What are California Marine Invasive Species?

Marine Invasive Species are animals or algae that have been translocated from their native region to California marine and estuarine waters. Invasive species are also called introduced, exotic, alien, nonindigenous or non-native.

Why are they a Concern?

The introduction of NIS can cause harm to the ecosystem by displacing native species, becoming a human health dangerby introducing new diseases, or cause economic havoc on commercial, agricultural, or recreational activities by clogging waterways, and impacting navigation and recreation.

A prime example of the harm an invasive species can cause is Caulerpa taxifolia, also known as Killer Algae, a strain of green seaweed believed to be released from an aquarium directly into a water body or through a storm drain. The Killer Algae forms a dense carpet on any surface including rock, sand, and mud displacing native plants and animals, disrupts the natural food chain, and seriously impacting recreational and commercial fisheries.

Although Caulerpa was successfully eradicated from California marine waters, few eradications have been attempted due to perceived challenges and high cost (over $7M was expended in the case of Caulerpa).

Number of Invasive Species Statewide

The map below shows that all major harbor areas and Bays in California have received significant NIS introductions. San Francisco Bay has the highest number of non-native species, followed by the Ports of Los Angeles/Long Beach. San Francisco Bay is a hotspot for invasions because it has high recreational boating and commercial shipping traffic, a history of oyster culture, and is adjacent to a highly urbanized area. Our research found that San Francisco Bay is a hub for the spread of NIS to the rest of the West Coast.

A map representing the density of invasive species per monitoring site statewide

Infamous Marine Invasive Species

Green Crab (Carcinus maenas)

Native to Europe and now established on the coast of North America as well as other non-native regions, the Green Crab is sold as fish bait in much of the world. It devours large quantities of prey, including native clams, oysters, mussels and crabs. The Green Crab can greatly affect the commercial shellfish industry (Smithsonian, National Museum of Natural History).

two Green Crabs in a person's hand
CDFW Image

Asian Kelp (Undaria pinnatifida)

The Asian Kelp is an invasive alga that is native to Japan, northern China, and Korea. Undaria, which was first found in Southern California and has since spread to San Francisco and Half Moon Bay, most likely came to the California coast by means of hull fouling. Undaria can out-compete native species by monopolizing resources, resulting in ecosystem alternation (Zabin et al 2009). Undaria continues to spread throughout California. In 2009, it was found in San Francisco Bay.

Map showing the spread of Undaria

Chinese Mitten Crab (Eriocheir sinensis)

Native to the Pacific Coast of China and Korea, the Mitten Crab was first introduced to the West Coast in 1991, likely by means of ballast water or possibly by intentional releases. According to Cohen and Weinstein (2001), it interfered with fish salvage operations, fish passage facilities, water treatment plants, power plants, and other facilities.

An image of the Chinese mitten crab Eriocheir sinensis
Imagine courtesy of the link opens in new windowSmithsonian Environmental Research Center

Clubbed tunicate (Styela clava)

The Clubbed tunicate is native to Eastern Asia and was first observed on the Pacific Coast in the 1930's. It was most likely introduced on the hulls of ships or with imported oysters (Clarke and Therriault 2007). Because of its hardy nature and ability to withstand salinity and temperature fluctuations, it has established a widespread non-native distribution. It out-competes native organisms for food in the water column and feeds on the larvae of native species causing population declines (Centre for Agriculture and Biosciences International 2015).

Styela clava (also known as a sea squirt)
Image courtesy of Wikimedia Commons link opens in new windowtaken by Matthieu Sontag, via Creative commons, CC BY-SA 3.0.

Common Invasive Species Pathways

Non-indigenous species are introduced to California waters via pathways (also known as vectors). Below are some of the most common pathways for introduction of new species into California

Vessel Biofouling

Fouling species can attach directly to ship hulls, along with mobile species that may be associated with them. Biofouling organisms are introduced to a new environment when they fall off the ship structure to release larvae. Vessel biofouling has been identified as one of the most important vectors for marine NIS introductions in several world regions, including California (Ruiz et al. 2011).

Ships hull getting cleaned in dry dock
Image courtesy of Wikimedia Commons created by link opens in new windowTemplar52 via link opens in new windowCreative commons.

Ballast Water

Ballast water is loaded on-board ships to adjust buoyancy and provide stability. The transport and discharge of ballast water to the port of call provides the opportunity for marine organisms to become established in a non-native region. It is estimated that more than 7000 species are moved around the world on a daily basis in this way (Carlton 1999).

Drawing of ballast water uptake and discharge
Image courtesy of Wikimedia Commons created by link opens in new windowMaxxl2 - Ballastwassertanks, via Creative commons, link opens in new windowCC BY-SA 3.0.


These non-native fish, invertebrates, and seaweeds were brought intentionally to California waters to establish new populations for fisheries or aquaculture.  Introduced fish often have unintended consequences for their new ecosystem, including parasites or disease. Fisheries also unintentionally bring NIS to California waters through accidental release of live fish, crustaceans and molluscs imported for human consumption, as well as accidental trans-location of species attached to aquaculture gear (floats, cages, etc).

photo of a non native fish swimming at the bottom of the ocean
Image courtesy of USFWS


Many aquatic organisms live on the shells of oysters. When oysters were trans-located to a new site to be farmed, the hitchhiking species were inadvertently introduced. The transfer of species through commercial oyster culture has declined over time, corresponding to the decrease in transfer of oysters and stocking activity in California.

Crassostrea gigas
Image courtesy of Wikimedia Commons User link opens in new windowMaxxl2-Ballastwassertanks through the Creative commons link opens in new windowCC BY-SA 3.0 License

Contact Information

Karen Bigham
Email: Karen.Bigham@wildlife.ca.gov
Phone: (916) 445-4562

Last update : 1/23/2023 1:56:06 PM

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