The Delta is considered one of the most invaded estuaries in the world. More than 250 alien aquatic and plant species have affected 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 biomass in the Delta. New invasive species will continue to arrive. Combined, these invasive species have created an environment uniquely hostile to native fishes.
- Of the 40 fish species now residing in the Delta, 28 are non-native. 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.
- Asian Clams (Corbula Amurensis) have significantly reduced the abundance of plankton, the base of the 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.