An ongoing decrease in oxygen levels underwater is an important component of the loss of marine life. More than 400 "dead zones" – where aquatic life can no longer survive – existed in the world’s oceans in 2007, according to a study led by a scientist at the Virginia Institute of Marine Science, compared to 150 in 2003.

Deprived of essential oxygen, these marine areas span 245,000 square kilometres and threaten vertebrate animals, with more than a third of marine mammals affected.

The phenomenon has been ongoing since the 1980s and is proliferating, while research on the subject lags behind.