I've written about the threat posed by invasive species to wildlife and, particularly, the role of ballast water discharges in spreading them. Well, now the poster child for aquatic invasive species, the zebra mussel, has been found in California (hat tip: Andrew Sharpless).
Zebra mussels, whose native habitat is the Black, Caspian, and Azov Seas, is thought to have first arrived in the United States around 1988 after hitching a ride in the ballast of oceangoing vessels traveling to the Great Lakes via the St. Lawrence Seaway. By 1990 zebra mussels has spread throughout the Great Lakes and, as you can see from the map below, have continued their march across the United States, in part due to their ability to live outside of the water for several days. 
Zebra mussels also cause widespread ecological and economic dislocation. Among other things, zebra mussels can substantially reduced the biomass of phytoplankton in waters that it infests, thus having ripple effects across entire ecoystems. The According to a 1995 Congressional study, the damage caused by zebra mussels to water pipes, power plants, navigational aids, and fishing gear, totaled at least $5 billion dollars, a figure that has only risen since. The continued spread of this invasive species across California and the Northwest is now probably unavoidable.