Application of a photo-identification based assessment model to southern right whales in South African waters

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University of Cape Town

The three-mature-stages (receptive, calving and resting) model of Cooke et al. (2003) is applied to photo-identification data available from 1979 to 2010 for southern right whales in South African waters. The 2010 number of females having reached the age at first parturition is estimated to be 1 205, the total population (including males and calves) 4 725, and the annual population growth rate 6.8%. The probability (average 11%) that a resting mature whale rests for a further year appears to vary annually, whereas the probability (7%) that a receptive whale rest (or aborts) rather than calves the next year appears to be constant. Information from resightings of grey blazed calves as adults with calves allows estimation of first year survival rate of 0.914 (compared to a subsequent annual rate of 0.987, and an age at 50% maturity of 6.4 years. However this suggests also that 27% (s.e. 6%) of grey blazed calves lose their marking before becoming adults. In contrast, the relative proportions of grey blazed animals amongst calves and amongst calving adults suggest rather a value of 10% (s.e. 8%). If the proportion losing markings is in fact 10%, first year survival rates estimate drops to 0.837 and the population growth rate to 6.4% pa.