Estimates of demographic parameters for southern right whales off South Africa from survey data from 1979 to 2006

Abstract

Aerial counts of right whale cow-calf pairs on the south coast of South Africa between 1971 and 2006 indicate an annual instantaneous population increase rate of 0.069 a year (95% CI 0.064, 0.074). Annual photographic surveys since 1979 have resulted in 1 968 resightings of 954 individual cows with calves. Observed calving intervals ranged from 2 to 23 years, with a principal mode at 3 years and secondary modes at 6 and 9 years, but these made no allowance for missed calvings. Using the model of Payne et al. (1990), a maximum calving interval of 5 years produces the most appropriate fit to the data, giving a mean calving interval of 3.16 years with a 95 % confidence interval of (3.13, 3.19). The same model produces an estimate for adult female survival rate of 0.990 with a 95% confidence interval of (0.985, 0.996). The Payne et al. (1990) model is extended to incorporate information on the observed ages of first reproduction of grey-blazed calves, which are known to be female. This allows the estimation of first parturition (median 7.74 years with 95% confidence interval (7.15, 8.33)). First year survival rate was estimated as 0.713 (0.529, 0.896) and the instantaneous population increase rate as 0.070 (0.065, 0.075). The current (2006) population is estimated as some 4 100 animals, or about 20% of initial population size: the latter parameter needs re-consideration.