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The Long Term Effects of Fishing Down the Food-Web and Oceanographic Variability on Seabird Diets in the California Current System, 1890-2004.

PI: Drs. Steven R. Beissinger, University of California and Benjamin H. Becker, Pacific Coast Science and Learning Center (CA)*

*External grantees

Seabirds depend upon the marine fish that they feed on for their survival. While humans have had dramatic impacts on coastal marine ecosystems worldwide, one of the most potentially dangerous has been the overfishing of higher trophic level species. This top-level overfishing has cascading impacts upon lower trophic levels and changes community structure, species dominance and ecosystem characteristics. This is part of a phenomenon called “fishing down the food web,” in which commercial fisheries fish out top predators and then shift to lower trophic level fishery resources. Because of this, trophic interactions observed today might be artifacts of structural changes to marine communities that were directly caused by overfishing.

The impacts of overfishing on fish populations are often difficult to distinguish from natural variability in fish stocks caused by ocean climate variation. The Institute for Ocean Conservation Science is supporting an investigation of how the trophic level of five marine seabird predators has varied over the past century, and whether this variation is primarily due to the overfishing of prey, cyclic changes in ocean temperature, or the interaction of both of these processes. Seabirds offer an opportunity to track changes in the marine food web because they are environmental samplers and their diets reflect the composition of the prey community.

The study is being conducted by Dr. Steven Beissinger, Professor of Conservation Biology at the University of California at Berkeley, and Dr. Benjamin Becker, Director of the Pacific Coast Science and Learning Center in Point Reyes, CA. They are using stable isotopes from seabird feathers as an indicator of trophic level. This study is unique in that it explicitly considers both oceanographic changes and fishing pressure, and attempts to tease apart their interactions during a 100-year “experiment” involving multiple marine system interactions for five seabird species. The species include both omnivores and birds dependent upon particular types of fish as prey. The marine system interactions being tested include era, season, and two climate conditions: Pacific Decadal Oscillation (PDO), a long-lived El Niño-like pattern of Pacific climate variability; and, El Niño-Southern Oscillation (ENSO), an oscillation of the ocean-atmosphere system in the tropical Pacific that impacts weather around the globe.

Past studies have shown shifts in marine community dominance occurring since the 1950s. This study will extend this understanding to seabirds and as far back to the 1880s using feather samples from museum collections. So far, stable isotope values for d15N and d13C (d stands for the greek symbol Delta, which is used for isotopes) have been characterized from over 1,900 birds dating from the 1880s to present in the California Current System. Nitrogen isotopes are useful for distinguishing trophic level, or how high up in the food web the species is feeding, with higher values indicating higher position. Declines in prebreeding trophic level (d15N) are apparent in four seabird species -- Marbled Murrelets, Tufted Puffins, Pelagic Cormorants, and Cassin's Auklets – which indicates that as time has progressed these species are preying on smaller and smaller fish, which may reflect a reduced availability of larger predatory fish due to recent overfishing. No such effect was observed in the Common Murre, however, which is the species with the broadest diet. Postbreeding diets of all species show less change over time. Analyses are in progress to distinguish between the effects of climate variability and fish harvest levels on the observed changes in seabird diet.

More information

Dr. Steven Beissinger's bio

Benjamin H. Becker, PhD
Dr. Becker is Director and marine ecologist at the Pacific Coast Science and Learning Center at Point Reyes National Seashore (National Park Service), California. He conducts research on marine mammals, seabirds, fishes, and marine invertebrates that aims to understand the relative effects of normal oceanographic variation (such as El Niño) versus human-induced pressures (overfishing, global climate change, disturbance) on marine ecosystems and species. Dr. Becker also works in stable isotope ecology, statistical analysis (frequentist and Bayesian) of complex, long-term datasets, and occasionally GIS modeling. As Director of the Pacific Coast Science and Learning Center, he oversees and coordinates research by scientists, and graduate, college and high school students that helps us
understand marine and coastal ecosystems. Additional interests include the development and implementation of marine science education and outreach programs for underrepresented students and the communication of science to both resource managers and the public. Dr. Becker currently coordinates the national Research Learning Center program for the National Park Service. He earned two Bachelor's degrees from UCLA(1994), a Master’s degree from Yale (1996), and a Ph.D. from UC Berkeley (2001).

Steven R. Beissinger, PhD
Dr. Steven R. Beissinger is a Professor of Conservation Biology in the University of California at Berkeley in the Department of Environmental Science, Policy & Management. He currently serves as the chair of this department and the Ecosystem Sciences Division. He earned a B.S. (1974) and M.S. (1978) in zoology at Miami University, and a Ph.D. in Natural Resource Ecology at the University of Michigan (1984).

Dr. Beissinger joined the faculty at Berkeley in 1996 after spending 8 years as a professor at Yale University and two years as an NSF Postdoctoral fellow in Environmental Biology at the Smithsonian Institution’s National Zoological Park in Washington D.C. Beissinger teaches undergraduate and graduate courses in conservation biology, demography and genetics of small populations, and behavioral and population ecology.

Professor Beissinger’s research has been conducted primarily with birds but has included work with plants, mammals, aquatic invertebrates, and herps. His current work focuses mainly on: (1) field studies of the ecology, demography and monitoring of endangered or exploited species; (2) demographic models of population viability and recovery, and (3) field studies of parental care strategies and mating systems. Field studies have included parrots, raptors (Snail Kites), passerines and seabirds (Marbled Murrelets) in the U.S. and internationally. This work has resulted in 100 articles in scientific journals, books and technical reports.
Dr. Beissinger's lab website at UC Berkeley

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