Behavioral Responses of Sea Turtles to Light Sticks Used in Longline Fisheries
During the past several decades, many sea turtle populations have declined significantly. In particular, nesting populations of leatherback (Dermochelys coriacea) and loggerhead (Caretta caretta) turtles in the Pacific Ocean have dramatically decreased. Turtle bycatch associated with pelagic longline fisheries has been implicated as a factor contributing to this decline in some populations.
Pelagic longline fishing involves a single main fishing line that can stretch over 50 km with thousands of individually hooked lines branching off from the main line. This fishing method is used in every ocean basin and commonly targets tunas, swordfish, and dolphin fish. In addition to these targeted fish, however, longline fisheries also inadvertently catch sea turtles.
Loggerhead and leatherback turtles are the species that most commonly come in contact with longlines. Turtles are often hooked in the mouth, throat, or digestive tract and subsequently drown when they are unable to surface to breathe. Turtles are also sometimes hooked in their flippers and carapaces, or become entangled in the lines.
Strategies to diminish the impact of longline fisheries on sea turtle populations have included seasonal and area fishing closures, attempts to decrease mortality of captured turtles through better handling practices, alteration of fishing methods, and changes in gear. In the North Atlantic swordfish fisheries, a reduction of sea turtle bycatch was achieved by using a special kind of hook with a circular shape.
An understanding of the sensory stimuli that attract turtles into the vicinity of longline sets may be useful in developing additional gear modifications that will reduce turtle bycatch. A common practice in some longline fisheries is to attach glowing lightsticks to the branch lines in order to attract fish into the vicinity of the baited hooks. Lights used include chemiluminescent lightsticks and battery-powered light-emitting diodes (LEDs). Although these lights are intended to attract fish, the possibility exists that they also attract sea turtles.
To investigate whether lightsticks used in longline fisheries attract turtles, we conducted experiments using an orientation arena and tracking system originally designed for studies on magnetic orientation of sea turtles. Juvenile loggerhead turtles were placed into a soft cloth harness and tethered to an electronic tracking system that monitored the direction that each turtle swam toward. Some turtles were tested with glowing lightsticks in the arena, whereas others were tested in the same tank in darkness as a control.
Results and Interpretation: The turtles consistently swam towards glowing light sticks of all colors and types. In contrast, they ignored lightsticks that were not illuminated. These findings imply that turtles are attracted to the illumination produced by lightsticks and suggest that such lights may be one factor that draws turtles into the vicinity of longlines. An important caveat, however, is that these experiments were carried out in the laboratory. Thus, additional studies under more natural conditions in the ocean will be needed to confirm the findings.
About the Research: The study described on this web page is based on the following published paper:
Wang, J. H., Boles, L. C., Higgins, B., and K. J. Lohmann. 2007. Behavioral responses of sea turtles to lightsticks used in longline fisheries. Animal Conservation 10: 176-182. [Download pdf] [National Geographic]
The lead researcher on this study was Dr. John Wang, a former graduate student and postdoc in the Lohmann Lab who is presently a researcher with the National Marine Fisheries Service in Hawaii. Dr. Wang has continued research on the responses of turtles to lightsticks, as well as numerous other innovative efforts to reduce turtle bycatch in fisheries throughout the world.
National Geographic News – “Glow Sticks May Lure Sea Turtles to Death”
TerraDaily, News About Planet Earth – “Light Sticks May Lure Turtles to Fishing Lines”