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A longline consists of a main line with individual branch lines (known as gangions, snoods or leaders) spaced at intervals along its length, running off a clip or swivel. Baited hooks are attached to each branch line - in coastal and inshore fisheries vessels may use much shorter main lines with fewer hooks, whereas in high seas fisheries main lines can extend for over 100km and deploy thousands of hooks. 

Longlines are classified based on where they are placed in the water column. They can be set to hang near the surface (pelagic longline) targeting fish such as tuna and billfish, or along the sea floor (demersal longline) for groundfish such as halibut or cod. Most commercial longline operations, particularly on the high seas, are fully mechanised – including baiting the hooks, shooting and hauling the lines, unhooking the fish and cleaning the hooks. Due to the nature of this fishing method, the catch of a longliner is often in better condition than that of a trawler, and fetches better prices at market.  

Compared to other fishing techniques, longline fishing has a minimal impact on sea-floor habitats. However, although longlines can employ some degree of selectivity, with hook size and bait determining the size and species (respectively) of fish caught, there are significant issues surrounding bycatch - particularly of pelagic sharks, turtles and seabirds. Within lucrative tuna and billfish fisheries, pelagic sharks such as the Blue Shark (Prionace glauca), Shortfin Mako (Isurus oxyrinchus) and hammerhead (Sphyrna spp.) are taken in large volumes as ‘wanted bycatch’ – although not the target species, their fins and meat are commercially valuable, so the bycatch is retained.

Related Links:

Fisheries and Aquaculture Organisation (FAO) website