Turtles migrate across seas extensively throughout their lives, and come into contact with a great deal of fishing activity, much of it potentially lethal. In the Mediterranean, twenty-one countries fish over an area that is less than 2 percent the total surface of the Pacific Ocean.[1]
In the first half of the 20th century, Mediterranean turtle populations were severely exploited by fisheries targeting turtles for trade. Nowadays, illegal consumption or trade of sea turtles is no longer a major conservation issue in the Mediterranean (unlike other parts of the world). Legislation today protects against the intentional harvest of turtles at sea. Yet, interaction with fisheries remains a significant threat to sea turtle populations. For example, it’s estimated that over 132.000 turtles are captured each year by this industry, causing at least 44.000 incidental deaths per year.[2] In shrimp fisheries, up to 90 percent of the catch is discarded, killing seabirds, dolphins, and sea turtles.[3]
This problem is called bycatch: living creatures that are caught unintentionally by fishing gear. Unlike the targeted species, bycatch is usually unwanted and unused, for instance if it is undersized or an endangered species, or if the fish catch exceeds the permitted quota. In that case, it is thrown back into the water, becoming a ‘discard’.
The long lifespan and slow reproduction rate of sea turtles make it difficult for populations to recover from these losses. Combined with a decline in suitable breeding areas, the current recruitment into the population from breeding is practically negligible compared to the number of turtles killed in the sea each year.[4]
Research indicates that turtles are more affected by fishing gears such as bottom trawlers, demersal longlines and set nets, and in particular by small-scale fisheries. Turtles get caught in nets or hooked on lines, where they often drown or sustain serious injury from ingesting the sharp hooks. There are three main types of fishing techniques employed by the Mediterranean fishing industries which cause incidental capture of sea turtles.
– Long-line fishing: each fishing unit consists of a floating ‘mother-line’, 300m long, from which hang the hooked ‘groundlines’, each 25m long, at 20m intervals. Units can be strung together in great chains. It is not unusual for 200 units to be strung together to reach a length of 60Km.
– Trawl-net fishing a boat drags an enclosed net behind it. The net forms a sock shaped enclosure catching any matter that enters it.
– Drift-net or gill-net fishing: uses a huge net, essentially an area of mesh; it is not fixed into any assembly. The mesh of the net allows the heads of the fish to pass through while trapping them at their gills. The net is usually dragged behind a single boat in a loop or may be fastened to two boats on a parallel course. This type of fishing is usually used to catch tuna, swordfish and albacore.
For more information visit EuroTurtle and the FAO’s dedicated webpage: http://www.fao.org/…
Sea turtle bycatch can be drastically reduced via technical measures.[5] Longlines using meat bait are especially hazardous, except to the herbivorous green sea turtle (Chelonia mydas). Changing our fishing techniques has huge potential to reduce fishery bycatch. Selective gear modifications present feasible solutions to sea turtle bycatch. Circle hooks can reduce incidental catches of marine turtles in longline fisheries by as much as 65 to 90 percent compared to traditional hooks. Dehookers and line cutters have been developed to remove longline gear without causing further injury to any sea turtle caught, and decrease post-release mortality. Not only this, but in the majority of experimental cases, using circle hooks even increases the catch of target species.[6]
Bycatch in longline fisheries can also be avoided by the baited hooks deeper than 100m, out of the ocean’s mixed species layer, and therefore out of reach of most bycatch, decreasing incidental catches and potentially increasing target catch and profits.[7]
Another form of turtle-friendly selective gear is the turtle exclusion device (TED). TEDs are hard or soft grid panels that allow the target fish to slip through into the net while directing larger creatures, like sea turtles, out an escape opening. For instance, in deadly shrimp fisheries, they are estimated to reduce turtle bycatch by 97 percent.[8]
[1] Wallace BP, Kot CY, DiMatteo AD, Lee T, Crowder LB, Lewison RL. 2013. Impacts of fisheries bycatch on marine turtle populations worldwide: toward conservation and research priorities. Ecosphere 4:art40.
[2] Casale, P. (2011), Sea turtle by-catch in the Mediterranean. Fish and Fisheries, 12: 299–316. doi: 10.1111/j.1467-2979.2010.00394.x
[3] Alverson D L, Freeberg M K, Murawski S A and Pope J G. (1994) A global assessment of fisheries bycatch and discards. FAO Fisheries Technical Paper No 339 Rome, FAO 1994
[4] Demetropoulos, A. 2000. Impact of tourism development on marine turtle nesting: strategies and actions to minimise impact. Convention on the Conservation of European Wildlife and Natural Habitats, Strasbourg, 6 September 2000. Council of Europe. 42 pp
[5] FAO. 2009. Guidelines to reduce sea turtle mortality in fishing operations. Rome. 128p.
[6] Read, A.J. 2007. Do circle hooks reduce the mortality of sea turtles in pelagic longlines? A review of recent experiments. Biological Conservation 135(2): 155-169.
[7] Secretariat of the Pacific Community. 2005. Set Your Longline Deep: Catch more target fish and avoid bycatch by using a new gear design. Pamphlet.
[8] Title 50, Section 223 – Approved TEDs. United States Code of Federal Regulations
