Influence of food supply and chlorinated hydrocarbon contaminants on breeding success of bald eagles.

Food supply and contaminants were investigated as possible causes of low bald eagle productivity near a bleached kraft pulp and paper mill at Crofton on Vancouver Island, British Columbia. Over a seven year period, 1992-1998, average productivity of five eagle territories situated south of the pulp mill at Crofton was significantly lower (0.43 young/occupied territory) than six territories north of the mill (1.04 young/occupied territory). A reference population of 32 territories located in Barkley Sound on the west coast of Vancouver Island demonstrated intermediary mean productivity (0.75 young/occupied territory). Measures of prey biomass delivered to nests were lowest south of the mill, and correlated significantly with nesting success. On average, measures of energy delivered to nests and a parameter determined to be related to prey availability, adult nest attendance time, accounted for about 70% of variability in nest success. Contaminant concentrations, including pulp mill derived polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), as well as dichlorodiphenyldichloroethane (DDE), polychlorinated biphenyls (PCBs), and calculated tetrachlorodibenzo-p-dioxin toxic equivalents (TEQs) were significantly greater in plasma samples of nestlings from south of the mill compared to the other two sites, but did not correlate significantly with individual nest success data. Nests south of the mill concentrate around Maple Bay, which appears to be a deposition area for contaminants transported by tides and currents from sources such as the pulp mill. Concentrations of DDE and PCBs in plasma of nestling eagles from south of the mill were less than the critical values estimated to affect production of young. For TEQs, there are no published critical values for plasma by which to compare our results. We conclude that less than adequate energy provisioning to nests, presumably related to low prey availability, was likely the main cause of poor nest success south of the mill site at Crofton. However, higher concentrations of both DDE and PCDD/F derived TEQs may have acted in concert with food stress to further reduce bald eagle productivity.