Proceedings - Teaching and Learning Centre - Simon Fraser ...

Fraser River Action Plan 3rd Research Workshop
Shorebird and Invertebrate Interactions
M.A. Sewell 1 and R.W. Elner 2
1 CWS/NSERC Research Chair in Wildlife Ecology
Simon Fraser University
2 Canadian Wildlife Service
Environment Canada
Each spring, the extensive intertidal mudflats of the Fraser River estuary are a stopover for millions of migratory
shorebirds, especially Western Sandpiper (Calidris mauri), enroute to breeding grounds in the north. Many of
these same birds and their young pass through the area again in the fall. Other migratory shorebirds, such as
Dunlin (Calidris alpina pacifica) overwinter around the estuary. Infaunal and epifaunal intertidal invertebrates
form a prey-base for many shorebirds and, also, fish and crab in the estuary. A series of sediment cores were
taken at various sites between Boundary Bay and Westham Island to examine, variously, community
composition, relative abundance and spatial and temporal patterns of invertebrates. The macrofaunal (>500
micron) component was numerically dominated by a few species: amphipods (Corophium spp.), podocopid
ostracods, polychaetes and the gastropod Batillaria zonalis. However, the highly patchy spatial distribution of
these invertebrates was a major barrier to establishing monitoring protocols and deriving relative abundance
estimates. Exclosure and random sampling experiments were performed to assess expected depressions in
macrofaunal invertebrate densities during the spring migration period. However, there was little evidence for any
reduction in invertebrate numbers due to shorebird predation. The possible reasons were:
1. the scale of patchiness prevented detection of any depression in prey density; and,
2. shorebirds are focusing on meiofaunal (