Purse seine fishing begins when fishers see a group of dolphins. Most dolphins in the eastern tropical Pacific Ocean travel in groups called pods ranging from a few to over one thousand individuals. Because dolphins breathe air, they must come to the surface periodically. When they surface they can be easily spotted by fishers who are looking for the tuna that are likely swimming below. Using speedboats and helicopters, the fishers chase and herd the dolphins into a small area. These chases take about 20 to 40 minutes but can sometimes last several hours. The fishers then put a mile-long net into the water that encircles the dolphins and the tuna swimming below. The top edge of the net is held at the water’s surface by large floats and extends down into the water several hundred feet like a fence. Using cables, the fishers close the net at the bottom, similar to tightening the drawstrings on a purse--hence the term "purse seine." This prevents the tuna and dolphins from escaping. The fishers then pull the tuna aboard the ship for processing and freezing, in preparation for canning. This entire procedure is called a "set."