Updated application of a photo-identification based assessment model to southern right whales in South African waters, focussing on inferences to be drawn from a series of appreciably lower counts of calving females over 2015 to 2017

Brandão, Anabela; Vermeulen, Els; Ross-Gillespie, Andrea; Findlay, Ken; Butterworth, Doug S

Updated application of a photo-identification based assessment model to southern right whales in South African waters, focussing on inferences to be drawn from a series of appreciably lower counts of calving females over 2015 to 2017

Report

2018

Abstract

This paper extends the analyses of Brandão et al. (2013) which applied the three-mature-stages (receptive, calving and resting) model of Cooke et al. (2003) to photo-identification data available from 1979 to 2012 for southern right whales in South African waters, by taking five further years of data into account. The lower counts of calving females over 2015 to 2017 are indicated to be a reflection of time variability in the probability that a resting whale rests another year, rather than of any mass mortality. The 2017 number of parous females is estimated to be 1 765, the total population (including males and calves) 6 116, and the annual population growth rate 6.5%. This reflects a small decrease to the 6.6% increase rate estimated previously; even in the instance of lesser numbers seen in recent years than estimated previously. Information from resightings of grey blazed calves as adults with calves allows estimation of first year survival rate of 0.852, a slight increase from the previous estimate of 0.850, compared to a subsequent annual rate of 0.988.

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