Age and Sex-ratios of Sea Ducks Wintering: Strait of Georgia, British Columbia, 2003, 2004, 2008, 2014, 2015

Abstract

Winter age and sex ratios provide valuable demographic data for sea ducks that are difficult to obtain by other means. Our objectives were to determine spatial, temporal, and density-related variability in age and sex ratios for five, and in proportions of adult males for 11, sea duck species that winter in the Strait of Georgia, British Columbia. Kmlong shoreline sections (n = 49-62) were surveyed in early February 2003, 2004, 2014, and 2015. Bufflehead (Bucephala albeola), Common Goldeneye (Bucephala clangula), Surf Scoters (Melanitta perspicillata) and Harlequin Ducks (Histrionicus histrionicus) were the most ubiquitous species in our study, occurring in 74 to 96% of km-sections; Hooded Mergansers (Lophodytes cucullatus), Long-tailed Duck (Clangula hyemalis) and Common Mergansers (Mergus merganser) were least common. Numbers of individuals counted were highest for Surf Scoters, White-winged Scoters (Melanitta fusca), and Harlequin Ducks with over 1000 individuals counted each year for Surf Scoters and Harlequin Ducks, and in most years for White-winged Scoters. Densities differed among years for White-winged Scoter, Bufflehead, and Red-breasted Merganser (Mergus serrator). Annual estimates for male age ratio (first year:adult male) varied significantly for Black Scoter (Melanitta americana; 0.071 to 0.170), Surf Scoter (0.064 to 0.139), Harlequin Duck (0.068 to 0.147), and Common Goldeneye (0.096 to 0.201). Regional differences in male age ratio were found for Barrow’s Goldeneye (Bucephala islandica; 0.029 to 0.144) and Common Goldeneye (0.060 to 0.178), and more complex interactions were found between regions by year for Surf Scoter. Sex ratios were less variable than age ratios and varied by year and region only for Common Goldeneye and Surf Scoter. Adult male proportions were correlated with but varied more than sex ratios and showed significant differences by year for Surf Scoter, Common Goldeneye, and Bufflehead and by region for Surf Scoter, Common Goldeneye, Bufflehead, Common Merganser, and Red-breasted Merganser. Based on previous research that calculated expected confidence limits from different numbers of occupied km-sections, the species-specific sampling intensity obtained in this study likely provided age ratio estimates with 95% confidence limits of ± 5% for Surf Scoter and ± 3% for Harlequin Ducks. Age ratios best serve as a relative index of recruitment because there is a demonstrated mismatch between known population trends and trends inferred from estimated age ratios and survival rates. Regional and density-related differences in age ratios, sex ratios, and adult male proportions indicated segregation and emphasize the need for broad-scale sampling to achieve representativeness. Inter-annual differences may indicate demographic changes but few comparative data exist, and several consecutive years of surveys are needed to provide baseline data