We indiscriminately kill sharks only for their fins. The sharks are still alive when they are finned. They get dragged onto boats, have their four fins sliced off and are thrown back into the ocean to either bleed to death or drown. Shark fin soup sells for up to $350 a bowl in Japan.
Many sharks are now facing extinction.
Sharks are predators at the top of the food chain. If they disappear there will be a massive increase in the number of smaller fish normally eaten by sharks. These smaller fish eat phytoplankton. Turns out that the phytoplankton supplies over 50% of the oxygen (by taking in CO2, incidentally) of the planet. Without sharks to keep the smaller fish population in check, we may be endangering our own source of oxygen.