Starving seabirds: unprofitable foraging and its fitness consequences in Cape gannets competing with fisheries in the Benguela upwelling ecosystem
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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.
This study was funded by the FitzPatrick Institute, DST-NRF Centre of Excellence at the University of Cape Town (South Africa) and the Centre National de la Recherche Scientifique (CEFE-CNRS, UMR5175 Montpellier, France). We warmly thank Pierre Nel and SANParks for accommodation and logistical support. We are also particularly grateful to Lea Cohen, Itai Mukutyu, Emilie Tew Kai, Timothée Cook, Bruce Dyer, Rabi’a Ryklief, Leshia Upfold and Plaxedes Vimbai-Rukuni for their help in the field, to Jonathan Green for essential input on seabird energetics and to Carl van der Lingen for comments upon an initial draft version.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
All experiments were performed under permits from South African National Parks with respect to animal ethics (No. CRC/2015/001—2002), and all protocols were validated by the French Direction des Services Vétérinaires (Permit No. 34-369).
- Ancel A, Horning M, Kooyman GL (1997) Prey ingestion revealed by oesophagus and stomach temperature recordings in cormorants. J Exp Biol 200:149–154Google Scholar
- Baillie J, Hilton-Taylor C, Stuart SN (2004) 2004 IUCN red list of threatened species: a global species assessment. IUCN, GlandGoogle Scholar
- Carson R (1951) The sea around us. Oxford University Press, New YorkGoogle Scholar
- Drent R, Daan S (1980) The prudent parent: energetic adjustments in avian breeding. Ardea 68:225–252Google Scholar
- Hockey PAR, Dean WRJ, Ryan PG (2005) Birds of Southern Africa. In: Hockey PAR, Dean WRJ, Ryan PG (eds) Birds of Southern Africa, 7th edn. Trustees of the John Voelcker Bird Book Fund, Cape TownGoogle Scholar
- Lovvorn JR, Gillingham MP (1996) Food dispersion and foraging energetics: a mechanistic synthesis for field studies of avian benthivores. Ecology 77:435–451Google Scholar
- Manly BFJ (1997) Randomization, bootstrap and Monte Carlo methods in biology. Chapman Hall, LondonGoogle Scholar
- Pettex E, Lorentsen S-H, Grémillet D et al (2012) Multi-scale foraging variability in Northern gannet (Morus bassanus) fuels potential foraging plasticity. Mar Biol 159:2743–2756Google Scholar
- Sauer WHH, Hecht T, Britz PJ, Mather D (2003) An economic and sectoral study of the South African fishing industry, vol 2: Fishery profiles. Report prepared for MCM by Rhodes University, GrahamstownGoogle Scholar
- Wilson RP, Pütz K, Grémillet D et al (1995) Reliability of stomach temperature changes in determining feeding characteristics of seabirds. J Exp Biol 198:1115–1135Google Scholar