Productivity, diets, and environmental contaminants in nesting bald eagles from the aleutian archipelago

Abstract

Abstract-We studied productivity, diets, and environmental contaminants in nesting bald eagles from the western Aleutian Islands, Alaska, USA, during the summers of 1993 and 1994. Productivity on Adak, Tanaga, and Amchitka Islands ranged from 0.88 to 1.24 young produced per occupied site and was comparable to that of healthy populations in the lower 48 United States. However, productivity on Kiska Island was depressed, averaging 0.67 young per occupied site. The lower reproductive success on Kiska was associated with elevated levels of dichlorodiphenyldichloroethylene and other organochlorine pesticides. Many of the organochlorine pesticides were elevated in bald eagle eggs from the four islands, but concentrations of these contaminants and Hg were significantly higher in eggs from Kiska Island than in eggs from the other islands. In contrast, polychlorinated biphenyl concentrations were higher in eggs from Adak, Amchitka, and Kiska (where military facilities have been installed) than in those from Tanaga (which has had little military activity). The most likely source of these contaminants in bald eagles was from their diets, which were variable spatially and temporally. Fish made up most (56%) of the eagles' diet on Adak and Tanaga Islands, followed by birds (25%) and mammals (19%). In contrast, birds comprised the majority (60%) of bald eagle diets on Amchitka and Kiska Islands, followed by mammals (30%) and fish (10%). The high proportion of seabirds in the diet of eagles from Kiska Island could be the major source of organochlorine and Hg contamination. Elevated concentrations of organochlorines in bald eagle eggs from the Aleutian Archipelago was surprising, because of the distance to agricultural areas. The results indicate that these contaminants can be transported long distances and affect wildlife populations in remote and pristine areas. We also discuss potential sources and transport mechanisms of these contaminants to the Aleutian Islands.

Elemental and Organochlorine Residues in Bald Eagles From Adak Island, Alaska

“…Historic military sites and activities in the Aleutian Islands may be a major contributor to elevated PCB residues in Aleutian Island sea otter and eagle populations (Estes et al, 1997). In addition, birds migrating from Asia may contain DDT-related compounds and thus may provide a source of secondary poisoning in Aleutian Island eagle populations that prey on them (Anthony et al, 1999). Contaminant sampling in resident and transitory prey species from Adak and other Aleutian Islands will help to distinguish between local and distant sources.…”

“…Summary statistics (geometric means and ranges) were determined for all analytes detected in Ն50% of the samples (Tables 1 and 2), with non-detect values substituted with one half of the detection limit (Frenzel and Anthony, 1989;Anthony et al, 1999). Statistical comparisons among collection years, age classes, and between sexes were performed on analytes detected in 70-100% of the samples within the same tissue type.…”

“…Top-level predators, such as bald eagles (Haliaeetus leucocephalus), are believed to be especially vulnerable to many contaminants and can be used as sentinel species for contaminated areas (Welch, 1994;Holl and Cairns, 1995). In Alaska's Aleutian Islands (USA), bald eagles prey primarily on fish and birds (Anthony et al, 1999), though community landfills may also represent an important food source, particularly during winter (Byrd et al, 1974; U.S. Department of Agriculture, 1990). Both natural and anthropogenic food sources are potential routes of contaminant exposure for these birds.…”

“…Of those, only two studies reported exclusively on Alaskan eagle populations (Estes et al, 1997;Anthony et al, 1999). Tissue studies of contaminants in bald eagles are also available, but are of limited applicability to Alaskan eagles (Kaiser et al, 1980;Anthony et al, 1993;Welch, 1994).…”

“…Of those, only two studies reported exclusively on Alaskan eagle populations (Estes et al, 1997;Anthony et al, 1999). Tissue studies of contaminants in bald eagles are also available, but are of limited applicability to Alaskan eagles (Kaiser et al, 1980;Anthony et al, 1993;Welch, 1994). This report examines contaminant residues in bald eagle tissues from Adak Island, which supports a non-migratory and reproductively stable eagle population (U.S. Department of Agriculture, 1990;Estes et al, 1997).…”