The Delta is considered one of the most invaded estuaries in the world.

More than 250 alien aquatic and plant species have impacted the Delta and at least 185 of these species have gained a foothold and are currently inhabiting – and altering – the Delta’s ecosystem. Invasive species represent at least 95 percent of the total biomass in the Delta. New invasive species will continue to arrive. Combined, these invasive species have created an environment uniquely hostile to native fishes.

Black Bass

Predator fish species, such as largemouth bass and striped bass, not only directly consume native species, they also compete with juvenile salmon and delta smelt for food. Of the 40 fish species now residing in the Delta, 28 are non-native.  Visit our Striped Bass and Predation pages and read Delta Watch for additional information of  non-native fish predation.  

Asian Clam

Asian Clams (Corbula Amurensis) have significantly reduced the abundance of plankton, the base of the Delta aquatic food supply and may affect the feeding efficiency and growth of delta smelt larvae. These voracious eaters of plankton have altered entire food webs, harming smelt, salmon and other native Delta species.

water hyacinth

Water Hyacinth is an extremely prolific aquatic invasive plant that can double in size every 10 days in hot weather and can quickly become a dense floating mat of vegetation up to 6 feet thick. Mats can attach to structures in the water, limiting access to boats, reducing swimming and fishing areas, and restricting water flow.

From an ecological perspective, water hyacinth can be very damaging to water quality by blocking photosynthesis.  Without this basic function, other plants in the ecosystem are unable to add oxygen to the water, reducing other aquatic life such as fish and other plants.  Further, water hyacinth can limit biological diversity by restricting other species of plants and blocking access to water for fish and wildlife or eliminating plants these organisms depend on for shelter and nesting.

The hyacinth has a strong hold on the Delta and is now believed to be impossible to eradicate.