Staging of Pacific flyway brant in relation to eelgrass abundance and site isolation, with special consideration of Humboldt Bay, California

Abstract

Spring distributions of black brant ( Branta bernicla nigricans) are closely related to that of their primary food plant, eelgrass ( Zostera marina). Using historical (1931–2001) brant data from Humboldt Bay, California, we show that proportional use of two main feeding areas in the bay strongly reflects food abundance. We used multiple regression to investigate whether the spatial relationship between brant and eelgrass holds at the flyway level. We related peak brant numbers at 11 staging areas in the Pacific flyway to the sites' Z. marina abundance and isolation from other important staging areas. We explained 90% of the variation in peak brant numbers across sites with these variables; isolated bays with high eelgrass abundance supported the most brant. Our results emphasize the importance of protecting large eelgrass habitats along the Pacific Coast, particularly in bays that are geographically isolated from other large staging areas.