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

International audienceFisheries are often accused of starving vulnerableseabirds, yet evidence for this claim is scarce. Foragingenergetics may provide efficient, short-term indicatorsof the fitness status of seabirds competing with fisheries.We used this approach in Cape gannets (Morus capensis)from Malgas Island, South Africa, which feed primarilyon small pelagic fish in the southern Benguela upwellingregion, thereby competing with purse-seine fisheries. Duringtheir 2011–2014 breeding seasons, we determined bodycondition of breeding adult Cape gannets and measuredtheir chick growth rates. In addition to these conventionalfitness indices, we assessed the daily energy expenditure ofbreeding adults using a high-resolution time-energy budgetderived from GPS-tracking and accelerometry data. Forthese same individuals, we also determined prey intakerates using stomach temperature recordings. We found that adult body condition and chick growth rates declinedsignificantly during the study period. Crucially, mostbirds (73 %) studied with electronic recorders spent moreenergy than they gained through foraging, and 80–95 % oftheir feeding dives were unsuccessful. Our results thereforepoint to unprofitable foraging in Cape gannets, witha longer-term fitness cost in terms of adult body conditionand reproductive performance that corresponds to a localpopulation decline. Based on this evidence, we advocatea revision of regional fishing quotas for small pelagic fishand discuss the possibility of an experimental cessation ofpurse-seine fishing activities off the west coast of SouthAfrica. These measures are needed for the ecological andsocio-economical persistence of the broader southern Benguelaupwelling ecosystem