Updated extension of a photo-identification based assessment model to southern right whales in South African waters to allow for the possibility of an early abortion of the calf in the model
Anabela Brand?o, Andrea Ross-Gillespie, Els Vermeulen, Doug S. Butterworth
This paper updates analyses (Brandão et al., 2020) of an extension of the model developed by Brandão et al. (2019) so as to now include sightings data for 2019 and 2020. Brandão et al. (2020) had been modified to include the possibility of an early abortion so that a pregnant (receptive) whale in year y can again be pregnant in year y+1 (the “delta-loop”). This was to be able to account for an increase in calving intervals that are dependent on environmental conditions, and in a way that differs from a change in the value of the β (whale rests for another year) or the γ (late abortion) parameters; this was to be able to explain the low number of sightings of females with calves observed over the 2015 to 2017 period. The further data now available show that following a large number of cow-calf pairs sighted in 2018, there were again low numbers of sightings for 2019 and 2020. From initial work, it has become clear that the low sighting probabilities estimated for 2015 to 2017 and for 2019 to 2020 are not eliminated by the incorporation of this “delta-loop” in the model. A weighted penalty function for the sighting probabilities is necessary to obtain recent sighting probability values in the region of earlier ones; this seems necessary for realism in circumstances where there has not been any marked reduction in the survey sighting effort over these recent years. A weight of 5.0 is able to achieve estimates close to the average of previous sighting probabilities for both periods of low sightings. Thus, low numbers of sightings of females with calves for five of the last six years can be explained by changes in reproduction-related demographic parameters without the need to postulate an increase in the adult mortality rate. Changing environmental (particularly feeding) conditions seem the likely cause, and to be associated with a changed distribution.