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National Marine Mammal Laboratory  (cont.)

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Harbor Seal Movements Between Haul-out Sites May Influence Population Estimates

Aircraft surveys are used to census Alaska harbor seals during the molting season to estimate population size.  Typically each haul-out site is counted 4 or more days to increase the precision of the counts.  Most statistical procedures used to estimate the population size of harbor seals assume that there is no movement between sites or that movement is negligible.   To test this, we radio-tagged 32 seals in Nanvak Bay (Bristol Bay), Alaska, just prior to aerial surveys in August 2000.  The number and location of tagged seals were recorded daily during the abundance surveys of the entire region.  We found that 24 of 32 tagged seals hauled out at sites other than Nanvak Bay. Seals moved in all directions from Nanvak, as far as 128 km, and the maximum distance between sites on successive days was 152 km.   Movements away from the tagging site complicate the estimation of the proportion of seals hauled out (for assessing the numbers missed during surveys).  For example, the average daily proportion of tagged seals hauled out at all sites was 55.7 percent, but the average proportion hauled out at the tagging site was only 34.9 percent.   If seal movements are correlated such that groups of seals move together among sites, these movements complicate the summary of survey counts.  If seals do move between haul-out sites and this movement is not accounted for statistically, the result will be overestimation of the number of seals present and underestimation of the correction factor (the factor used to correct for the number of animals in the population not present and therefore not seen). Both of these errors will lead to overestimates of population size.

By Dave Withrow.


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