This week, in this article we discuss how can we save marine turtles from longline fishing, since many species of marine turtles are endangered due to accidental captures.
Loggerhead sea turtle (Caretta caretta) and leatherback sea turtle (Dermochelys coriacea) are the marine turtles most captured with superficial longline fishing (Gilman et al. 2006), but are also captured the other species (Polovina et al. 2003). Despite accidental captures of this species are strange, the worldwide whole has an important effect (Lewison et al. 2004). Here, we are focusing in the measure to reduce these accidental captures in the loggerhead sea turtle for the huge available bibliography.
Loggerhead sea turtle (Caretta caretta) (Picture from DeviantArt).
THE LONGLINE FISHING
Longline fishing is a type of fishing that consists on a main line from which puts up hooks with bait. It’s one of the most ancient fishing systems that are known. The main line can measure between some hundreds of meters to 50-60 km, with a distance between hooks from 1 meter to 50 m. Despite of being considered the most selection fishing, because depending on the bait and the hook size it is possible to catch one type of fish or another, it is not free from accidental captures, among which we can find sea birds and marine turtles.
Longline fishing, despite of being very selection, captures marine turtles (Picture from Sea Turtle Conservancy).
HOW CAN WE SAVE MARINE TURTLES FROM LONGLINE FISHING?
Reduction of the fishing time
If fishers reduced the time in which the longline is in the water, it would be reduced the accidental captures of loggerhead sea turtle, but this doesn’t happen with leatherback sea turtle (Watson et al. 2005). The problem is that is not economically possible for the reduction in the goal species captures.
Change of the hook
Changes in the hooks are the most effective. Wider hooks reduce turtle’s captures and the proportion of the animals that swallows the hook without compromising the commercial viability the swordfish in the North Atlantic (Gilman et al. 2006), but these doesn’t happen in other fisheries. The shape determines the place where the hook gets hooked: while circular hooks gets hooked in the mandible or in the mouth, J hooks gets hooked internally. The change to a circular hook reduces the captures and the mortality after the freeing (post-release death) in the loggerhead sea turtle because they usually are captured with they bite the bait and this get hooked more externally and it is easier to free them (Gilman et al. 2006; Bolten & Bjorndal 2005; Watson et al. 2003). The change in the shape is effective in certain fisheries and areas, like for example in the swordfish (maintaining the captures (Piovano et al. 2009)) an the blue shark in Azores (Bolten & Bjorndal 2005). For this reason, circular hooks don’t reduce the captures of goal species and suppose a low-cost investment, but complicate their removal and they usually are more breakable than J hooks (Gilman et al. 2006). In conclusion, the use of circular hooks in the swordfish fishery in the Mediterranean and the Northwest Atlantic can suppose an easy and cheap technique to reduce marine turtle captures (Piovano et al. 2009; Watson et al. 2005; Gilman et al. 2006, 2007). The direct mortality produced for the hooks is reduced, so the 80% of the freed turtles are alive, but the post-release death depends on the position of the hook (Camiñas & Valeiras, 2001).
Hook types. (A) Circular hook and (B) J hook(Picture from Cicmar).
Change of the bait
Bait is another important factor. When the bait is fish, the captures of loggerhead turtles are reduced compared to the use of squid, and the captures of swordfish become bigger (Watson et al. 2005). The reason is that they feed on fish doing small bites and this prevent from being swallowed, while squid is more resistant and they swallow the bait completely (Watson et al. 2003, 2004). In the Mediterranean and Northwest Atlantic, using mackerel maintains the swordfish’s captures and reduces the captures of loggerhead sea turtles (Alessandro & Antonello 2010; Watson et al. 2005; Gilman et al. 2006, 2007), but reduces the captures of the Atlantic bluefin tuna (Rueda et al. 2006; Rueda & Sagaraminaga 2008). Using baits with different colours don’t seem to be a good measure because don’t prevent from capturing them (Swimmer et al. 2005; Watson et al. 2002).
Change of the depth of fishing and the distance to the coast
Loggerhead sea turtles usually dive over the 40 m deep, maximum until 100 m (Polovina et al. 2003). For this reason, if the longline was placed under the depth of more abundance, the captures would be reduced (Rueda & Sagarminaga 2008). The problem is that the goal species captures would be reduced too depending on the fishery (Gilman et al. 2006) and, moreover, if they got hooked, they wouldn’t be able to breath in the surface and they would die. According to fishers, the hooks closer to the buoys capture more turtles because they are in swallower depths (Watson et al. 2002). So, these secondary lines should be longer. The turtle captures also depend on the distance to the coast (Báez et al. 2007), and the longline should be place further than 35 nautical miles and the captures of swordfish captures don’t be affected (Alessandro & Antonello 2010).
Elimination of light sticks
Light sticks should be banned because increase their capture (Alessandro & Antonello 2010).
Change of the fishing areas
Marine turtles gather in areas, so one capture increases a lot the probability of capturing more. For this reason, a good measure should be the communication between ships and the shift in the areas (100 km away) during a period of time (for example, one week) (Gilman et al. 2007). This would be very effective, but suppose an extra gas expense and the reduction of the time that fishers are fishing due to the journeys. Another measure could be the permanent or seasonal closure of areas, but this is economically infeasible.
Sea temperature monitoring
The capture rate of loggerhead sea turtles increases when the temperature is over 22ºC, while the capture of swordfish increases under 20ºC. For this reason, it should be better to fish in waters under 20ºC (Watson et al. 2005). However, in this case the fishing pressure on the swordfish should be controlled.
A good manage tool is the presence of observers on board a ship, like in the longline swordfish fleet in Hawaii (Gilman et al. 2007). Fisher observers record the number of fishing devices, the fishing days, the fishing position and the number of captured turtles (Álvarez de Quevedo et al. 2010).
A good manage tool is the presence of observers on board a ship (Picture from Journal of Applied Ecology).
HOW DO TURTLES HAVE TO BE FREED?
Turtles have to be freed using the right device to remove the hook and, in the case that it is not possible, the line has to be cut as closer to the hook as possible because this reduces the mortality because the line can affect the intestines (Casale et al. 2007).
To free the turtles, the line has to be cut as closer to the hook as possible (Picture from Greenpeace).
The effectiveness and the commercial viability of the strategies to avoid the capture of loggerhead turtles depend on the fishery, the size of the animal, the goal species and other differences between fleets (Gilman et al. 2006, 2007). The combination of circular hooks and fish like a bait is very effective in reducing the captures of turtles without affecting the goal species. This changes, together with tools to remove the hooks and the lines, reduce the accidental captures and the post-release deaths.
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