Why are there few fish in the sea

The ocean takes up 70 percent of the Earth's surface, but contains only 15 percent to 25 percent of the Earth's total estimated species. John Wiens, a professor of ecology and evolution at Stony Brook University in New York conducted a research which said Freshwater fish could have diversified from saltwater ancestors, only to see those ancestors wiped out in ocean extinctions. Such extinctions would free up space for some freshwater fish to evolve, once again, to thrive in the ocean.

"Looking at a group in which all these species are aquatic … helps us to isolate what's special specifically about the ocean," Wiens said.
He and his co-authors pulled information on all living fish species from a comprehensive database called FishBase. Next, they combined that information with a family tree of ray-finned fish that shows relationships between groups and clades (groupings of organisms consisting of an individual species and all its descendents). The researchers also put together a tree for known fossil fishes.