Giant kelp rafts wash ashore 450 km from the nearest populations and against the dominant ocean current

On 9 August 2020, two local marine naturalists (authors W. Marshall-Grey and J. Rankin) on the south coast of New South Wales, Australia noticed a significant amount of a large unfamiliar kelp washed up on a local beach. A browse through Graham Edgar's iconic marine guidebook for temperate Australia (Edgar, 2012), followed by some quick confirmations via phone and email, revealed that the unfamiliar seaweed was giant kelp (Macrocystis pyrifera, Figure 1): a species whose closest known populations are >450 km away to the south (in Tasmania and western Victoria) and whose transport to New South Wales would have required oceanic rafting over several weeks and hundreds of kilometers against the prevailing south-flowing East Australian Current (Figure 2). Subsequent community-led searches over the following days confirmed four more locations of often-substantial amounts of giant kelp wrack, as well as many more anecdotal and unconfirmed accounts.