The calm, shallow waters of coastal estuaries are easily polluted by nutrient-rich sewage. When estuary waters become overnutrified as a result, algae proliferate. The abundant algae, in turn, sometimes provide a rich food source for microorganisms that are toxic to fish, thereby killing most of the fish in the estuary.
Fish in an estuary that has been polluted by sewage are generally more likely to die from pollution than are fish in an estuary that has been polluted in some other way can be properly inferred from the information above.