oxygen depletion in oceans, caused mainly by agricultural run-off and pollution, creates dead zones where sea creatures can suffocate. A recent study claims that dead zones can also lead to male-dominated fish population, which can drive a species to extinction.

Researchers from the City University of Hong Kong raised zebra fish, a hardy species popular in aquariums, in tanks with normal and low (hypoxic) oxygen concentrations. They found low levels of dissolved oxygen