US Fish & Wildlife Service


Date of this Version



Published in Contaminant Report R6/SI5M/97, 1-44, (1997)


The purpose of this study was to evaluate concentrations of arsenic, mercury, selenium, and chlorinated hydrocarbon compounds in interior least tern eggs from the northern Great Plains states from 1992 through 1994. The Environmental Contaminants Specialists in Kansas, Nebraska, South Dakota, North Dakota, and Montana agreed on a standard protocol for collection and analyses of eggs during the study period.

Addled, fiooded, or abandoned eggs collected during the study period were submitted for chemical analysis by the Environmental Contaminants Specialist in each state.

A total of 104 eggs were analyzed for arsenic, mercury, and selenium; 78 of them also were analyzed for chlorinated hydrocarbons.

Concentrations of some contaminants, particularly mercury, were difficult to interpret. Therefore, we also evaluated recent least tern productivity in the northern Great Plains states.

Arsenic was detected in only 13 of the 104 eggs analyzed, and was unlikely to have detrimental effects on least tern reproduction.

The geometric mean mercury concentrations for individual states each year were below 0.50 μ-g/g fresh weight (a concentration that is known to affect other species), but I I % of the eggs contained mercury at more than that concentration.

Only 20 (19%) of the eggs contained selenium at less than the 3 μ-g/g dry weight concentration currently considered to be safe for avian reproductive success. Twenty-six percent of the eggs contained more than 5 μ-g/g dry weight. Thus, selenium likely is affecting least tern nesting success in the study area.

Cyclodienes in 36 (46%) of the eggs analyzed for chlorinated hydrocarbons equaled or exceeded the concern level of 0.10 μ-g/g wet weight. The concentrations of oxychlordane and heptachlor epoxide, the most toxic components of technical chlordane were low in all eggs. Dieldrin and chlordane compounds and metabolites were ubiquitous in the tern eggs, and might be affecting least tern reproduction in the study area.

DDT was detected at very low concentrations in some of the 1992 eggs, which shows that the terns had very recently been exposed to DDT. It was not detected in 1993 or 1994 eggs. DDT compound concentrations found are not likely to have detrimental effects on the population.