4.1 Survival
Adult survival of seabirds is generally high (Dias et al. 2019).
Long-term mean adult survival rates for penguins usually exceed 0.8,
with most (9 of 13 species reported) above 0.85 (Bird et al.2020). Even in a population of Magellanic penguins (S.
magellanicus ), declining at ~1.3% per annum, adult
survival was >0.8 in 23 of 25 study years (92%; Gownaris
& Boersma 2019). Based on this, African penguin survival rates in this
study appear to be relatively low compared to other penguin species,
especially at Robben Island (0.77 ± 0.02) where survival was only
>0.8 in 2 of 7 years (28%). However, our estimates remain
consistent with previous estimates for African penguins (e.g., Wolfaardtet al. , 2008; Sherley et al. , 2014) and represent the
first survival estimates from African penguins where no individuals were
tagged with potentially harmful flipper bands. Within this study, we
found that changes in survival are predominantly driven by changing prey
(sardine) abundance, with lower abundance underpinning lower survival at
Robben Island and higher survival at Stony Point. Understanding the
drivers of these colony-specific differences is fundamental to
successful future population management of African penguins.
The finding that survival declines along with sardine abundance at
Robben Island is consistent with previous literature (Sherley et
al. 2014; Robinson et al. 2015). This response underlines the
recent concern for the long-term viability of the colony at Robben
Island (e.g. Sherley et al . 2018), given the low and declining
availability of sardine to seabirds off western South Africa (Robinsonet al. 2015, Crawford et al . 2019). However, the
increasing survival rates with decreasing food abundance in individuals
at Stony Point are more surprising. This may be explained by the
presence of additional factors (e.g., predation, density dependence)
which can impact survival differently across colonies (Weller et
al . 2016); i.e., food availability may not currently be the dominant
external driver of variation in survival at Stony Point. Alternatively,
this may be explained by limitations within our analysis; for example,
the fisheries data we used index fish abundance across a large area of
South Africa’s coastal waters (Coetzee et al. 2008) and may not
necessarily adequately reflect localised food availability at both
colonies. Eastward displacement of sardine and anchovy in the Benguela
upwelling system is driving decreased food availability for seabirds in
the Western Cape (Crawford 2007, Crawford et al . 2019). With
Stony Point located >70km south-east of Robben Island,
localised food availability may be higher around Stony Point because of
this displacement. Supporting this, a recent study found adult African
penguins in Western Cape colonies situated further east (i.e., Stony
Point) had a higher body mass compared to those further west (i.e.,
Robben Island) (Espinaze et al. 2020). However, further research
with colony-specific estimates of prey availability (e.g., Campbellet al . 2019) is required to confirm this, alongside additional
monitoring of external factors (e.g., predation rates) that may be
driving inter-colony differences in survival.