Browsing by Author "Altwegg, R"

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    Decomposing the variance in southern elephant seal weaning mass : partitioning environmental signals and maternal effects
    (2015) Oosthuizen, W C; Bester, M N; Altwegg, R; McIntyre, T; de Bruyn, P J N
    Predator populations are likely to respond to bottom-up processes, but there remains limited understanding of how wide-ranging marine predators respond to environmentally driven temporal variation in food availability. Widespread declines of several Southern Ocean predators, including southern elephant seals Mirounga leonina, have been attributed to decreases in food availability following environmental changes. We used linear mixed models to examine temporal process variance in weaning mass (a key fitness component) of southern elephant seals at Marion Island over a 27-year period (1986– 2013). We quantified the contribution of within- and between-year covariates to the total phenotypic variance in weaning mass and determined whether the observed reversal of population decline was associated with a continued increase in weaning mass, suggesting improvement in per capita food availability to adult females. Weaning mass initially increased rapidly with maternal age, but reached an asymptote when females were nine years old. Longitudinal data examining between-individual maternal differences suggested latent, age-independent maternal influences on weaning mass. Between-year differences accounted for only 6% of the total phenotypic variance in weaning mass. We found no evidence for a systematic trend in weaning mass, but model predicted weaning mass was 8.70 kg (95% CI ¼ 2.14– 14.73) lower during the 1980s, suggesting that food limitation may have been most severe during these years when the population was declining. Model support for a population size effect was entirely driven by the low weaning mass and comparatively high (but declining) population size from 1986 to 1988; subsequent variation in population size had no detectable influence on weaning mass. Remotely sensed chlorophyll-a concentration within the seals’ foraging distribution explained 45% of the between-year variation (1998–2013, n ¼ 9) in weaning mass, with higher weaning mass in years of positive chlorophyll-a anomalies. Environmental variation associated with variability in the Southern Annular Mode poorly predicted temporal variation in weaning mass. Our long-term data on elephant seal weaning mass provides a perspective on variation in food availability in a pelagic environment which is poorly known. Examining the long-term regionally specific effects of environmental variability aids our understanding of how these predators interact with their environment.
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    Restoration of oiled African penguins Spheniscus demersus a decade after the Apollo Sea spill
    (2008) Wolfaardt, A C; Underhill, L G; Altwegg, R; Visagie, J
    The bulk ore carrier Apollo Sea sank south-west of Dassen Island off western South Africa in June 1994, oiling approximately 10 000 African penguins Spheniscus demersus, most of which were collected from Dassen Island. A total of 4 076 de-oiled penguins was released with flipper bands. From 1994 to 2005, follow-up research using re-sighting and capture-mark-recapture methods indicated that about 73% of the de-oiled penguins observed back at Dassen Island attempted to breed, and were thus successfully restored into the breeding population. For de-oiled breeders, the median interval between their first recorded sighting and first recorded breeding attempt was 11 months, indicating a short-term delay in restoration. At least 45% of the de-oiled breeders were still being re-sighted five years after their release, and a minimum of 4% survived into their ninth year. These results represent the most successful restoration estimates anywhere in the world. The proportion of de-oiled juvenile penguins re-sighted back at Dassen Island and recorded breeding was lower than that of birds in adult plumage. De-oiled non-breeders spent significantly more time along the shore and less time within breeding colonies than de-oiled breeders. The mean proportion of de-oiled breeders that abstained from breeding each year during the study period was greater than expected. There was a negative relationship between breeding and subsequent survival and breeding, suggesting a cost of reproduction for de-oiled birds.