Hii Mafrum
Just to add to Fiona's answer:
There are about 60 genus and 350 species of sharks, in all seas and a wide variety of depth. All the sharks , from the smallest to the largest, are predators. They eat all the sea creatures (Most of them are, even, cannibals) - :fishes,crabs,molluscs and sea-mammals.
So, - even if they would extinct (all of them ??) It Would not change very much the ecosystem, because thier "food" would have now more animals to eat themselves, they will increase in number, and make a new equilibrium in their environment. The lump sum of the fishes that the world use as food will not change much.