ABSTRACT: We compared the foraging ecology of Cape gannets Morus capensis attending 2 colonies of equivalent size, yet with contrasting diet and population trends. One colony, on the west coast of South Africa, is decreasing in size and its occupants feed mainly on fishery wastes, whereas the other colony, on the south coast of South Africa, is growing and its occupants feed exclusively on natural prey (pelagic fish). In October and November 2005, we examined the diet, at-sea behaviour, and energy requirements of breeding gannets using direct observations, miniaturised GPS loggers, and time-depth recorders attached to foraging adults. Concurrent hydroacoustic surveys allowed us to assess the distribution and abundance of their preferred prey (the sardine Sardinops sagax and anchovy Engraulis encrasicolus). Birds from the declining west coast colony foraged in areas containing very low abundances of pelagic fish. They fed primarily on low-energy fishery discards. They increased their foraging effort and exploited a greater area than birds from the growing colony, which took advantage of abundant pelagic fish stocks in their foraging range. A marked eastward shift of pelagic fish initiated in the late 1990s has resulted in the shortage of natural prey to Cape gannets on the west coast, strongly suggesting that the local population trend is driven by food availability during the breeding season. A bioenergetic model showed that enhanced availability of low-energy fishery discards does not seem to compensate for the absence of natural prey. Added to the predation pressure by the Cape fur seal Arctocephalus pusillus and the great white pelican Pelecanus onocrotalus, those threats weigh heavily on a vulnerable seabird population.