[tgarr]

So you're saying then that oxygen levels are key. But why hasn't anyone detected lower oxygen levels in the ocean? Or have they? Do you have any information on this?

[pointsal]

factory fishing ships (not meager boats)...using the latest satellite technology and mega nets....there's a clue about fish decline. Let's all go back to subsistence and only hunt/fish our local area.

[Argus]

No, it's not oxygen levels in the ocean, but carbon dioxide levels in the atmosphere, that cause the greenhouse effect. Phytoplankton are found closer to the surface of the ocean, because they use sunlight for photosynthesis.

[dirtboy]

So you're saying then that oxygen levels are key. But why hasn't anyone detected lower oxygen levels in the ocean? Or have they? Do you have any information on this?

There actually was an article about this poster on FR a few days ago - they have detected a decrease in deep-sea oxygen levels. I did a bit of Googling to check out the premises and learned something new, although I should have figured this out from other sources - cold water holds more dissolved oxygen than warm water, which is the opposite of how dissolution normally works.