MEPS 505:281-293 (2014)  -  DOI: https://doi.org/10.3354/meps10762

Changes in prey availability impact the foraging behaviour and fitness of Cape gannets over a decade

Lea A. Cohen1,*, Lorien Pichegru2,3, David Grémillet1,4, Janet Coetzee5, Leshia Upfold6, Peter G. Ryan1

1DST/NRF Centre of Excellence at the Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch 7701, South Africa
2DST/NRF Centre of Excellence at the Percy FitzPatrick Institute of African Ornithology, Department of Zoology, Nelson Mandela Metropolitan University, Port Elizabeth 6031, South Africa
3Seabird Division, BirdLife South Africa, PO Box 515, Randburg 2125, South Africa
4Centre National de la Recherche Scientifique, CEFE-UMR 5175, 1919 Route de Mende, 34293 Montpellier cedex 5, France
5Branch: Fisheries Management, Department of Agriculture, Forestry and Fisheries, Private Bag X2, Rogge Bay 8012, South Africa
6Oceans and Coasts, Department of Environmental Affairs, Private Bay X2, Rogge Bay 8012, South Africa
*Corresponding author:

ABSTRACT: Seabirds respond to environmental changes by adjusting their breeding and foraging strategies, but this behavioural flexibility has limits. Cape gannets Morus capensis breeding in the southern Benguela on Malgas Island off South Africa’s west coast have experienced large fluctuations in natural prey availability over the past decade, linked to environmental change and localised overfishing. When small pelagic fish are unavailable, breeding gannets increase their consumption of low-quality fishery discards (primarily hake Merluccius spp.). To investigate the limits of foraging flexibility of breeding gannets facing variable prey availability, we monitored foraging behaviour, nest attendance, adult body condition and chick growth between 2002 and 2012, along with diet composition and prey abundance (through annual hydroacoustic assessments) during the birds’ breeding season. The combined biomass of sardine Sardinops sagax and anchovy Engraulis encrasicolus within the Malgas gannet colony’s foraging range varied tenfold across the study period and was positively correlated with the proportion of these high quality fish in the gannets’ diet (17 to 90%). Foraging effort increased and nest attendance decreased with decreasing sardine/anchovy consumption. Adult body condition was negatively impacted by increases in hake in the diet. Chick growth was lowest when low sardine and anchovy composition was coupled with an increase in adult foraging effort, suggesting a limit to behavioural compensation for food shortages. This long-term study demonstrates the consequences of variable prey levels for Cape gannet behaviour and fitness. These results highlight the need for detailed investigations of seabird–fishery interactions, and the necessity to limit fishing within Cape gannet foraging ranges during years of low natural prey abundance.


KEY WORDS: Seabirds · Chick growth · Fishery discards · Pelagic fish · Foraging effort


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Cite this article as: Cohen LA, Pichegru L, Grémillet D, Coetzee J, Upfold L, Ryan PG (2014) Changes in prey availability impact the foraging behaviour and fitness of Cape gannets over a decade. Mar Ecol Prog Ser 505:281-293. https://doi.org/10.3354/meps10762

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