Great Blue Herons, a Monitoring Species?

Great Blue Heron photo: Klamath Basin RefugeThe 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.

Biologist Climing to Nest Photo: USFWS Oregon State OfficeMethods: 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.

Results:
Carp Photo: USFWS/Duane RaverContaminants in Eggs and Prey:
Eggs from Ross Island had the highest mean concentrations of all measured organochlorines with the concentrations of DDE being significantly greater at Ross Island than at Fisher and Samish. Concentrations of trans-nonachlor were significantly greater at Ross Island compared to Bachelor, Fisher, and Samish.  Total PCB concentrations were also significantly higher at Ross Island than at most other sites. Organochlorines concentrations were also highest in heron prey from Ross Island with total PCB concentrations being 4.8 to 31 times greater compared to all other sites.  The PCB concentrations were highest in large-scale suckers, carp, peamouth chub, and trout. All sites had significantly reduced shell thickness compared to the pre-DDT mean. It was also observed that  shell thickness and DDE concentration in individual eggs were negatively correlated. Ross Island showed the most shell thinning being two and three times greater than at Bachelor and Karlson respectively.

GBH colony Molalla Photo: USFWS Oregon State OfficeReproductive Success:
Clutch size did not differ among sites. However, in 1994 hatching success was significantly greater at Karlson compared to Molalla and Samish.  Karlson also had the highest fledging success during both years.  Fledging and reproductive success were not correlated with concentrations of DDE, however, the data suggested that there may be a correlation with TCDD concentrations since deformities were found in pipping embryos from 4 of the 6 sites.  A larger sample size would clarify this relationship. Karlson had the highest percentage of failed nests.  Most nests at Karlson either fledged three to four chicks or completely failed where at all other sites they hatched and fledged between one to three eggs and chicks.  Site means for nest failure were positively correlated with mean TCDD concentration.  In addition, individual fledge success was negatively correlated with TCDD concentrations detected in eggs from the same nests.

Biomagnification Factors:
Karlson had the greatest biomagnification factor (increase of tissue accumulation in species higher in the natural food chain as contaminated food species are eaten) for all contaminants except PCB 169.

GBH colony Photo: USFWS Oregon State OfficeConclusions: 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. 

Learn more by reading the following full report:
Thomas, CM and Anthony RG.  Environmental Contaminants in Great Blue Herons (Ardea Herodias) from the lower Columbia and Willamette Rivers, Oregon and Washington, USA.  Environmental Toxicology and Chemistry, Vol 18, No. 12, pp 2804-2816, 1999.

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