Decline of Canada geese ( Branta canadensis ) and common goldeneye ( Bucephala clangula ) associated with a collapse of eelgrass ( Zostera marina ) in a Nova Scotia estuary

Abstract

Mean numbers of migrant Canada geese (Branta canadensis) in Antigonish Harbour in the southern Gulf of St. Lawrence (Canada) during October to December were similar (approx. 450-500 birds) for the period 1998-2000. Similarly, during this period, geese used two foraging sites. However, in 2001, the average number of birds decreased by half and the primary foraging sites were used only rarely. This coincided with a decline of about 95% in the biomass of roots and rhizomes of eelgrass (Zostera marina) that occurred between October 2000 and 2001. Eelgrass is the principal food of geese in this estuary. In addition, there was a reduction of around 50% in the numbers of common goldeneye (Bucephala clangula), which feed on invertebrates associated with eelgrass. Lower than usual weekly abundances of geese and goldeneye are probably the result of an unusually short residence time in the estuary, rather than a decline in the total number of visiting migrants. We attribute these changes in the distribution and abundance of geese and goldeneyes to the dramatic decline in eelgrass.

Effect of grazing by Canada geese Branta canadensis on an intertidal eelgrass Zostera marina meadow

“…The Fishing Island case study represents one of the few accounts of eelgrass decline due to Canada goose grazing in the Gulf of Maine region. Other studies in this region have reported how eelgrass availability can affect goose populations (Seymour et al 2002, Hanson 2004. In contrast, our results show that Canada geese can have a substantial effect on eelgrass populations.…”

“…The recovery of seagrass during the summer months re-establishes the food resources such that waterfowl return on a seasonal basis. The dependence of many waterfowl species on recurring seagrass populations has been well documented (Ganter 2000, Moore et al 2004, and waterfowl have been known to alter their migration routes when seagrass resources become unavailable (Seymour et al 2002).…”

“…In North America, eelgrass Zostera marina L. meadows are considered important staging areas for black brant geese Branta nigricans L. on the west coast (Wilson & Atkinson 1995, Ward et al 2003, Moore et al 2004, and for brant B. bernicla L. and Canada geese B. canadensis L. on the east coast (Seymour et al 2002, Hanson 2004. The length of stay of waterfowl at staging areas is often closely correlated to the available seagrass resources at that site (Wilson & Atkinson 1995).…”

Codium fragile: rhizomatous growth in the Zostera thief of eastern Canada

“…This was reported for the rhizomatous coenocyte, Caulerpa cupressoides (West) C. Agardh (Williams 1984;Williams and Fisher 1985). Seymour et al (2002) documented a 95% decline of Zostera in Antigonish Harbour, and noted ca.50% declines in two migrating bird species that either feed on the rhizomes and roots (Canada geese) or feed on invertebrate populations in eelgrass beds (common goldeneye). The cause of the eelgrass decline was not determined; however, there was no evidence of eelgrass wasting disease or anthropogenic eutrophication.…”

“…The collapse of an eelgrass population was recently described from Antigonish Harbour in the southern Gulf of St. Lawrence (Seymour et al 2002). Although the cause of this decline was not established, the impacts of the invasive green crab, Carcinus maenus L., were hypothesized as the source of the destruction.…”