Starving seabirds: unprofitable foraging and its fitness consequences in Cape gannets competing with fisheries in the Benguela upwelling ecosystem
Résumé
Fisheries are often accused of starving vulnerable
seabirds, yet evidence for this claim is scarce. Foraging
energetics may provide efficient, short-term indicators
of the fitness status of seabirds competing with fisheries.
We used this approach in Cape gannets (Morus capensis)
from Malgas Island, South Africa, which feed primarily
on small pelagic fish in the southern Benguela upwelling
region, thereby competing with purse-seine fisheries. During
their 2011–2014 breeding seasons, we determined body
condition of breeding adult Cape gannets and measured
their chick growth rates. In addition to these conventional
fitness indices, we assessed the daily energy expenditure of
breeding adults using a high-resolution time-energy budget
derived from GPS-tracking and accelerometry data. For
these same individuals, we also determined prey intake
rates using stomach temperature recordings. We found that adult body condition and chick growth rates declined
significantly during the study period. Crucially, most
birds (73 %) studied with electronic recorders spent more
energy than they gained through foraging, and 80–95 % of
their feeding dives were unsuccessful. Our results therefore
point to unprofitable foraging in Cape gannets, with
a longer-term fitness cost in terms of adult body condition
and reproductive performance that corresponds to a local
population decline. Based on this evidence, we advocate
a revision of regional fishing quotas for small pelagic fish
and discuss the possibility of an experimental cessation of
purse-seine fishing activities off the west coast of South
Africa. These measures are needed for the ecological and
socio-economical persistence of the broader southern Benguela
upwelling ecosystem.