Trends in Numbers of Winter Bald Eagles at Lake Coeur D'alene in Idaho

Abstract

During their winter migration, bald eagles (Haliaeetus leucocephalus) stop at Lake Coeur d'Alene, Idaho to eat spawned kokanee salmon (Oncorhynchus nerka). Long-term data on these migrating bald eagles provide a unique opportunity to investigate their trend from 1974 to 2010. We counted bald eagles during 12 weeks from November through February of each year. They arrived in mid-November, peaked in number in late-December, and departed in January. We recorded the fewest bald eagles, latest arrival dates, and slowest rates of arrival (i.e., linear slope between peak numbers of bald eagles with dates of their first and peak occurrences) during the 1980s. Numbers of bald eagles rebounded during the 2000s, with the most bald eagles, earliest arrival dates, and fastest rate of arrival. To explore trends at a regional scale, we utilized data from the Midwinter Bald Eagle Surveys along with available salmon data. We found that Lake Coeur d'Alene had an average 34% more midwinter numbers of bald eagles and an average 74% less biomass of kokanee salmon during the 2000s than the 1980s. Neighboring Lake Pend Oreille had an average 48% fewer midwinter numbers of bald eagles and an average 58% less biomass of kokanee salmon during the same time period. These results indicate that active management of kokanee salmon may increase bald eagles at Lake Pend Oreille but not necessarily at Lake Coeur d'Alene. Numbers of bald eagles at Lake Coeur d'Alene have been largely influenced by a combination of growing nesting populations and shifting migration patterns.