Thousands of dead fish have washed up on shores across the Bay Area in recent weeks. A red tide is killing everything from anchovies to sharks. Preventing a similar disaster may cost the region billions of dollars. In late July, Mary Spicer noticed that the water lapping around her kayak started to turn red. A few weeks later it was dark brown. … Jon Rosenfield, senior scientist with environmental group SF Baykeeper, says Heterosigma may be killing fish in two ways: It can produce a toxin that is deadly to fish, but it can also result in low dissolved oxygen levels in the water, which can be deadly.