Trophic interactions among piscivorous birds and critical fishery resources of the Chesapeake Bay region
Dr. Greg Garman, director of VCU’s Center for Environmental Studies and Cathy Viverette, CES research associate, are participating in a multi-institution and cross-disciplinary study of the trophic relationships among fish-eating birds and selected fishery resources within the Chesapeake Bay and its major tributaries. Project partners include Dr. Bryan Watts of the W&M-VCU Center for Conservation Biology, James Uphoff of the Maryland Department of Natural Resources and Dr. Stephen Macko of the Department of Environmental Sciences at UVA.
In spite of significant population growth and geographic expansion by piscivorous (fish-eating) birds in the Chesapeake Bay and its tidal tributaries, the impact of avian predation and competition on marine, estuarine and riverine fish assemblages has not been quantified or incorporated into ecosystem models. Similarly, the potential role of fishery population dynamics in regulating populations of bird species that are of national conservation concern has never been evaluated within the region. In fact, Chesapeake Bay ecosystem models typically ignore avian predators and competitors, and fishery stock assessments for the region generally fail to incorporate these potentially important ecological interactions. Several ecologically, culturally and economically important Chesapeake Bay fishes, including Atlantic menhaden (Brevoortia tyrannus) and American shad (Alosa sapidissima), contribute substantially to the diets of fish-eating birds. Thus, piscivorous birds, which are used widely as a sentinel species for tracking ecosystem health elsewhere, may be useful indicator species for fishery population status and trends in the Chesapeake Bay.
This ongoing study includes an historical analysis of bird and fishery interactions across long (decadal) and short (seasonal) temporal scales, using existing and unpublished datasets for major predator and prey species. The research is working toward developing a novel, fishery-independent tool for stock assessment of Atlantic menhaden and American shad by identifying diagnostic isotopic markers that will allow tracking of Atlantic menhaden and alosine population trends using feathers from sentinel bird species such as ospry. The goal of this research is to allow natural resource managers to track and forecast responses of avian and fish communities to management and restoration policies and efforts and to contribute substantially to the overall accuracy of Chesapeake Bay ecosystem models.
Retrospective isotopic analysis
An analysis of stable isotopes in feathers collected from bald eagles and osprey from the Chesapeake Bay from 1850 to 2009 is being conducted to estimate historical trends in the contribution of anadromous fishes to the diets of these birds. Feathers from bald eagles and osprey from museum specimens and active nests are being analyzed for C, N and S stable isotopes. We are especially interested in evaluating the hypothesis that upstream migrations of anadromous clupeid fish represent, at least historically, an ecologically important seasonal energy subsidy in the form of marine-derived organic matter to piscivorous birds nesting within the Bay’s tidal tributaries.
Prey consumption by piscivorous birds
Diet analysis is being conducted for osprey, double-crested cormorants and brown pelicans in nesting colonies in a variety of locations in the Chesapeake Bay and its tributaries. We are assessing diets by systematically walking through colonies when young nestlings are on nests and recording the species, number and size of regurgitated fish associated with the nests of each species.
Species-specific biomarkers for Atlantic Menhaden and other target fish
We are conducting studies to determine if osprey feathers contain biomarkers, such as fatty acids, specific to Atlantic menhaden and other commercially important fish. Included in this work are experiments whereby we are supplementing some osprey nests with menhaden or shad, while other nests receive no food supplement. Feathers of the young birds are then collected and analyzed for potential biomarkers. Establishing specific biomarkers for menhaden or shad would help with Bay wide monitoring of the health of their populations and their importance in the Bay food web.