"Even minuscule amounts of BP's crude oil has affected fish in profound ways in the Gulf of Mexico—even when oil in the water was nondetectable. This according to a paper in early view in PNAS (Proceedings of the National Academy of Science)."

"The problems showed up as genetic responses in liver tissue and as aberrant protein expression in gill tissues—and they lived on in fish even after their environment looked and tested clean."

Julia Whitty reports for Mother Jones September 26, 2011.
[2]
SEE ALSO:

"Researchers Find Signs of Toxic Oil Effects in Louisiana Marsh Fish" (McClatchy) [3]

"Oil Spill Affected Gulf Fish’s Cell Function, Study Finds" (Green/NYT) [4]

"Trace Amounts of Crude Oil Harm Fish" (Nature News) [5]