Aerial surveys are most commonly used to estimate the abundance of walruses. However, walruses are definitely one of the most difficult species to survey, because of their clumped distribution throughout remote areas, and their simultaneous occurrence on sea ice, land and in the water at some times of the year. In addition, large annual variations in sea ice distribution and extent affects their distribution and makes survey planning challenging.
When they are hauled out on land or ice floes, walruses tend to lie piled on top of one another, making it hard to see all individuals, or even to count them in a photograph (Stewart et al. 2013c). If walruses are disturbed for some reason at their haulout shortly before the survey, a large number of animals can be dispersed and not counted. Since walruses also spend time in the water, correction factors must be applied to any count to correct for the proportion of the population unavailable to be counted during the survey. These factors are difficult to obtain simultaneously with the survey. Because of these difficulties, walrus abundance estimates therefore tend to be imprecise and probably underestimate the true stock size (negative bias) in most cases (Heide-Jørgensen et al. 2013).
Walrus occur in a complex environment of ice, water and land, that makes them difficult to survey, and often in compact groups.
Top photo: J. Blair Dunn, Fisheries and Oceans Canada; Bottom photo: Rob Stewart
Read much, much more on the historical population sizes on the Stock Status page.