Brazilian officials have been quick to blame recent rain water, which they say caused the water temperature to change drastically. They believe this phenomenon and the increase in decaying matter in the waters changed oxygen levels, leading to the death of many innocent fish. However, the State Environmental Institute and the Secretariat of State for the Environment are doing further testing. Because blaming this many fish deaths on rainwater sounds like something out of the Old Testament, rather than an actual scientific statement.
The bay, unsurprisingly, has a history of pollution. CNN reports that just last year, biologists announced that rivers leading into the bay were filled with a superbacteria known to cause urinary, gastrointestinal and pulmonary infections. Sounds like a great place to go for a dip... if you think the East River is a little too clean.