ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on November 2, 2006
ICES Journal of Marine Science: Journal du Conseil 2007 64(1):169-177; doi:10.1093/icesjms/fsl011
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Trends in numbers of Cape gannets (Morus capensis), 1956/1957–2005/2006, with a consideration of the influence of food and other factors
1 Department of Environmental Affairs and Tourism, Marine and Coastal Management, Private Bag X2, Rogge Bay 8012, South Africa
2 Avian Demography Unit, Department of Statistical Sciences, University of Cape Town, Rondebosch 7701, South Africa
3 Ministry of Fisheries and Marine Resources, PO Box 394, Lüderitz, Namibia
4 Department of Environmental Affairs and Tourism, Marine and Coastal Management, Private Bag X2, Rogge Bay 8012, South Africa
5 Environmental and Coastal Management, PO Box 77000, Nelson Mandela Metropolitan University 6031, South Africa
Correspondence to R. J. M. Crawford: tel: +27 21 4023140; fax: +27-21-4217406; e-mail: crawford{at}deat.gov.za
Crawford, R. J. M., Dundee, B. L., Dyer, B. M., Klages, N. T., Meÿer, M. A., and Upfold, L. 2007. Trends in numbers of Cape gannets (Morus capensis), 1956/57–2005/06, with a consideration of the influence of food and other factors – ICES Journal of Marine Science, 64, 169–177.Cape gannets (Morus capensis) breed at six colonies in Namibia and South Africa. Population size averaged about 250 000 pairs over the period 1956/1957–1968/1969 and about 150 000 pairs from 1978/1979 to 2005/2006. Over the whole 50-y period, numbers at the three Namibian colonies fell by 85–98%, with greater proportional decreases in the south. There were increases at two South African colonies between 1956/1957 and 2005/2006. The colony at Lambert's Bay increased between 1956/1957 and 2003/2004, but attacks by Cape fur seals (Arctocephalus pusillus) on birds at nests caused abandonment of the entire colony in 2005/2006. Long-term changes at colonies are thought to be largely attributable to an altered abundance and distribution of prey, especially sardine (Sardinops sagax) and anchovy (Engraulis encrasicolus). In both Namibia and South Africa, the numbers of Cape gannets breeding were significantly related to the biomass of epipelagic fish prey. Over the 50-y period, there was also a marked similarity in the proportions of gannets and epipelagic fish in the Benguela system, which were present in Namibia and South Africa. In the 2000s, there was an eastward shift in the distribution of sardine off South Africa and a large increase in the number of gannets breeding at South Africa's easternmost colony. When sardine were scarce off South Africa, gannets fed on anchovy, but off Namibia anchovy only temporarily and partially replaced sardine. Ecosystem management measures that might improve the conservation status of Cape gannets are considered.
Keywords: anchovy, Arctocephalus pusillus, Cape fur seal, Cape gannet, distribution, food, long-term change, Morus capensis, sardine
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