Organochlorine Contaminants in 
Eggs of Double-Crested Cormorants

Double Crested Cormorant Colony Photo:  USFWS Carol SchulerThis investigation was designed to determine the concentrations of organochlorine contaminants in eggs of double-crested cormorants nesting on East Sand and Rice Islands in the lower Columbia River.  This study compared concentrations between the islands and to a reference area (Hunters Island, located just off the southern Oregon Coast).  It evaluated the toxicological significance of the compounds and examined the accumulation pattern of contaminants in the eggs.  East Sand and Rice Islands are exposed to numerous environmental contaminants from the Columbia River.  The river drains an area encompassing 260,000 square miles, and receives contaminants through permitted municipal and industrial discharges, urban and industrial non point pollution, agricultural runoff, accidental spills of oil and hazardous materials, and atmospheric deposition.  Exposure to contaminants carried by the river threatens the viability of fish and wildlife in the lower Columbia River and on the Lewis and Clark National Wildlife Refuge (NWR).  Previous studies have documented a variety of environmental contaminant problems affecting fish and wildlife resources.  In these studies contaminants including dioxins, furans, polychlorinated biphenyls (PCBs), organochlorine pesticides,and their metabolites have been found in fish and wildlife from areas on or near the Lewis and Clark NWR at potentially hazardous concentrations.

Double Crested Cormorant nest Photo:  USFWS Carol SchulerMethods: The double crested cormorants are useful indicators of contaminant conditions in upper food-chain level species so the information gathered in this study can be used to assess possible effects to other fish-eating birds such as bald eagles.  Eggs were collected during incubation from Rice Island in 1990 and from both Rice and East Sand Islands in 1991, 1993, 1994, and 1995.  In 1993, eggs were also collected from the reference colony at Hunters Island.  The eggs were measured for length, width, whole egg mass, and volume.  Eggshell thickness was measured on 68 eggs which had been rinsed with water and air dried for a minimum of 30 days.  Eggshell thinning was determined as the percent difference in thickness between each eggshell and the mean eggshell thickness determined for museum specimens collected in the Northwest prior to 1947.  Between 10 and 22 eggs each year were analyzed for organochlorine pesticides, TCDD, TCDF, total PCBs, a variety of PCDDs and PCDFs at a variety of laboratories.

Results: Total PCBs and p,p'-DDE were the most elevated contaminants in eggs from the Columbia River Islands during all years of the study.  Concentrations of p,p'-DDT, p,p'-DDD, dieldrin, HCB, heptachlor epoxide, and cis-nonachlor were also consistently above detection limits.  Eggs from Rice Island exhibited higher total PCB concentrations than eggs from East Sand Island in 1995, 1994, and 1991.  Eggs from Rice Island also had higher concentrations of p,p'DDE, p,p'-DDT, cis-chlordate/octa-chlordane, alpha-BHC, HCM, and mirex in 1995.  Overall the mean concentrations appeared to decline for most organochlorine pesticides from 1990 to 1995.  Concentrations well above detection limits were noted for TCDD, pnCDD, HxCDD, HpCDD, OCDD in the eggs.  Only one furan (PnCDF) was well above detection limits.  Maximum concentrations of TCDD were found in eggs from both Rice and East Sand Islands in 1993 while Rice Island eggs exhibited higher concentrations of TCDD, OCDD, and TCDF in 1995.  Neither egg length nor shell thickness  were different between the two islands.  However, the shells from both islands exhibited mean thickness below the pre-1947 mean.  Eggshells were up to 14.5 percent and 31.3 percent thinner in eggs from East Sand and Rice Island compared to pre-1947 eggs.  It was also observed that shell thickness and p,p'DDE concentrations in individual eggs were correlated.

Double crested Cormorant nest in Lower Columbia River Photo:  USFWS Carol SchulerDiscussion and Conclusions: Concentrations of p,p'DDE, total PCBs, mercury and some dioxin-like compounds (TEQs) accumulated in eggs of double-crested cormorants from Rice and East Sand Islands; whereas 1993 samples from the reference colony were significantly lower or below detection limits.  Nearly all contaminant concentrations in Rice Island eggs either appeared higher, or were significantly higher, than values in eggs from East Sand Island.  One explanation for the observed difference in the amount of contaminants between the two islands is due to their location.  Rice Island is located 22 miles above the mouth of the estuary and  contaminants entering the estuary could be deposited within the feeding area of cormorants nesting on this island.   East Sand Island is 6 miles up the river and the cormorants nesting there  may feed more in the open ocean and contact less contaminated prey.  The primary exposure for the cormorants is the bioaccumulations of contaminates through ingestion of contaminated prey.  Eggs collected in 1991 from both islands exhibited mercury concentrations within a range associated with reproductive impacts for some avian species.  Maximum concentrations of TCDD or TEQs in eggs from Rice Island also exceeded concentrations associated with reproductive impacts in birds from the Great Lakes and concentrations affecting brain asymmetry measurements.  Results indicate that lower Columbia River cormorants, especially birds nesting on Rice Island, are exposed to concentrations of contaminants that decrease shell thickness, adversely impact developing embryos, or elicit egg mortality some individuals.  However, contaminant concentrations since 1994 appear to be well below concentrations impacting double-crested cormorants at the population level as compared to other field studies from the Great Lakes Region.  Results of this study indicate contaminants are near effect-threshold concentrations and a relatively small increase in mean egg burdens could impact the cormorant population.  Contaminants released during large dredging projects could result in increased egg burdens and further impact reproduction.


  1. Ensure that adequate vegetative buffers exist on any land managed by the refuge that supports agriculture or pasture to prevent the erosion soil associated with DDT or its metabolites from entering the waterways.
  2. Continue population monitoring or aerial nest counts of cormorants.  Since they are an indicator species for the region, hatching and nesting success should be monitored to see if dioxin-like chemicals are influencing egg mortality.
  3. Continue monitoring contaminants in cormorant eggs on a periodic basis to more closely examine trends over time and to better evaluate the risks to mammalian and avian predators from consuming cormorant eggs or young.

Learn More by reading the following full report:
Buck J. and Sproul E.  Organochlorine contaminants in Double-crested cormorants from Lewis and Clark National Wildlife refuge in the Columbia River Estuary.  U.S. Fish and Wildlife Service, Final Report.  October 18, 1999

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