Starving seabirds: unprofitable foraging and its fitness consequences in Cape gannets competing with fisheries in the Benguela upwelling ecosystem

Abstract

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.