Nature Notes: Non-Native Organisms in the Bay

by James Knuckey, Teaching Intern

Marine animals and plants that are not historically found in the San Francisco Bay regularly find their way into local ecosystems.

How? Some of these species were intentionally introduced, such as the Striped Bass (Morone saxatilis), which was introduced from the Atlantic Ocean in 1879, for recreational fishery purposes.

Even though these intentional introductions occur, they are not the most common. Far and away the most common vector for biological introductions in the San Francisco Bay is through global shipping.

One of the primary ways that non-native organisms arrive in the Bay is in ballast tanks. Ballast tanks are compartments within a ship that are filled with water. By adding this water, the ship’s weight is increased, thereby lowering the ship’s center of gravity and providing greater stability. Without ballast water the ship could flip over or break up while at sea. Ships suck water into the ballast tanks while docked at a port, and often discharge the water just prior to arriving at their destination port. Often the ports where these large, oceanic ships take on ballast water are in another country, and foreign organisms are sucked up with the water, and then transported around the world (in this case, to San Francisco Bay).

Non-native organisms that arrive in these tanks are incredibly harmful to the native community, and often reshape the entire ecosystem. In San Francisco, non-native organisms are known to cause lower levels of plankton, which other native species, such as the endangered Delta Smelt (Hypomesus transpacificus) feed on. They have also been linked to clogged water pumping stations, destroyed levies (from burrowing non-native species), and damage to local fish populations, sometimes resulting in the failure of local fisheries. Unfortunately, non-native organisms can thrive in the Bay because they have no natural predators in this new environment.

In some areas of the San Francisco Bay 90% of the biomass is non-native and there are estimates that 300 non-native species now reside in the Bay. Many establish themselves within a couple of years. The abundance of these species makes San Francisco Bay the most invaded estuary/bay in the world. Luckily, the government of California is taking action.

California has the strictest rules on ballast water in the nation, which has led to a significant decrease in the arrival of new organisms. Ships are required to either discharge their ballast water 200 miles from the shore, or to treat their water before discharging it into the Bay. In 2010, the California government signed a law that ships cannot discharge water that may contain organisms larger than 50 microns (the size of a grain of sand). By 2020, there will be a law which stops the discharged ballast water from containing any detectable organisms at all.

Researchers are currently working on systems that can be placed in ballast tanks that would kill all of the waterborne organisms. Some of the techniques that are currently being tested are: treatment with chemicals, ultraviolet light, heat, or pumping the water into onshore treatment plants. So luckily, there is hope for stopping the invasive species that are arriving in the Bay. We may not be able to entirely eradicate the non-native species that have already taken up residence here, but we can halt the arrival of more species that threaten the natural ecology of the San Francisco Bay. 


Miller, Amy. San Francisco Bay Invaders. QUEST Northern California.

California Department of Fish and Game Marine Invasive Species Program.

Staff Writers. Researchers calculate the global highways of invasive marine species. Terra Daily.

Hidden Cargo: How Invasive Species Sneak into the Bay. Bay Area Monitor.

Bass, Striped. International Game Fish Association.