USE OF RATIO ESTIMATORS TO ESTIMATE THE SIZE OF COMMON EIDER POPULATIONS IN WINTER

Surveys of common eiders (Somateria mollissima) are often conducted in winter because elders tend to congregate in small discrete areas duri ng this period. Evaluations of population size are usually based on vi sual estimates, the reliability of which is difficult to assess. In wi nter 1989, we tested a method of surveying elders in the Mingan Archip elago and Anticosti Island region in Quebec. The survey consisted of 2 consecutive overflights of the study area at high and low altitude wh ere photographs were taken and visual estimates were recorded. We esti mated the size of the population using a 3-step process. First, we est imated the number of adult males using a ratio estimator that combined high-altitude photo counts and visual estimates; the former was consi dered an accurate count. The ratio of females and immatures to adult m ales was then estimated from photos taken from low altitude. Subsequen tly, a capture-recapture model was used to estimate the number of elde rs in small flocks. The proposed method produced estimates with an acc eptable level of precision: 77,627 elders (CV = 13%) on 2 February and 92,247 elders (CV = 21%) on 28 February. This precision suggests that the method can be useful in monitoring common elder populations in wi nter, thus facilitating the management of this species. The proposed m ethodology is unusual in field biology because sampling cannot be cont rolled; thus, the estimation is relying on a model instead of a predef ined sampling design.