Natural markings and their use in determining calving intervals in right whales off South Africa
Since 1979, 245 right whales (excluding calves) have been individually identified in aerial photographs taken annually on the South African coast, using variations in dorsal pigmentation and callosity patterns. White or grey blazes (or both) occurred dorsally in 16,7% of individuals, one form of which (partial albinism) occurred in 3,6% of the calves born but may be sex linked. In 19 animals carrying dorsal marks that were resighted two to seven years later, changes in the appearance of the callosities were noted in 95% of individuals, 58% showing pronounced changes. Definite matches between years were made for 139 cows with calves. The average per capita calving interval, adjusted for incomplete spatial and temporal coverage, was 3,183 ± 0,091 (SD) years, or a calf production rate of 0,314 ± 0,009 (SD) per adult female per year. From a simple model it is shown that rates of population increase as high as that observed (6–7%) can only be maintained with a calf production rate of this order if the age at first parturition is relatively low and/or the adult survival rate high.