What is longline ?

Longline is a passive fishing technique using baited hooks. The full fishing gear consists of various branch lines - between 100 and 200 - which hang from a main line ( kept on the surface by buoys ) onto which the hooks are attached. In the case of the Spanish longlining fleet working in the Mediterranean, the toal length of the gear can exceed 40 kilometres and use more than 2,000 hooks, while the large industrial fleets are capable of setting 100 kilometres of longlines.

The hooks are baited with small pelagic fish ( sardinella, mackerel, etc. ), cephalopods or artificial bait ( occasionally luminous ), to attract the swordfish. But other carnivorous species, such as tuna, dolphinfish and sea turtles, can also be caught by this method of fishing.

What is the target species of longliner fleets ?

The majority of longliner fleets working in the North Atlantic specialise in catching swordfish, tuna and occasionally sharks. Sea turtles are an unwanted by-catch, as are many other species such as rays, sunfish, palometas, dolphinfish and certain species of marlin, tuna and sharks.

Although some of these species are commercialised along with the target species, others are thrown back overboard.

Why are turtles caught ? 

The bait used to catch fish by longliners also attracts other carnivorous marine creatures such as seabirds and sea turtles. In the case of the latter, with the exception of the green sea turtle ( Chelonia mydas ), which is herbivorous, all the other species can fall victim to these hooks, particularly if they are used for fishing in surface waters they are easier for the turtles to spot and access.

On biting or swallowing the bait, the hook ends up caught in the mouth or oesophagus of these animals. Extraction can be more or less difficult depending where the hooks is located, so it is not unusual to see turtles that have repeatedly been caught on longlines over the same season, with various hooks and fishing wire sticking out from their mouths.

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