Second North American Sea Duck Conference - Patuxent Wildlife ...

SECOND NORTH AMERICAN SEA DUCK CONFERENCE
POPULATION TRENDS OF SEA DUCKS WINTERING IN FOUR BAYS OF KODIAK
ISLAND, ALASKA – A TWENTY YEAR ANALYSIS
Denny Zwiefelhofer¹, Douglas J. Forsell², Mike Keim 3 , and Joel Reynolds 3
¹USFWS, Kodiak National Wildlife Refuge; Kodiak, AK; Denny_Zwiefelhofer@FWS.gov
²Chesapeake Bay Field Office, Annapolis, MD;
3 USFWS, Division of Natural Resources; Anchorage, AK
Shipboard surveys of marine birds wintering near shore on Kodiak Island have been conducted each
February since 1986 on two western shore bays, Uyak and Uganik, and since 1991 on two eastern
shore straits, Eastern and Western Sitkalidak Strait. Each survey revisited a fixed set of standardized
300 m wide strip transects, each of ten-minute duration at 9 knots running speed. The annual surveyed
area covered approximately 200 km 2 , or 17 percent of the surface area of the bays and 13 percent of the
shore. Birds on the water were counted from the flying bridge of a 15 meter vessel using 8x40 or 10x50
binoculars (pre-1996) or 12x36 stabilizing binoculars (post-1996). The same observer conducted all
surveys. Survey observations were analyzed using generalized estimating equations to account for the
longitudinal study design, i.e. repeated surveys of the same transect through time. Separate analyses
were conducted for eight sea duck species at each survey area, estimating annual mean density on the
sample transects, temporal trend in mean density, annual mean probability of occurrence on the sample
transects, and temporal trend in probability of occurrence.
The predominantly resident breeding species, Barrow’s goldeneye (Bucephala islandica), harlequin
ducks (Histrionicus histrionicus), common mergansers (Mergus merganser), and red-breasted
mergansers (Mergus serrator), all exhibited increasing or relatively stable winter population densities
and occurrence probabilities. Common mergansers’ extreme rate of increase suggested a recently
increasing influx of winter migrants of unknown origin. Of the winter migrant species, surf scoters
(Melanitta perspicillata), black scoters (Melanitta nigra), and white-winged scoters (Melanitta
fusca) all exhibited relatively stable or declining densities and occurrence probabilities, with spatially
contrasting trend patterns suggesting possible differential harvest pressure. The other winter migrant
species, long-tailed ducks (Clangula hyemalis), exhibited increasing or relatively stable winter
densities and occurrence probabilities at all survey areas.
122 ANNAPOLIS, MARYLAND, USA NOV. 7-11, 2005