Counting seals: estimating the unseen fraction using a covariate and capture-recapture model

Abstract

We evaluated the influences of environmental covariates on the proportion of harbor seals (Phoca vitulina richardsi) ashore on Tugidak Island using a series of photographic capture-recapture experiments and ground based counts. We used general linear models to examine the significance of combinations of covariates including quadratics terms. Time of day, tide height, rate of tide change, surf, and wind speed significantly influenced the number of seals ashore during the molting period. The model including all significant covariates best explained the probability of seals being hauled out. We observed a decline in the local population using the haulout suggesting that seasonal migration affects the number of seals ashore. The relationship between covariates and the number of seals hauled out on Tugidak Island differ in some respects from those reported at other sites in Alaska, implying that a region wide application of a single correction factor to counts of hauled out harbor seals may not adequately account for seals at sea.