The diet of the kelp gull is very varied, and includes molluscs, fish, worms, echinoderms, arthropods (sometimes including swarming termites), reptiles, amphibians, birds, small mammals, and berries (2) (4) (6). Sickly lambs and young poultry are also sometimes taken, and the kelp gull may even attack and kill adult birds as large as geese (2). It also scavenges on rubbish, sewage, carrion, and fish offal, and can often be seen following boats at sea or foraging around abattoirs, fish or seafood factories, or sewage outlets (2) (4) (6). Food may sometimes be stolen from other birds such as terns and penguins (2) (4), and in some areas the species associates with southern right whales (Eubalaena australis), capturing prey items stirred up by the whales (2). The kelp gull is able to dive briefly to obtain food from below the water surface, and has also been seen to drop molluscs onto hard surfaces to break open the shells (2) (4).
The kelp gull breeds between September and January, in colonies numbering from one to several hundred pairs (2) (6). The species is monogamous, with pair bonds usually maintained from one breeding season to the next, and strengthened by courtship feeding during winter months. Both adults help to incubate the eggs and raise the chicks (4). The nest, built on bare rock, sand or mud, often in a well-vegetated site, is a large and bulky structure, constructed from dried vegetation or seaweed (2) (4) (6). Around 3 eggs are laid (2) (4), which hatch after 24 to 30 days, the young kelp gulls fledging approximately 7 weeks later (2). Kelp gulls begin to breed from around three to four years old, and may live for 20 years (4).