SIZE-CLASS SEGREGATION OF BOWHEAD WHALES DISCERNED THROUGH AERIAL STEREOPHOTOGRAMMETRY

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Abstract

Bowhead whales (Balaena mysticetus) summering in the eastern Beaufort Sea in 1983 were measured through aerial stereophotogrammetry. Photos were taken from a turbine Aerocommander 690 with wingtip-mounted, calibrated 35-mm cameras. Photos were measured on an analytical plotter with scale data provided by radar altimeter and focal length of the lenses. Coefficient of variation of known-sized targets 12 m long was 1.7 percent (n= 25). Potential duplicate photographs of whales were removed from consideration through an algorithm based on whale swim speed and elapsed time between sightings. Significant segregation by bowhead whale length was found between four broad regions (P < 0.001, ANOVA, Chi square). Also an inverse correlation appeared between longitude and size of animals (P < 0.001), with the larger animals occurring farther east. This unequivocal size-class segregation confounds an accurate assessment of overall size-class composition of the population as well as recent attempts to determine calving rate from aerial survey data.

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