Analysis of Reef Fish Diet and Trophic Position at Natural and
Artificial Reefs in the Northeastern Gulf of Mexico
Joseph H. Tarnecki1, William F. Patterson III1, Dustin Addis1,2
1University
of West Florida – Department of Biology – Pensacola, Florida; 2Florida Fish Wildlife Research Institute –St. Petersburg, Florida
Email: [email protected], [email protected], [email protected]
INTRODUCTION
Figure 2. Plots of percent diet by prey category mass for predominant reef fish taxa.
Figure 5. Estimate of the %pelagic versus benthic production consumed by reef fishes
Unhatched bars = natural reefs; hatched bars = artificial reefs. Samples size is above bar.
based on muscle 34S values estimated with IR-MS.
Artificial reefs have been widely deployed on the northern Gulf of Mexico
1 2
ecologically, as compared to natural reefs and with respect to fish, is to examine
habitat effects on fish trophic ecology1. Therefore, we tested for differences in fish
diet and trophic position among reef fish species that occurred on both habitat types
as part of an ongoing study aimed at examining differences in ecological and fishery
functions of natural versus artificial reefs. Analyses conducted included gut content
analysis of captured fishes, along with C, N, and S stable isotope analysis of muscle
1 1
1
3 6
2
1 2
85 69
17 6
19 26
2 5
5 2
2 2
100
Fish
Decapoda
Stomatopoda
Cephalapoda
Gastropoda
Amphipoda
Other Inverts
Zooplankton
80
60
40
20
0
per erjack
p
a
n
mb
yS
A
a
r
G
%Pelagic
Production
reefs. A key component to understanding how artificial reef systems function
Percent Mass
(GOM) shelf but little is known about how they function ecologically versus natural
1 4
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80
60
40
20
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Natural
Artificial
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samples with isotope ratio-mass spectrometry (IR-MS). Gut content analysis is useful
for determining specific, recent (hours to days) diet, but limitations are that
 Variability in δ13C, δ15N and δ34S in fish muscle samples is consistent with the variable
differential digestion/evacuation rates typically exist for soft versus hard bodied prey,
diets observed for many species (Fig. 3), although small stomach sample sizes for many
and the fact that fish coming from depth often have distended stomachs thus no prey
species likely failed to capture the full variability in diets (Fig. 2).
are present2. No details as to specific diet can be inferred from muscle stable isotope
analysis, but one can infer trophic position from
15N,
while
13C
and
34S
values are
useful for estimating the source of primary production and whether fish forage on
 IR-MS analysis revealed a range in TLs were present among sampled fishes (Figs. 3A,
4), with lane snapper, amberjack and gag feeding at the highest level and bank seabass
and other invertivore/piscivores approximately 0.5 TL below those fishes.
pelagic or benthic prey3.
CONCLUSIONS
Stomach content data for some species examined in this study, such as red
snapper, vermilion snapper, and red porgy, provide insight into their predominant prey,
breadth of diet, and trophic position. Sample sizes for most other species are currently
too small to draw much inference with respect to their trophic ecology, and even for
Within species, fish tended to feed at higher TLs at artificial versus natural reefs, but
the three species listed above small sample sizes would limit statistical power in
mean difference was only 0.1 TL between habitat types.
testing for differences in diet between natural and artificial reefs. Perhaps it is no
 δ34S values indicate most fishes had similar mixes of pelagic versus benthic prey at
coincidence that the three species with the broadest observed diets also had the
 Fish sampled at natural (n = 30) and artificial (n = 31) reefs in northern GOM (Fig. 1)
natural versus artificial reefs, yet estimates of source C indicate a higher percentage of
highest sample sizes for stomach content analysis.
Stomachs extracted, fixed in 10% formalin, and preserved in 70% isopropyl alcohol
phytoplankton versus benthic microalgae C at artificial reefs.
