Fishing in the middle of the ocean?
As others here have already pointed out, the middle of the ocean is somewhat ;ike a desert. There is LOTS of life there, but most of it is just around microscopic in scale.
As others have noted, predators like the billfish (swordfish, marlins, sailfish, etc.) will follow prey fish that migrate through or that collect around large floating accumulations of flotsam (clumps of detached kelp that is floating around, garbage from human vessels, sections of old net that have broken off, trees blown into the sea during big storms, etc.). These floating masses can be small, like someone’s ice cooler that got bumped overboard; or they can be quite large, perhaps covering an acre of surface and hanging dow into the water 50 to 100 feet. Just this bit of shade and increased surface area where things can attach helps generate a huge diversity of tiny critters that make good food for small fish. These little “islands” of detritus and junk attract large numbers of small fish and sometimes serve as breeding grounds for some smaller species. These smaller fish then attract larger predators like the billfish and sharks, and occasionally they attract tuna that are passing by.
If you wanted to fish out in the open ocean, these “junk” structures would be the place to start.