Which Invasive Seaweeds Do We Monitor During Reef Check California Surveys and Why?

Sargassum muticum Photo by: Mike Schwalbach

Each month, Reef Check will answer a technical question regarding the monitoring protocol of our coral reef or rocky reef programs. If you have a question you would like answered, please email rcinfo@reefcheck.org.

Reef Check California – Which invasive seaweeds do we monitor and why?

During Reef Check California (RCCA) surveys, not only do we count canopy-forming seaweeds on our transects, we also search for selected invasive species of seaweeds. Invasive species are species that have been introduced to an area in which they were historically not present, and do or are likely to have adverse ecological or economic effects. Invasive species are often found to spread rapidly and can alter the structure of communities by ‘outcompeting’ native species for space, food or light in the case of algae. Because they are new to an area they may not have natural predators that keep their populations in check. Invasive seaweeds are very common in areas that receive a lot of boat traffic because they are usually spread in ships’ ballast waters, or as fouling agents on a boat. Therefore they often start out in bays, harbors, and marinas.

When invasive species are detected soon after arrival there is a higher chance to eradicate them before they cause much of an ecological effect. Therefore, RCCA records and reports the four most concerning invasive species of algae in California if they are found anywhere during a survey dive. Two closely related species that we monitor, Sargassum muticum and Sargassum filicinum, are fairly widespread in Southern California. These are native to Japan and outside of their original ranges they are found to be highly invasive, clogging marinas and boat pipes, and covering otherwise diverse reef ecosystems. They disperse successfully due to numerous airbladders covering their branches, and their abilities to live and grow as unattached mats and fragments. Currently there are no removal efforts underway in California for these species.

Another seaweed that we monitor is the only invasive kelp (a class of large brown algae) in California, Undaria pinnatifida (aka Wakame –an ingredient of miso soup). This species, recently introduced from Japan, has been found inside harbors all along the California coast, and only on a small area of natural reef on Catalina Island. There have been numerous efforts to eradicate Undaria by manual removal within several harbors.

The invasive seaweed of greatest interest in California is Caulerpa taxifolia. This small green alga was historically a favorite in home aquaria because of its beauty, fast growth, and hardiness. Irresponsible dumping of aquarium water has been a primary dispersal technique of this invasive. It has caused widespread ecological and economic damage in waters it has invaded, particularly within the Mediterranean Sea. Small infestations in California, in Carlsbad and Huntington Harbor, took 6 years and over $7 million to eradicate.

Monitoring the presence of these species in California helps marine managers to study their effects on the environment and assess the need to take action. Early detection of the spread of an invasive is critical for eradication efforts to be successful. Therefore, the more educated divers we have in water the better able we are to detect such a change.

Sargassum filicinum Photo by: Colleen Wisniewski	Undaria pinnatifida Photo by: Peter Haaker	Caulerpa taxifolia Photo by: Rachel Woodfield