Modeling variability in replicated surveys at aggregation sites

Surveys of aggregation sites can provide unbiased estimates of annual trend s in population size if the proportion of the population counted at these s ites does not vary systematically among years. However, counts at these sit es tend to be highly variable and resulting trend estimates typically have poor precision. I developed an index based on a simple parametric model for counts of Pacific walruses (Odobenus rosmarus divergens) at haul-out sites in Bristol Bay, Alaska that accounted for the general temporal pattern of variability in the proportion of the population at the sites. Simulations s uggested that an index based on mean annual counts was a more sensitive ind icator of trend than the model-based index or the currently used index base d on maximum annual counts. The model-based index may be more useful for ot her situations where timing of the aggregation peak is more variable.