Estimating the abundance of pack-ice seals off east Antarctica

A survey was undertaken to provide estimates of the abundance of crabeater, Ross and leopard seal populations in 1,500,000 km2 of pack-ice off east Antarctica between longitudes 60-150oE. Sighting surveys were undertaken along almost 10,000 km of survey transect from an icebreaker and two helicopters to estimate the density of seals hauled out on the ice in survey strips. The probability of detecting seals in survey strips was estimated using double observer line transect methods, and satellite-linked dive recorders were deployed on a sample of seals to estimate the probability of seals being hauled out on the ice at any time of day. Due to non-random placement of survey transects, model-based inference, involving the fitting of a density surface as a function of geographic covariates, was used to extrapolate estimated densities in surveyed areas to the entire survey region. Estimating uncertainty in abundance estimates included consideration of uncertainties in species identification, estimation of detection probability, estimation of haulout probability, and extrapolation from sampled strips to the entire survey region.