Longline fishing is a commercial fishing technique. It uses a long line, called the main line, with baited hooks attached at intervals by means of branch lines called snoods. A snood is a short length of line, attached to the main line using a clip or swivel, with the hook at the other end. Longlines are classified mainly by where they are placed in the water column. This can be at the surface or at the bottom. Lines can also be set by means of an anchor, or left to drift. Hundreds or even thousands of baited hooks can hang from a single line.
In South Africa there is two types of commercial longline fisheries in operation. The pelagic longline industry catch tuna and swordfish by setting hooks near the surface of the water. The demersal longline industry catch Hake and Kingklip by setting hooks on the sea floor.
Seabirds are most vulnerable to mortality on longline hooks during the short period between hooks leaving the vessel and sinking beyond the diving range of foraging seabirds. Mitigation measures are designed to prevent contact between seabirds and hooks during this critical period. The period during which bait are available to birds is determined by the sink rate of the line, the diving ability of the bird species present and the use, or not, of seabird deterrents. Seabirds can also be hooked and potentially injured during line hauling.