *° *f." 
a ■ £* m m 
* ■ 
* o 
□ □ 
cP 
A. 
D o d d o D 
C3 production 
N 
8 13 C(%°) 
~ I 
-20 
marine 
primary 
production 
Autochthonous 
" production 
Figure 2 
8 15 N vs. <5 13 C values for the four guilds and anadromous Alosa spe- 
cies, showing that most resident freshwater fishes are approximately 
two trophic levels above primary producers (C3 or autochthonous 
production), in contrast to the Alosa spp., whose <5 15 N reveals that 
they are one trophic level above marine primary production. Boxes 
indicate the isotope signature of C3 terrestrial plant primary pro- 
duction, freshwater autochthonous production, and marine primary 
production. Alosa spp. are 13 C-enriched relative to most freshwater 
residents, reflecting marine primary production. 
20 
15 
_ 10 
o 
c o 
-t 
0 
-5 
-30 -25 -20 -15 
8 13 C(%o) 
Figure 3 
<5 34 S vs. 8 13 C values for the four guilds and anadromous Alosa spe- 
cies, with boxes to indicate the isotope signature of C3 terrestrial 
plant primary production, freshwater autochthonous production, 
and marine primary production. Alosa spp. are highly 34 S-enriched 
relative to most freshwater residents, reflecting marine sulfate 
(which becomes incorporated into primary producers and Alosa 
spp. while they grow in the Atlantic Ocean). Predators are the only 
guild showing elevated <5 34 S, indicating the incorporation of marine 
protein derived from Alosa spp. 
■ 
generalist 
* 
planktivore 
A 
carnivore 
G 
predator 
□ 
anadromous 
Marine protein 
O 
O * A 
O □ 
„ a D a 
D Ib (5 n D □ 
o 
A O 
CP 
A° 
A A 
□ A 
O □ . 
o o 
& n 
■ /-V 
OA 
0®, 
C3 production 
Autochthonous production 
of <5 15 N values among all fishes was similar 
to that observed for 6 13 C (10%c). The anad- 
romous fish had the lowest <5 15 N values and 
generally grouped between 12%c and 13%o; 
however, their values were not lower than 
generalists or carnivores. The predators were 
the most 15 N-enriched of any group (Table 2). 
There were no significant differences among 
the d 15 N values for carnivores, generalists, 
and planktivores (Table 2). 
Sulfur isotopes were hypothesized to be 
the most useful for tracing marine protein 
into freshwater, owing to extreme differ- 
ences between the <5 34 S of marine plankton 
and various sulfur sources in freshwater. 
Predator fishes and anadromous Alosa spp. 
showed elevated 34 S signals relative to other 
resident freshwater fishes, indicating that 
the predators incorporated Alosa spp. sulfur 
(protein). The range of <5 34 S values among all 
the fish captured was from approximately 
0 %o to 20%c, a considerably larger range than 
observed for the other two isotopes (Table 
2, Fig. 3). Significant differences were ob- 
served in 6 34 S among several of the separate 
groups. Anadromous species were highly 34 S- 
enriched relative to all resident freshwater 
fish (Table 2, Fig. 2), although the striped 
bass (40 cm total length (TL)) had values 
between 0.3%c and 6.4%c, the lowest of the 
anadromous <5 34 S values. Predators were the 
most 34 S-enriched of the resident fish, fol- 
lowed by planktivores (a trend also observed 
for d 13 C). Carnivores and generalists were 
the most 34 S-depleted of the guilds and were 
not significantly different from each other 
(Table 2). Sulfur was the only stable isotope 
that completely separated the anadromous 
Alosa spp. from the full time freshwater resi- 
dents. All of the Alosa spp. individual values 
were 34 S-enriched and outside the ranges 
observed in the other groups (Table 2). 
Fatty acid analysis 
Fatty acid (FA) isotope values show that some 
predators derive fats from anadromous fish 
and that there is a large variation among FA 
isotope values. FA <5 13 C values were deter- 
mined for one alewife (anadromous), one giz- 
zard shad ( Dorosoma cepedianum, a native 
freshwater planktivore), and two blue catfish 
( Ictalurus furcatus, an introduced piscivorous 
predator). For the blue catfish bulk <5 13 C and 
6 34 S values from muscle tissue showed that 
one individual (A in Table 3) was significantly 
13 C and 34 S-depleted relative to the other. 
This was also the case for the respective 6 13 C 
values of their individual FAs. The anadro- 
mous alewife and the more 13 C-enriched blue 
