LOW REPRODUCTIVE RATES OF LAKE SUPERIOR BALD EAGLES - LOW FOOD DELIVERY RATES OR ENVIRONMENTAL CONTAMINANTS

Reproductive rate (productivity) of bald eagles (Haliaeetus leucocepha lus) nesting on the shores of Lake Superior was significantly less tha n that of neighboring eagles nesting in inland Wisconsin (1.0 vs. 1.3 young per breeding attempt, 1989-1993), and at other inland lake/river ine habitats in the Great Lakes Basin. It is possible that the current causes of low productivity on Lake Superior might include exposure to organochlorine contaminants and/or low food availability. Levels of d ichloro diphenyl-dichloroethylene (DDE) and total polychlorinated biph enyls (PCBs) in addled eggs and eaglet blood from Lake Superior and in land Wisconsin reference sites were measured. Food delivery rates by p arent eagles to nestlings, a possible index to food availability, were quantified at both locations. Concentrations of both DDE and total PC Bs in addled eggs declined significantly front 1969 to 1993 (p < 0.001 , p = 0.006 respectively), and current concentrations of DDE are at or below the no observable adverse effect level (NOAEL) for reproductive impairment. Concentrations of DDE and total PCBs in plasma were great er in individual nestlings from the shores of Lake Superior than in ne stlings at inland locations (18.9 mu g/kg vs. 3.0 mu g/kg DDE, p < 0.0 01, and 109.1 mu g/kg vs. 42.6 mu g/kg, p = 0.002), but were not corre lated to the 5-year average history of productivity for the territory (p > 0.05). Food delivery rates by parent eagles to nestlings at Lake Superior were 56% lower than those to inland nestlings (2.16 vs. 4.87 prey items per day, p = 0.002). Food delivery rates were significantly correlated to average 5-year productivity for inland Wisconsin refere nce sites (p < 0.001, r(2) = 0.90), although not for Lake Superior sit es (p = 0.593). It is concluded that it is likely that the current low productivity of Lake Superior eagles is at least partly attributable to low food availability, but some other factor, possibly PCBs, may al so contribute to low productivity.