Every year, dissolved oxygen in large portions of the sound falls almost to zero, killing thousands of fish. On any given day about a third of the nation's shellfish waters are closed because of pollution. Researchers have found cancerous liver tumors in as many as a fourth of bottom-dwelling fish. Biologists have reported that sole collected in polluted parts of Boston Harbor and Washington State's Puget Sound carry high levels of toxins in their tissues. Sewage sludge in concentrations as low as one part in a thousand can kill herring and cod eggs.
Pollutants also kill marine species indirectly by degrading their habitat.
Some scientists believe worldwide die-offs of the tiny creatures that create coral reefs are related to pollution or some other human impact. The coral reefs are particularly important because they are perhaps the most species-rich areas in the ocean. Many fish species use such reefs as nursery areas for young and as feeding grounds.