METHODS
Muscle stable isotope ratios provide much greater information about the trophic
ecology of sampled reef fishes. The three fish that predominantly had fish prey in their
Prey items enumerated and identified to the lowest taxonomic level possible
stomachs, gag, amberjack, and gray snapper, also were estimated to feed at the
Figure 3. Scatterplots of
amphipods, gastropods, cephalopods, stomatopods, decapods, other inverts, and fish
A) δ15N vs. δ13C (± SE)
Muscle samples dried at 60°C for at least 24 h; δ13C, δ15N and δ34S analyzed with
depicting interspecific
isotope ratio mass spectrometry and reported in standard delta notation
B) δ34S vs. δ13C used
estimated4:
TLfish = 1 +
(15Nfish
Percent pelagic (versus benthic) derived production
(34Sfish - 34Sbenthic) x 100
%Pelagic
Production =
(34Spelagic - 34Sbenthic)
Figure 1: Study reef
sites (n=61) located
in the northeastern
GOM. Natural reefs =
green; artificial reefs
= pink.
pelagic prey sources.
estimated5:
14
range in d15N values. The omnivorous red snapper had d15N values right in the center
of the overall range, consistent with its broad diet. Among the piscivores, gag is
13
inferred to have had the highest percentage of benthic prey based on its low d34N
12
to infer benthic vs.
– 7.1)/3
highest trophic level based on muscle d15N values, and fishes that principally had
A
invertebrate or zooplankton prey in their stomachs clustered at the lower end of the
values, while the planktivore/invertivore vermilion snapper and the piscivore scamp fed
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-17.5
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-15.5
almost exclusively on pelagic prey.
Overall, study results do not indicate substantial intraspecific differences in diet
, where 34Spelagic = 19.5‰, 34Sbenthic = 15‰
20
19
34S ‰
Fish trophic level (TL)
trophic differences and
15
15N ‰
Diet sorted into 8 categories for diet comparisons by %mass: zooplankton,
Pensacola Bay
Mobile
Bay
or trophic position at natural versus artificial reefs. Lastly, data presented here provide
B
a baseline to assess whether organic carbon from the Deepwater Horizon oil spill
18
becomes transferred from biodegrading bacteria into the marine foodweb, as well as if
17
other trophic effects in the reef fish community occur due to the spill.
16
Natural
15
Artificial
14
-18.5
ACKNOWLEDGMENTS
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-16.5
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-15.5
We thank the Florida Research and Wildlife Research Institute (FWRI) for
 C‰
13
funding. We also thank Cpts. Seth Wilson, Johnny Green, Sean Kelly, and Gary Jarvis
for their participation in sampling, along with the numerous volunteers who aided
Area
Shown
Gulf of Mexico
Gulf of Mexico
Figure 3. Estimated trophic level of fishes based on analysis of 15N in muscle tissue
sampling and sample processing.
samples with IR-MS.
REFERENCES
33 fish species were sampled from June 2009 to September 2010; 77% (657/855) of
stomachs had prey present, 47% (406/855) had identifiable prey.
 Among the predominant fish sampled, fish dominated prey observed in gag, lane
snapper, and amberjack stomachs; vermilion snapper and red porgy fed on the
lowest trophic level; and, red snapper had the most diverse diet (Fig. 2).
Trophic Level
RESULTS
3.9
Natural
3.6
Artificial
3.3
1.
Bohnsack JA. 1989. Are high densities of fishes at artificial reefs? Bull. Mar. Sci. 44:631-645.
2.
Hyslop EJ. 1980. Stomach contents analysis-a review of methods and their application. J. Fish
Bio. 17:411-429.
3.
Fry, B. 2006. Stable Isotope Ecology. Springer. New York, NY. 308 pp.
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Fredriksen, S. 2003. Food web studies in a Norwegian kelp forest based on stable isotope
analysis. Mar. Ecol. Prog. Ser. 260:71-81.
5.
Rooker JR, Turner JP, Holt SA (2006) Trophic ecology of Sargassum-associated fishes in the
Gulf of Mexico determined from stable isotopes and fatty acids. Mar. Ecol. Prog. Ser.
313:249-259.
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