Invasive Kelp Spreads into New Territory

June 2017 - Do you know that seaweed from your miso soup? Tasty as it is, Wakame, or Undaria pinnatifida, is not only known as a celebrated staple of some Asian cuisines. It is also a notorious marine invader.
Person leaning over the side of a dock, looking at Undaria plants clinging to the side, just below the water line
A dock in Ventura Harbor with an Undaria infestation just below the waterline. Undaria was first discovered in Ventura Harbor, the mainland home of Channel Islands National Park, in 2008.
Undaria is a large, golden-brown kelp native to Japan, Korea, and China. It doesn’t travel fast or far on its own, but it is a skilled hitchhiker. As ship and boat traffic has increased around the world, it has caught rides to distant shores on boat hulls or in ballast water. If conditions are right in the new harbors it reaches—and a wide range of conditions will do—it will spread. Before long it can cling to new boat hulls, clog docks, smother fishing gear, disrupt marine farming, or even alter marine ecosystems. Such impacts have earned it a spot among the world's 100 worst invasive species.

Undaria was first found in California in 2000 in Long Beach Harbor. Since then it has been inadvertently carried by boat to other harbors up and down the California coast. It hitchhiked to Catalina Island and Channel Islands Harbor early on. In 2008, it was discovered in Ventura Harbor, the mainland home of Channel Islands National Park. Soon after, the park created outreach materials such as dockside posters to educate local boaters on how to recognize and prevent the spread of this invader. The park’s islands seemed to remain infestation-free. Then, in June 2016 Kelp Forest Monitoring Program (KFMP) biologists discovered the invader during their annual survey of a monitoring site on the north side of West Anacapa Island. An even more recent Undaria discovery took place at San Clemente Island to the south of the park this June.
Undaria plants growing along a Kelp Forest Monitoring Program transect on the seafloor.
Undaria plants were discovered in June 2016 during an annual Kelp Forest Monitoring Program survey of a monitoring site on the north side of West Anacapa Island. The infestation is the first to be recorded in Channel Islands National Park islands.

NPS

The initial discovery at Anacapa Island consisted of about 100 individual Undaria plants, including reproductive adults. KFMP follow-up surveys revealed a much larger infestation along much of the island’s north side. Because of its extent, biologists think that the infestation likely began several years ago. They now worry that Undaria may increase its rate of spread around Anacapa Island. Due to its quick growth and large size, it could compete with native seaweeds for space and light, changing the island’s marine ecosystem.
Undaria plants dangling from the hull of a boat raised out of the water
Here, Undaria hangs from the stern of the Sea Ranger II. Boat sterns are most susceptible to infestation since they are less likely to be exposed to high water movement. Boaters can help by regularly checking and cleaning their vessel hulls, especially right before a visit to the Channel Islands.

As KFMP continues to assess the infestation, they have also evaluated potential control and extermination actions. Part of their evaluation has involved measuring a few tagged Undaria plants in Ventura Harbor. This gives biologists a sense of local growth rates. Although the data is limited, so far it seems that the local lifecycle is quite fast. The young plants studied reached maturity in as little as 2-3 months, and mature plants started dying at about four months of age.

The KFMP has also looked at efforts to control Undaria elsewhere in California and around the world. They have found that any attempt at eradication or long-term control is highly unlikely to succeed, regardless of effort or cost.

Dense stands of Sargassum horneri covering the sea floor
Sargassum horneri first began spreading in Southern California in 2003. It now dominates large swaths of kelp forest habitat on several of the Channel Islands.

NPS / Brett Seymour

The good news is that boaters can still help protect the Channel Islands from additional Undaria infestations by regularly checking and cleaning their vessel hulls, especially right before an island visit.

While Undaria is a recent arrival to Channel Islands National Park, it is not the first marine invasive species to take hold in park waters. Devil weed (Sargassum horneri) is an example of another Asian kelp that began spreading rapidly in Southern California in 2003. It now dominates large swaths of kelp forest habitat on several islands. Through their suite of monitoring protocols, the KFMP is continually documenting native and invasive species alike. In time, KFMP data will also help us understand how invaders like Undaria are impacting Southern California’s kelp forest community.

For more information on how to recognize and respond to Undaria, check out this fact sheet or visit http://undaria.nisbase.org/.


Prepared by the Mediterranean Coast Inventory and Monitoring Network.

Last updated: June 29, 2017