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Updated application of a photo-identification based assessment model to southern right whales in South African waters to include data up to 2018

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The assessment of southern right whales in South African waters is updated to include data from 2018. After three preceding years of very low sightings of females with calves, 2018 saw these numbers increasing to reach a record level of 426. This pattern of results is best explained by variation with time in the probability that a resting female rests again the following year, where this probability increased substantially after 2008, but seems to have fallen back to earlier levels from 2017. Another surprising feature of the data is one extremely high and three extremely low estimates of the probability of sighting a female with calf after 2013. The low estimates do not seem compatible with near unchanged survey conduct for the years concerned, so that a penalty term is added to the assessment to force these to be closer to earlier values. The resultant population trajectory suggests that an annual increase rate of 6.1% from 1979 to 2008 which dropped to 3.9% for the following decade, with the current abundance (including calves) totalling 5838 whales.

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Department of Mathematics and Applied Mathematics, University of Cape Town