Caspian terns on the Great Lakes: organochlorine contamination, reproduction, diet, and population changes, 1972-91

The Great Lakes of North America are one of the most heavily contaminated freshwater ecosystems in the world, yet they support at lest one-third of North America's breeding Sterna caspia. To assses the health of this breeding population in relation to organochlorine contaminant (OC) levels, the authors studied 16 colonies (6800 nests) throughout its Great Lakes range in 1991. A wide range of OCs was detected in eggs from each colony, but at markedly lower concentrations than recorded in previous studies. Eggs from Saginaw Bay Lake Huron and Green Bay (Lake Michigan), both industrialized areas, were the most heavily contaminated. Eggshells were as thick as pre-DDT. Clutch size, hatching success, and overall reproductive output were high relative to other studies, and there was little evidence for adverse biological effects, at the population level, of current contaminant loads. Wasting syndrome was identified in chicks at Saginaw Bay, associated with the highest TCDD toxic equivalent (TEQ-1.6 μg.kg^-1 wet weight) in egg pools from 10 Great Lakes colonies. Diet differed among lakes, although alewife Alosa pseudoharengus and Centrarchidae dominated overall. The Great Lakes breeding population of Caspian terns has at least doubled since the late 1970s, with average annual increases higher on Lake Ontario (23%) than on Lake Michigan (6%) or Lake Huron (0.3%). Contaminants now seem to exert a relatively small influence on this population. -from Authors<p></p>