A purse seine is a huge wall of netting that is set in a circle around a school of fish. Once the net is set around the school, a line is pulled and the net closes at the bottom. Purse seining cannot select for a certain type of fish—whatever is encircled by the net is captured. Unwanted fish, turtles, or marine mammals may also be caught. In general, however, fishers try to avoid unwanted species by refraining from setting nets when they are visible from the boat deck. A low percentage of fish caught using a purse seine are bycatch.