Excerpts:

Not only does warm water hold less dissolved oxygen than cool water, it also tends to divide into layers that don’t readily mix. According to one recent study, the ocean has been losing oxygen since the mid-1980s, likely because rising temperatures have impeded circulation

 “When oxygen goes way down, it’s effectively habitat loss,” Levin says. “They might move north, they might move upslope into shallower water.” Species that can’t easily relocate, like muck-dwelling invertebrates, may perish.

The cruel corollary to deoxygenation is that warmer waters also drive up animals’ metabolic rates, forcing them to use more oxygen to breathe. As Curtis Deutsch,  a chemical oceanographer at the University of Washington, puts it, “They need more, at the same time that they have less.”

Yale School of Forestry and Environmental Studies

http://e360.yale.edu/features/feeling-the-heat-warming-oceans-drive-fish-into-cooler-waters