The Cape Gannet is strictly a marine bird. Within its normal range Cape Gannets are restricted to the continental shelf, at no more than 100 kilometres from the coast. Occasionally these large seabirds have been recorded on oceanic waters.

They are fish eating birds like most seabirds, that plunge and dive from great heights into the ocean and grab their prey. They favour sardines and other shoaling pelagic fish such as Pilchards. They eat around 300 grams of fish per day.

These birds are very powerful fliers who glide the air thermals frequently flapping to accelarate their flight. This technique of flap-gliding is a more energy consuming flying technique than the other dynamic souring that is used by other large seabirds, such as the Albatross. Cape Gannets are generally quiet at sea, however, when in colonies they will emit a rasping call ‘arrag arrah’.

Cape Gannets are not strictly migratory birds and the majority will remain within 500 kilometres of their breeding site year round, with the possibility of some adult male Gannets continuing to use the breeding grounds as roosting sites throughout the non-breeding season.