MacAvory et al.: Anadromous fish as marine nutrient vectors 
171 
Table 3 
Fatty acid (FA) 6 13 C values for Rappahannock River fish. Means ± 1 Standard Deviation. (n=3). Values are corrected for CH40H 
derevitization. FAs show that carbon from anadromous fish has been incorporated by Ictalarus furcatus but not by other resident 
fishes. Bulk isotope values show trends similar to the FAs and are as follows: alewife A. pseudoharengus, 6 13 C -19.3%e, 6 15 N 
11.9%c, 6 34 S 17.1%e; blue catfish Ictalarus furcatus (A) d L3 C -26.0%e, d 15 N 13.3%e, 6 34 S 6.1%o; 7. furcatus (B) 6 13 C -19.3%e, 6 15 N 
16.6%o, d 34 S 10.8%o; gizzard shad Dorosoma cepedianum 6 13 C -21.5%e, d 15 N 14.5%o, 6 34 S 10.2%c. 
Fatty acid 
Alosa pseudoharengus 
alewife ( %c ) 
Ictalurus furcatus 
blue catfish (%<?) 
A Ictalurus furcatus 
blue catfish (%c) 
B Dorosoma cepedianum 
gizzard shad (%o) 
12:0 
-22.4(0.4) 
-28.5 (0.5) 
-22.5(0.9) 
-27.4(1.0) 
14:0 
-27.4(1.8) 
-33.6(0.9) 
-26.9 (0.6) 
-25.5 (1.4) 
16:1 
-26.8 (0.8) 
-35.4(0.6) 
-25.6(0.7) 
-27.4(0.6) 
16:0 
-22.1 (0.1) 
-30.3(0.2) 
-23.3(0.3) 
-25.7(0.6) 
18:1 
-23.3(0.6) 
-30.5 (0.6) 
-24.5 (0.7) 
-28.7(0.4) 
18:0 
-19.9(1.8) 
-28.8(0.7) 
-20.4 (1.1) 
-23.5 
catfish (B) had d L3 C FA values that, for the most part, 
overlapped with each other. Their 16 and 18 carbon 
length FAs were generally 13 C-enriched relative to the 
gizzard shad and the second blue catfish (A) (Table 3). 
For all fish, except gizzard shad, the saturated 12:0, 
16:0, and 18:0 FAs were more enriched (2%c to 6%c) than 
the 14:0, 16:1 and 18:1 FAs. 14:0 FAs are not elongated 
to 16 or 18 carbons in animals, which is why they are 
13 C-depleted relative to saturated 16:0 and 18:0 (see 
Discussion). For the gizzard shad, the 12:0 FAs were 
2 %c depleted relative to the 14:0 FAs. The blue catfish 
(B) with low 6 13 C and 6 34 S bulk values, generally had 
more 13 C-depleted FAs than other fishes. There was up 
to a 10 %o range among the FAs within an individual fish, 
with unsaturated FAs 13 C-depleted relative to saturated, 
and longer saturated chains being generally 13 C-depleted 
relative to shorter chain FAs (Table 3). 
Discussion 
The fact that the anadromous Alosa spp. were the most 
13 C-enriched of the groups examined was expected 
because they retain the 13 C-enriched (relative to fresh- 
water) signal of marine carbon fixation (Garman and 
Macko, 1998, MacAvoy et al., 2000, Hoffman et al., 
2007). High 6 13 C in freshwater systems with anadro- 
mous fish does not necessarily indicate trophic status 
(Garman and Macko, 1998; MacAvoy et al., 2000; Greg- 
ory-Eaves et al., 2007). The 13 C-enriched predators 
(mostly piscivorous catfish) show a wide range in 6 13 C, 
from -16 to -27%o (white perch also show elevated <5 13 C 
relative to most resident freshwater fish, but they also 
are 34 S-depleted, indicating that their carbon signature 
reflects their status as a secondary carnivore, not marine 
carbon). The most 13 C-enriched of the predators reflect 
the consumption of marine material, probably spawn- 
ing adult Alosa spp., which had the most 13 C-enriched 
values of any prey item found. A number of predators, 
however, clearly derive very little carbon from marine 
migrants; they are strictly freshwater feeders, as shown 
by their 13 C-depleted carbon isotope values. Among the 
remaining three guilds, the planktivores (within which 
the anadromous Alosa spp., mainly filter feeders, were 
not included) were the most 13 C-enriched, driven largely 
by the migratory and filter-feeing gizzard shad (Jenkins 
and Burkhead, 1993). Gizzard shad 13 C enrichment prob- 
ably reflects consumption of autochthonous production 
and not marine derived nutrients, because the gizzard 
shad 6 34 S are too low to reflect substantial marine mate- 
rial (Table 2 and see below). The 6 13 C range among the 
resident freshwater fishes suggest, not surprisingly, 
that both autochthonous and allochthonous production 
contribute to carbon fixation in this tidal freshwater 
stream. Indeed, in the York River estuary, a few kilo- 
meters south of the Rappahannock River, Raymond 
and Bauer (2001) estimate that between 38% and 56% 
of dissolved organic carbon was derived from internal 
(autochthonous) sources. 
Only a small percent of the residents show an ex- 
clusive allochthonous signal in the region of the Rap- 
pahannock River examined, and most of the resident 
freshwater fish show an autochthonous <3 13 C signature, 
which is characteristic of small tributaries close to the 
main stem of a large piedmont river. The d 13 C range 
of allochthonous productivity in Virginia tidal fresh- 
water streams is between -25%e and -28 %c (Garman 
and Macko, 1998; Hoffman et al., 2007). Because C0 2 
solubility is limited in water, systems dominated by au- 
tochthonous production tend to be 13 C-enriched relative 
to C3 plants that appear in small streams dominated 
by C3 allochthonous production (Michener and Schell, 
1994). Garman and Neilson (1992) note that the pres- 
ence of gizzard shad and detritivores in Virginia tidal 
freshwater suggest that autochthonous production is 
important in these systems relative to non-tidal areas 
upstream, where fishes primarily consume terrestrial 
arthropods (Garman, 1991). Most of the guilds exam- 
ined in this study reflected the predominance of autoch- 
thonous production and have d L3 C values that are lower 
