Undaria pinnatifida
(Asian Kelp)

Citizen Science - Get Involved!

Have you  seen this kelp?

mature Undaria pinnatifida kelp in San Francisco Bay area
Undaria pinnatifida sporophyll
immature blade of Undaria pinnatifida
 Top image: Mature Undaria pinnatifida with pinnae dividing blade and sporophyll present at bottom of stipe. Middle image: Closeup of sporophyll from a mature Undaria plant.  Bottom image: immature, undifferentiated (without pinnae) blade of Undaria.  Photos © Marine Invasions Research Laboratory at SERC

Undaria pinnatifida is a quick-growing non-native kelp, also known as Asian kelp, which arrived to Southern California in 2000 and quickly spread north to Monterey. Uncontrolled, it can become a fouling species on ship hulls, nets, fishing gear, moorings, ropes, and other marine structures and is capable of having a profound influence on the ecosystem. Recently, small populations have been found in San Francisco Bay and Pillar Point Harbor in Half Moon Bay.

Because of its prolific growth and large size, Undaria can quickly foul natural and man made structures, causing economic and ecological damage. It also competes for light and space with native populations of marine algae, plants and animals, drastically affecting native ecosystems. While native to Japanese and Asian waters, Undaria has invaded European waters, New Zealand, California and Mexico.

Scientists from SERC, UC Davis, the Monterey Bay National Marine Sanctuary, NOAA, the California State Lands Commission, and US Fish and Wildlife Service have been working to remove the kelp by hand from infested marinas. SERC scientists are also continuing to look for new populations at Bay Area marinas and are spreading the word to boaters via posters, fliers, marina newsletters and the press.  If you are interested in assisting us with removals, please click here for Undaria removal events.

How can I help?
We have a new citizen science website where you can learn how to undentify Undaria, participate in removal efforts and upload removal data.  Boat owners, harbor visitors and divers can inspect vessel hulls, boat slips, moorings, docks and piers. Please photograph any suspected Undaria and email to zabinc@si.edu. If you can safely remove the kelp, please do so, photograph and keep in a plastic bag in the refrigerator for identification purposes until scientists can respond to your information.

Undaria is a large golden-brown seaweed that can measure between 3-9 ft. Young plants have an undivided, wide flat blade, while mature plants are distinguished by pinnae or “fingers” dividing the blade, and a fluted, ruffled, spiraling reproductive structure called the sporophyll that forms around the main stem or stipe near the holdfast of the kelp. This sporophyll is not found in native kelp populations. Both the young and mature plants have a midrib, which is not present in most native kelp. The combination of the color, shape, midrib and sporophyll, along with a crinkled texture, distinguish it from related native kelps.

Undaria grows in the low intertidal zone to depths of approximately 15 feet and tolerates a wide range of temperatures, although it grows best in cooler temperatures. A rapid growing and hardy species, it can colonize almost all hard surfaces including rocks, wood, plastic, rope, buoys, docks, pilings and vessel hulls as well as natural substrates such as rocky reef habitats and shell. In sheltered waters it can quickly form large kelp forests blocking out sunlight to native kelp populations and other marine algae and wildlife.

native laminarian kelp (left) and mature invasive Undaria (right) from San Franscisco Bay
Native Laminarian kelp (left) and mature Undaria pinnatifida (right).  Note the smooth, dark green blade of the native kelp as compared with the brown wrinkled appearance of Undaria.  In this case, both the native and invasive kelp has a midrib.  Photos © Marine Invasions Research Laboratory at SERC


References and links




Contact us to join the volunteer effort to detect and control this invasive kelp!

© SERC Marine Invasions Research Laboratory 2009