>90% of it comes from plankton.
Safety in numbers.
The first point is that predators can’t congregate in huge numbers. They are dispersed around the ocean. I an average predator eats 100 fish a day, and the ocean as a whole contains a hundred million million predators, then if the fish dispersed they would all be eaten in one day. In contrast, any given area of ocean can only contain a few thousand predators. That means that only a few hundred thousand fish get eaten each day, rather than billions. That’s a huge difference.
The second point is that large numbers of fish mean large numbers of eyes watching out for predators. Isolated fish are going to be picked off immediately. Large numbers will see the predators coming and take evasive action.
The third point is that the large numbers are confusing to predators. A single fish is easy to chase down. When it swims into a school, it gets lost and the predator has to try to acquire another target to pursue.
Each species is specialised, but it’s the hunting technique, prey location, defensive strategies etc. that are specialised, not the prey.
Dolphins are specialist social hunters. They rely on teamwork to capture food and for defence against their own predators. Dolphins are also very energy hungry: it takes a huge amount of food to keep one dolphin alive, and they live for a very long time. They also need to breathe air, so they are restricted to the uppermost levels of the oceans. So dolphins exist in small numbers in tight-knit groups close to the surface and always need to be travelling between food sources.
Fish such as tuna will congregate in schools, but they aren’t social and they don’t actively defend each other against predators. They also need a lot less food than dolphins. Sharks take that to the extreme, with very low food requirements and an ability to eat almost anything, so they can simply cruise the oceans and hope to run into food.
Birds are even more energy hungry than dolphins, but they can compensate by being able to cover vast amounts of territory to locate prey. They can only hunt in the top few metres of the ocean.
And so on an so forth for all the predators. Even on land most predators are like that. Coyotes, foxes, hawks, cats etc. don’t have wildly divergent prey types. The difference specialisation is in hunting technique and other survival strategy, which prevents their niche from completely overlapping.
Not until you get to really huge sizes, as in several tonnes. At that level the ability to isolate oxygenated and deoxygenated blood becomes much more favourable, but at the size of birds and dolphins lungs don’t have any inherent advantage over gills, which is why the vast majority of marine life in that size range still have gills. If you collected all the individuals or all the species in the in that size range, 99% of them would have gills, so clearly lungs aren’t inherently advantgeous.
Penguins also commonly do it.
It’s not well suited to fish because it involves lifting the front end out of the water and maintaining an air bubble over the fron of of the body. Of course fish need to keep their mouths in the water in order to be able to breathe, which kinda preclude porpoising.