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Trophic Structure of Midwater Fishes Over Cold Seeps in the North Central Gulf of Mexico

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Trophic Structure of Midwater Fishes Over Cold Seeps in the North Central Gulf of Mexico TROPHIC STRUCTURE OF MIDWATER FISHES OVER COLD SEEPS IN THE NORTH CENTRAL GULF OF MEXICO Jennifer P. McClain-Counts A Thesis Submitted to the University of North Carolina Wilmington in Partial Fulfillment of the Requirements for the Degree of Master of Science Center for Marine Science University of North Carolina Wilmington 2010 Approved by Advisory Committee Steve W. Ross Lawrence B. Cahoon Chair Joan W. Willey Accepted by Dean, Graduate School TABLE OF CONTENTS ABSTRACT....................................................................................................................... iv ACKNOWLEDGMENTS ................................................................................................. vi DEDICATION.................................................................................................................. vii LIST OF TABLES........................................................................................................... viii LIST OF FIGURES ........................................................................................................... xi INTRODUCTION ...............................................................................................................1 METHODS ..........................................................................................................................4 Study Area................................................................................................................4 Sample Collection ....................................................................................................5 Dietary Analyses ......................................................................................................6 Gut Content Analyses ...................................................................................6 Stable Isotope Analyses ................................................................................7 IsoSource Mixing Model ............................................................................10 Trophic Position Analyses......................................................................................10 Statistical Analyses.................................................................................................11 RESULTS ..........................................................................................................................13 Catch Data ..............................................................................................................13 Gut Content Analyses.............................................................................................13 Diet composition .........................................................................................13 Factors influencing diet composition..........................................................17 Stable Isotope Analyses..........................................................................................19 Trophic Position Calculations ................................................................................23 ii DISCUSSION....................................................................................................................24 Diet composition ....................................................................................................24 Spatial and Temporal influences on diet ................................................................30 Additional insight with SIA....................................................................................32 Site differences............................................................................................32 Diet variations .............................................................................................33 Methodology...........................................................................................................34 Interesting Note ......................................................................................................35 CONCLUSIONS................................................................................................................36 LITERATURE CITED ......................................................................................................37 iii ABSTRACT Midwater fishes are an important component of pelagic food webs and provide insight into energy utilization and movement through the water column. In this study, the diets of midwater fishes collected over cold seep habitats were examined to determine general feeding patterns and whether size, depth, time of day or location affected diet composition within fish species. The base of the midwater food web was also examined to determine whether chemosynthetic energy in benthic cold seeps was incorporated into the midwater fish community. Discrete depth Tucker trawling was conducted in August 2007 over three cold seep habitats (> 1000 m) in the north- central Gulf of Mexico. Surface sampling was also conducted to provide a prey base (zooplankton and POM) for stable isotope analyses (SIA). Gut content analysis (GCA) and SIA (δ13C and δ15N) in conjunction with IsoSource software were utilized for diet reconstruction and to determine trophic positions. SIA also aided efforts to determine chemosynthetic influences on the midwater food web. GCA was performed on 31 species in the five most abundant families (Gonostomatidae, Myctophidae, Phosichthyidae, Sternoptychidae and Stomiidae), with midwater fishes classified into one of three guilds: piscivore, large crustacean consumer, or zooplanktivore. SIA was performed on 6 fish families (Gonostomatidae, Myctophidae, Phosichthyidae, Sternoptychidae, Stomiidae, and Melamphaidae), 13 invertebrate categories, and 3 primary producers (POM, Sargassum spp. and detritus), and classified all fishes as zooplanktivores. Using IsoSource, more precise contributions of individual prey taxon were documented, which did not always support results from GCA. Size, depth, time of day and location did not affect diet composition within a species; however migration trends suggested competition may be reduced by feeding over a range of depths and over a 24 hour period. Significant differences in trophic position calculations between GCA and SIA highlighted the iv importance of using multiple techniques to describe trophic structure, as each method characterized the diets differently. v ACKNOWLEDGMENTS This project was largely funded by the Department of the Interior U.S. Geological Survey under Cooperative Agreement No. 05HQAG0009, sub agreement 05099HS004. I thank the crew of the R/V Cape Hatteras and all scientific personal for assisting with fishing operations and sample processing. S. Artabane, A. Quattrini, and A. Roa-Varon assisted with fish identifications and C. Ames assisted with invertebrate identifications. Guidance and support during stable isotope analyses were provided by Drs. A. Demopoulos and C. Tobias, and K. Duernberger. I would also like to thank S. Artabane, T. Casazza and A. Roa-Varón for their assistance in dissecting and processing fish stomachs. Special thanks to my committee, Drs. S. Ross, L. Cahoon, and J. Willey, for their guidance and support during the duration of this project. I would additionally like to thank my advisor, Dr. S. Ross, for setting me up with this project and Dr. L. Cahoon for his assistance with statistics. Finally, thanks to S. Ross, T. Casazza, A. Demopoulos, A. Quattrini, L. Truxal and M. Carlson for their suggestions and edits provided throughout the writing process of this thesis. vi DEDICATION I would like to dedicate this thesis to my parents, who encouraged my early passion in marine science and gave me the confidence to follow my dreams and overcome any obstacles. Your constant love and support was unwavering and because of that, I can present this Masters project. vii LIST OF TABLES Table Page 1. Surface and midwater stations sampled over three cold seep sites (AT340, GC852, and AC601) (see Fig.1) in the Gulf of Mexico (9-25 August 2007)................................................................................................48 2. The total number of all midwater fishes, invertebrates and autotrophs examined in dietary analyses from the North-central Gulf of Mexico ......................................................................................................55 3. Results of ANOSIM comparing effects of size, time of day, depth and location on the general prey categories consumed for each fish species.............................................................................................................58 4. Percent volume and frequency of prey items consumed by Chauliodus sloani collected from three sites in the Gulf of Mexico (AC601, GC852, AT340) separated by time of day..............................................59 5. Percent volume and frequency of prey items consumed by Gonostoma elongatum collected from three sites in the Gulf of Mexico (AC601, GC852, AT340) separated by time of day ................................60 6. Percent volume and frequency of prey items consumed by Stomiidae collected from three sites in the Gulf of Mexico (AC601, GC852, AT340) separated by time of day..............................................62 7. Percent volume and frequency of prey items consumed by Cyclothone alba collected from three sites in the Gulf of Mexico (AC601, GC852, AT340) separated by time of day..............................................63 8. Percent volume and frequency of prey items consumed by Cyclothone braueri collected from
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