Gillnets are entangling nets suspended vertically in the water column at variable lengths. These "net walls" maintain their vertical orientation by flotation devices on the top (float) line and weights or anchors on the lower (lead) line. Gill nets may be deployed in all parts of the water column by adjusting the flotation or weights used. On the sea floor bottom gill nets are set to catch demersal fishes, in mid-water they target mid-water fishes, and surface driftnets are used to capture both surface and mid-water species. Fish are entangled in the nets by their gills. The holes in the mesh are variable in size; larger meshes allow individuals of smaller sizes to escape the nets. Gillnets are used in coastal and pelagic waters, sometimes unanchored (drift nets), and can be set at multiple depths. The time that nets remain in the water varies between several hours to several days. Nets may be anchored by weights or floats. Trammel nets are multi-layered gillnets. Fish are ensnared in the middle layer that has the finest mesh size. They are generally set near the sea bottom. Encircling gillnets are employed in shallow water and set in a circle. Fishers create a disturbance in the water that drives the fish into the nets where they become entrapped.