Great Blue Herons, a Monitoring Species?
The purpose of this study was to determine whether Great Blue Herons would serve as a good monitoring species for contaminants in piscivorous (fish eating) birds from the Columbia and Willamette basins. To be a good indicator species they should have a wide distribution, high food-chain status, nest fidelity, and low sensitivity to contaminants. Great Blue Herons on the lower Columbia and Willamette Rivers meet all four criteria.
Methods: Juvenile herons may feed in grasslands along the west coast, but adult females typically feed in estuaries marshes and intertidal beaches so chemical residues in eggs are therefore most representative of contamination in aquatic ecosystems. In this study, Great Blue Heron eggs were collected from six colonies in Oregon and Washington during 1994 and 1995. These colonies were on Bachelor and Fisher Island located on the lower Columbia River, Karlson Island in the Columbia River Estuary, Molalla State Park and Ross Island colonies located on the lower Willamette River, and Samish Island in Puget Sound. Samish Island was used as a reference site since a previous study had detected low concentrations of most organochlorines in heron eggs (Cobb 1994 and 1995.) Five eggs were collected per colony. The eggs were dried and then the eggshell thickness was measured. In 1994, 30 eggs were analyzed for organochlorines, congener-specific PCBs, polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and trace elements. In 1995 thirty more eggs were analyzed for organochlorines, non-ortho-chloro-substituted PCBs, dioxins, and furans. Heron reproductive success was also studied. The colonies were visited three times each month during the nesting season, and the chicks were located at all visible nests. The observations continued for 9 weeks past the estimated peak of hatching and reproductive success was reported as the number of chicks fledged per number of occupied nests.
Conclusions: The results supported the hypothesis that environmental contaminants were elevated in great blue herons from the lower Columbia and Willamette Rivers since the concentrations of DDE detected in heron eggs during this study were generally above those detected in recent studies on herons in other areas of the Pacific Northwest. However, adverse effects of DDE on hatchability or reproductive success of great blue herons are unlikely at the concentration observed during this study. The total PCB concentrations at Ross Island in 1994 also exceeded levels detected in previous piscivorous bird studies on the Columbia River. However, great blue herons appear to have a lower sensitivity to dietary PCBs. One egg from Ross Island had total PCBs exceeding the critical values associated with impaired embryo health in chickens and the nest still fledged two chicks. The presence of embryo deformities which have typically been linked to toxicity of TCDD-like compounds and positive correlations between nest failure and TCDD concentrations demonstrated that contaminants were impacting individual herons on the Columbia and Willamette Rivers. However, at the present time, the hatch and fledge rates at all of the sites were similar to those calculated for most other colonies in which reproduction was not impaired so it appears that the contaminants do not impair great blue heron reproduction at the colony level in the Columbia and Willamette Rivers only at the individual level. Results from this study have helped demonstrate that the herons can be used as an indicator species of environmental contamination.
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