Black Brant Winter and Spring-Staging Use at Two Washington Coastal Areas in Relation to Eelgrass Abundance

Abstract

We monitored numbers of Black Brant (Branta bernicla nigricans) in Washington from fall 1980 through spring 1992 at Willapa Bay, and from fall 1986 through spring 1993 in the Dungeness area. We estimated brant use by converting the counts into use days. Coincidentally we also monitored variations in the extent of eelgrass (Zostera marina) beds by remote sensing techniques. At Willapa, brant use was positively correlated with the total extent of eelgrass beds and negatively correlated with the extent of oyster beds that were located within eelgrass beds, and where eelgrass had been removed by mechanical means. A 52% decline in brant use was associated with a 22% decline in eelgrass. At Dungeness there was a significant negative trend in spring-staging brant use. Overall a 63% decline in brant use coincided with a 31% decline in eelgrass. The Dungeness eelgrass beds may have declined because of natural factors. In both areas, brant use during the spring-staging period was more related to eelgrass extent than brant use during the winter months. These results suggest that Black Brant use in coastal Washington is limited by eelgrass availability. Immatures averaged 10.4% of the population at Willapa and 9.9% at Dungeness and are amongst the lowest reported. A shortage of eelgrass during the critical spring-staging period may have led to reduced endogenous reserves and associated low reproductive success of Black Brant that staged in coastal Washington. The shortage of eelgrass may have contributed to the observed southward shift to Mexico by wintering brant