Aleteo, known as finning in english, is an illegal practice used to hunt sharks in Costa Rica for their fins. Shark fins are believed to be medicinal or delicassies in Asia, and have led to the creation of a clandestine fishing industry in Costa Rica, where sharks are abundant on the Pacific Coast.
The government has responded by making finning illegal, however due to unexplainable reasons several loopholes exist in the law, that permit finning to continue to this day.
Aleteo (finning) is a fishing technique where the shark is caught, its fins cut off, and the body thrown back into the water. Unable to swim, the sharks eventually drown or bleed out. Catching a shark is not illegal in Costa Rica: several varies of shark are caught and sold for their meat. But finning the shark and keeping only the fins is illegal.
This condition creates a legal loophole through which finners operate. They'll catch the shark and bring it back to shore whole, then cut off the fins and discard the body (which usually doesn't sell easily). The fins are classified as legal product, but the end result is the same: a dead shark, killed only for its fins.
Through the loophole, the amount of fins that can be procured is very small. Fishing boats can only carry a few whole sharks, and normally at the end the cost of fuel and salaries is too high compared to what the black market will pay for the fins. For this reason, traditional illegal finning is still used in Costa Rica, and the fins are either swapped on the sea with other boats, or hidden inside other cargo the boat might be carrying.
An excellent documentary on finning, Shark Water, can be found on Youtube.