Migratory Patterns of American Shad (alosa Sapidissima) Revealed by Natural Geochemical Tags in Otoliths

Geochemical signatures in the otoliths of diadromous fishes may allow for retrospective analyses of natal origins. In an assessment of river-specific signatures in American shad (Alosa sapidissima), an anadromous clupeid native to the Atlantic coast of North America, stable isotope and elemental ratios in otoliths of juvenile American shad produced accurate natal tags from 12 rivers. Significant inter-annual variability in geochemical signatures from several rivers was detected, due largely to differences in 8180 values among years. The database was further expanded to include 20 rivers from Florida to Quebec, encompassing all major spawning populations. This task was accomplished by collecting juvenile otoliths along with water samples from rivers where juveniles were not sampled. Regressions between otolith and water chemistry for those rivers where both were collected showed significant relationships for Sr:Ca, Ba:Ca, 86180, and 87Sr:86Sr ratios but not for Mg:Ca or Mn:Ca. Despite reducing the combined signature to only four chemical ratios, cross-validated classification accuracies of knownorigin juveniles averaged 93%. Ground-truthed signatures were used to classify migrants of unknown origins. Adults returning to spawn in the York River were classified according to their otolith composition. Only 6% of spawners originated from rivers other than the York, supporting the hypothesis that most American shad spawn in their natal river. Of remaining spawners, 79% originated from the Mattaponi River and 21% from the Pamunkey River. The results suggested that while most American shad home to their natal river there is less fidelity to individual tributaries, allowing subsidies to subpopulations with persistent recruitment failure. Otolith signatures were also used in mixed-stock analyses of immature migrants along the coast of Maine in the spring and Minas Basin in the summer. Mixed-stock compositions showed remarkably low diversity and were dominated by fish from the Shubenacadie and Hudson rivers, with an increasing proportion of Potomac River fish over time. In contrast to results from adult tagging studies, southern stocks were virtually absent. These data suggest ontogenetic shifts in migratory behavior. The thesis concludes with a report that water contributed 83% of Sr and 98% of Ba in the otoliths of a marine fish. Thesis Supervisor: Simon R. Thorrold Title: Associate Scientist, Woods Hole Oceanographic Institution ACKNOWLEDGEMENTS This thesis could not have been possible without the invaluable support from many people. Foremost, I thank my advisor Dr. Simon Thorrold for providing me with an exciting project in which I could explore my interests and the guidance, instruction, and insight to ensure my research was successful. Many thanks also to Dr. Jesus Pineda, Dr. Karin Limburg, Dr. Ed Boyle, and Dr. Gil Rosenthal who served on my thesis committee and offered invaluable advice. This work was funded by a variety of agencies. Support was given by National Science Foundation grants to Dr. Thorrold (OCE-0215905 and OCE-0134998). I received additional support from the Woods Hole Oceanographic Institution (WHOI) Academic Programs Office, the American Museum of Natural History Lerner-Gray Fund for Marine Research, a SEASPACE, Inc. Research Scholarship, and a WHOI Ocean Life Institute Research Grant. Support in the logistics and execution of the work came from numerous sources. Fish were provided by Nate Gray (Maine Department of Marine Resources, Brian Smith (New Hampshire Fish and Game Department), Tom Savoy (Connecticut Department of Environmental Protection), Kathy Hattala (New York State Department of Environmental Conservation), Russ Allen (New Jersey Department of Environmental Protection) , Bob Sadzinski (Maryland Department of Natural Resources), Pete Kornegay and Kevin Dockendorf (North Carolina Wildlife Resources Commission), Doug Cooke (South Carolina Department of Natural Resources) Don Harrison (Georgia Department of Natural Resources), and Rich McBride (Florida Marine Research Institute). Brian Watkins and Kristen Delano (Virginia Institute of Marine Science, VIMS) provided technical support. Water samples were collected with help from Joel Hoffinan, and Nick Trippel. Thanks also to Lary Ball, Dave Schneider, and Scot Birdwhistell at the WHOI Plasma Mass Spectrometry Facility, and Dorinda Ostermann at the WHOI Micropaleo Mass Spectrometry Facility for help with sample analysis. Louis Kerr (Marine Biological Laboratory, Woods Hole) provided microscopy facilities for hatchery screening. Oxygen isotopic ratios in water samples were analyzed by Dr. Peter Swart (University of Miami). Statistical assistance was given by Dr. Vicke Starczak. A special thanks to all current and former members of the Fish Ecology Laboratory at WHOI, including Harvey Walsh, Kelton McMahon, and Leah Houghton; Jennifer FitzGerald provided vital technical and emotional support in my early days at WHOI, and Dr. Travis Elsdon was an outstanding intellectual mentor and an even better friend. Special thanks to Dr. John Olney, whose encouragement and collaboration helped convince me I was on the right career path; and Dr. Lauren Mullineaux, who taught me by example how to balance excellence in teaching, research, and lab management in a successful scientific career. And of course I could not have survived without the love and support of my many friends, including Gareth Lawson, Carly Strasser, Daniel Ohnemus, and Regina Campbell-Malone, my graduate school confidante and best buddy. George Ward earns special commendation as an unfailingly patient and helpful physical oceanographer and peerless stepfather. This work is dedicated to my parents, Judy Walther and David Mahler, who taught me the passion and wonder of science.