Low Reproductive Rates of Lake Superior Bald Eagles: Low Food Delivery Rates or Environmental Contaminants?

Abstract

Reproductive rate (productivity) of bald eagles ( Haliaeetus leucocephalus) nesting on the shores of Lake Superior was significantly less than that of neighboring eagles nesting in inland Wisconsin (1.0 vs. 1.3 young per breeding attempt, 1989–1993), and at other inland lake/riverine 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 dichloro-diphenyl-dichloroethylene (DDE) and total polychlorinated biphenyls (PCBs) in addled eggs and eaglet blood from Lake Superior and inland Wisconsin reference sites were measured. Food delivery rates by parent eagles to nestlings, a possible index to food availability, were quantified at both locations. Concentrations of both DDE and total PCBs in addled eggs declined significantly from 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 greater in individual nestlings from the shores of Lake Superior than in nestlings at inland locations (18.9 μg/kg vs. 3.0 μg/kg DDE, p < 0.001, and 109.1 μg/kg vs. 42.6 μg/kg, p = 0.002), but were not correlated 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 reference sites (p < 0.001, r 2 = 0.90), although not for Lake Superior sites (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 also contribute to low productivity.