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 4 100 80 60 40 20 0 r Gag nappe eS Lan r k ppe berjac a n yS Am Gra ate ish abass mp per ner per per Porgy t f a u p n p r c m a a e u o S To Gr Sn Sn ed eR rigg ank Se d n d u R l T e e o i B l y R R B mi Gra Ver Natural Artificial r Gag nappe eS Lan r r r y mp per ass tate fish org upe Runne nappe p r b m a e o P a r Sca o n g e T G S e kS Red ilion S Trig n Blu y a Red Red a B m Gr Ver 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 -18.5 -18.0 -17.5 -17.0 -16.5 -16.0 -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 -18.0 -17.5 -17.0 -16.5 -16.0 -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. 4. 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. 3.0 2.7 2.4 2.1 k per p rjac a e n b yS Am Gra r Gag nappe eS Lan h er gy mp r per per ner p ass tate a o u p n p rfis b c m a a e o u P a r S o n n g e R T G S e kS Red ilion S Trig n Blu y a Red Red a B m Gr Ver
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