Common loon mortality in marine habitats

Abstract

The common loon remains remarkably anonymous and biologically little-known during its winter season. A major portion of the breeding population winters off coasts of the United States: Atlantic, Gulf, and Pacific. Adult common loons are flightless for a few weeks in midwinter during a simultaneous molt of their wing feathers. This apparently makes loons especially sensitive to environmental disturbance, because they are among the most commonly recorded dead and dying birds on coastal beaches from winter through spring. Loons' large size, slowness to die, and tendency to beach themselves as a last resort also render their mortality more visible. Contributing causes of mortality include storms, food limitation, entanglement in fishing nets, oil, and possibly, other toxic substances. Occasional, unpredictable die-offs of many hundreds (1993) or even thousands (1983) of emaciated loons have occurred in southeastern U.S. waters. Loons offer a prime opportunity for study of this unexplained marine mortality. In the 1980s, neurotoxic bioaccumulated mercury was hypothesized as a cause of mass mortality, but data on the mercury and selenium loading of loons and other long-lived marine carnivores, and on the marine mortality patterns of loons and other seabirds, indicate that mercury is not the cause. Algal neurotoxins in marine food chains have caused some seabird die-offs and are one useful hypothesis for future research. Interdisciplinary study of unusual seabird mortality events can provide a major paradigm for the study of marine environmental health.Key words: common loon, mortality, mercury, biotoxin, bioindicator, marine environment.