Light attractors, including chemical lightsticks and battery-powered light-emitting diodes (LEDs), are attached near baited hooks on branchlines to attract fish. For example, in the western and central Pacific Ocean, lightsticks are used by some longliners targeting swordfish. Line is usually set in the late evening and soaked overnight. Strategies are needed to make longline lightsticks less attractive or invisible to sea turtles and sharks.
Another important consideration is the contribution of lost or discarded chemical lightsticks to marine pollution, both the plastic and the toxic contents.
Laboratory studies have shown that sea turtles are drawn to the types of light attractors used in longline fishing. They appear to be attracted to glowing green, blue and yellow chemical lightsticks, as well as to orange LED-based 'Electrolume' lightsticks.
Strategies that may make lightsticks less attractive or invisible to turtles, but still able to be perceived by finfish require further research, but include:
- placing shades on the lightsticks (turtles spend most of their time higher in the water column than where swordfish hooks soak; shading means less light would reach upwards to the turtles),
- making the lights blink (at a very fast interval, so that finfish can see the light but not turtles),
- using attractors of specific colours emitting light at wavelengths not visible to turtles,
- using lightsticks that generate specific wavelengths that might repel turtles.
Also to be considered are:
- daylight conditions under which light attractors are still visible to turtles,
- intensity of lunar illumination
- the variation in visual capabilities between different turtle species.
Sharks and stingrays
Little is known about how sharks and stingrays respond to the types of light attractors used by longliners.
Surveys of fisher knowledge of shark avoidance strategies have shown that many believe that using lightsticks increases shark catch rates (though without consensus on which colours are the most attractive to sharks). In contrast, recent trials in the Mediterranean Sea found that light attractors did not influence stingray bycatch.
Chondrichtyans are known to have highly developed non-visual senses, olfaction in particular, which are used in finding prey. This may impact upon the importance of light attractors in influencing shark catch rates and indicates the need for further research.
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