Bull kelp is a common name for the brown alga Nereocystis luetkeana which is a true kelp in the family Laminariaceae.
Species in the genus Durvillaea are also sometimes called "bull kelp", but this is just a shortening of the common name southern bull kelp. Durvillaea is a genus in the order Fucales and, though superficially similar in appearance, is not a true kelp (all of which are in the order Laminariales).
Bull kelp and southern bull kelps-
Bull kelp (Nereocystis luetkeana) at Caspar Point, California
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Southern bull kelps (Durvillaea) in Otago, New Zealand
