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The middle of the oceans are the deserts of the marine ecosystem. Nutrients are washed into the sea via rivers, so coastal waters are rich in nutrients and support complex food webs. The middle of the oceans are poor in nutrients (and shelter for fish!), and don't have many fish.

The middle of the ocean is an entirely different eco-system than the continental shelf areas.

Probably a weak analogy but think forest vs plains (if not desert). The coasts, fed by nutrient rich rivers is where a lot of the life is.

There is no such thing as an unlimited supply of fish. Even if there were as many fish in the middle of the ocean as near the shore we would deplete it quickly.

Ocean ecosystems are as different as terrestrial ecosystems. There are forests, and deserts also. Life is patched and not always easy to found.