The best method of monitoring ice seal populations in shifting packice is by aerial censuses. However, there are a number of problems with this method that are difficult to address in the packice and are best addressed on more accessible ice seals such as the Weddell seal. Counts made during the moult are affected by the fact that Weddell seals can leave the fast ice to forage. Seal numbers on the fast ice will therefore differ depending upon the proximity and availability of prey species, with lower numbers being recorded on the fast ice the further the seal has to travel from the ice to find food. In any census the subgroup of the population that is being counted must be known. Southern elephant seals and Weddell seals haul out to moult in sequences on the basis of age, sex and reproductive condition. This leads to periods of high and low counts through the moult. Examination of data on crabeater seals indicates that a putative 60% decline in numbers may have been due to counts being made at different times of the moult and therefore with different subgroups of the population being counted.