Browsing by Author "Vermeulen, Els"

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    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
    (2018) Brandão, Anabela; Vermeulen, Els; Ross-Gillespie, Andrea; Findlay, Ken; Butterworth, Doug S
    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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    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
    (2021) Brandao, Anabela; Ross-Gillespie, Andrea; Vermeulen, Els; Butterworth, Doug
    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 y (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.
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    Visual health assessment of parous female southern right whales (Eubalaena australis) off the southern Cape coast, South Africa
    (2019) Hörbst, Sandra; Moloney, Coleen; Vermeulen, Els
    The long-term monitoring of the South African southern right whale population has revealed a decrease in sightings along the coast and an increase of calving intervals within the last few years, causing changes in the species´ demographics causing a decrease in annual population growth rate (6.5% y -1 ). Because reproductive success is linked to body condition, the purpose of this study was to conduct a visual health assessment based on overhead photographs from the annual aerial southern right whale surveys and detect potential links between visual health condition of parous females and the increased calving intervals. Additionally, it was aimed to find relationships between visual health of parous females and environmental indices of the Southern Ocean, representing food availability. To allow global comparison of the results, the method used for the visual health assessment was developed in collaboration with international southern right whale researchers from Australia and South Africa. The health indices were adapted from a visual health assessment method developed for northern right whales and include body condition, skin condition, the presence of cyamids around the blowholes and rake marks on the skin. The results showed that these health variables derived from overhead photographs were sufficient to detect visual health changes over time. Within the study period (2005 – 2017), there were two years in which whales had a significant decreased visual health (i.e. increased health score) than average; 2008 and 2014. No direct link between the observed health condition and calving intervals could be found, possibly due to the lack of data on calving intervals post-2014 as well asthe assessment of breeding females only (i.e. breeding females are in good enough conditions to reproduce). However, significant relationships were found between visual health and Southern Ocean productivity (p< 0.001) and climate indices (p < 0.05) with a 0-year lag. These results clearly indicate a link between southern right whale visual health condition and Southern Ocean food availability in one feeding ground, suggesting that this may be the primary feeding ground for parous females during pregnancy. Understanding the links between visual health, reproductive success and climate/food availability helps to understand changes in the population’s demographics and to predict the resilience of the species. Additionally, the standardization of the method allows for global comparison.