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       Introduction and Background
    Methods and Results
    Conclusions

Introduction and Background:

         Longline fisheries are a significant cause of mortality for several species of sea turtles.  Pelagic-stage loggerheads are sometimes hooked in the mouth or digestive tract (see Figure 1) and can also become entangled in the longlines.  Thus, the potential for drowning arises as soon as a turtle enters the vicinity of a longline set.  An improved understanding of stimuli that induce turtles to approach longline sets may therefore be useful in ongoing efforts to minimize the effects of longline fisheries on sea turtle populations.

        A common practice among longline fishermen is to attach glowing light sticks to their lines to attract fish.  Whether such light sticks also attract sea turtles has not been investigated.  As a first step toward determining what features of a longline set attract turtles, we conducted experiments with small, pelagic-stage loggerheads to determine if they are attracted to glowing light sticks.

         Although several colors of light sticks are used by fishermen, green appears to be the most common choice.  In initial experiments we tested the responses of young loggerheads to glowing green light sticks under both laboratory conditions (in a lightproof room) and under the less controlled but more natural lighting regime of a night sky.  Turtles were found to orient strongly toward the light sticks under both conditions.

         Because hatchling loggerheads crawling on beaches are known to avoid yellow light with a wavelength of about 560-600 nm, we reasoned that this aversion might persist when the turtles are older.  To investigate the possibility that light produced by yellow light sticks might be unattractive to young pelagic-stage turtles, we conducted additional trials with two different brands of yellow light sticks.

        The emission spectra of the light sticks used in these experiments can be found in Figure 2.
 
 
 

Methods and Results:

        Small pelagic-stage loggerheads were collected in the Gulf Stream approximately 25 miles offshore of Cape Canaveral, FL.  The turtlesí straight carapace length (SCL) ranged from 44 mm to 50 mm.  All experiments were performed at night.

         Turtles were tested in a water-filled orientation arena 1.83m in diameter.  Each turtle was placed in a lycra harness and tethered to a rotatable tracker-arm attached to a computerized tracking unit consisting of a digital encoder and a data acquisition computer.  This tracking system continuously monitored the direction toward which the turtle swam (Figure 3).  Each turtle was permitted to swim for 10 minutes in the presence of either a glowing light stick or a light stick that had not been activated.  At the end of the trial the mean angle (average direction) of the turtle was calculated using all data collected during the test period.  The position of the light stick within the tank was varied between trials; for purposes of analysis, however, the data were normalized so that 0° represented the position of the light stick.

        Reading the circular graphs:  Each red dot represents the mean angle of orientation (average direction) for a single turtle.  The line that connects the red dot to the center of the figure is proportional to the turtleís r-value.  Lines extending to the edge of the circle indicate highly consistent headings.  Shorter lines indicate less consistent headings, implying that the turtle changed course frequently or circled the arena instead of maintaining a steady course toward one direction.
 
 

Experiment 1: Do sea turtles orient toward green Snaplight light sticks in darkness?

         Two groups of turtles were tested in an indoor, lightproof room.  Turtles in the experimental treatment were placed in the orientation arena with an activated green Snaplight light stick in either the east or west side of the tank. Turtles in the control treatment were placed into the tank under identical conditions, except that an unactivated light stick was used.  Results are shown in Figures 4 and 5.
 

        Results from the Hotelling test indicate that the turtles in the control treatment were not significantly oriented as a group.  In the experimental treatment, the turtles were significantly (p<0.005) oriented with a mean angle of 356.3°.  The control and experimental treatments were significantly different (F-test, p<0.02).  These results indicate that turtles oriented toward green Snaplight light sticks only when they were activated.
 
 

Experiment 2:  Do sea turtles orient toward green Snaplight light sticks under night sky light conditions?

        This experiment was identical to Experiment 1 except that it was performed outdoors under a night sky to more closely simulate lighting conditions that turtles encounter in the open ocean.  Results are shown in Figures 6 and 7.

        Results from the Hotelling test indicate that the turtles in the control treatment were not significantly oriented as a group.  In the experimental treatment, the turtles were significantly (p<0.001) oriented with a mean angle of 343.8°.  The control and experimental treatments were significantly different (F-test, p<0.001).  These results indicate that turtles oriented toward green Snaplight light sticks despite the presence of ambient light in the night sky.
 
 
 

Experiment 3:  Do sea turtles orient towards yellow Snaplight light sticks and towards yellow Coghlan light sticks?

         Two groups of turtles were tested in the presence of glowing Snaplight yellow light sticks or glowing Coghlan yellow light sticks.  These trials were carried out indoors in the lightproof room.  Results are shown in Figures 8 and 9.
 

       Turtles tested in the presence of yellow Coghlan light sticks were significantly oriented (Hotelling test, p<0.025) in the approximate direction of the light sticks.  In the yellow Snaplight treatment, the turtles where not significantly oriented.
 
 
 
 

Conclusions:

    1.) Young pelagic-stage loggerheads were strongly attracted to glowing green light sticks both in a completely dark environment and under a night sky.

    2.) Turtles were not attracted to light sticks that had not been activated, indicating that light itself functions as the attractant, rather than chemicals or other non-visual cues associated with the stick.

    3.) Although turtles as a group were weakly attracted to glowing yellow Coghlan light sticks, several individuals swam consistently away from the light.  Moreover, turtles were not significantly oriented as a group when tested in the presence of yellow Snaplight light sticks.  These results suggest that yellow light sticks may be less attractive to young pelagic-stage loggerheads than are green light sticks.

    4.) Because turtles captured on longlines are considerably larger than the turtles used in these initial experiments, additional research is needed to determine if the responses of young pelagic-stage turtles persist in older animals.  The methodology developed for this study can be readily adapted to studies with larger turtles.
 
 

Acknowledgements:

    We thank M. Salmon and J. Wyneken for helpful advice and for providing summer research space at Florida Atlantic University.  We also thank B. Witherington for supplying us with the sea turtles, L. Avens for providing critical aid with graphics, and A. Wang for valuable comments.  We give special thanks to R. Boettcher, Topsail Sea Turtle Hospital, and Bridges Professional Park Animal Hospital for generously providing the radiograph.  Lastly, we thank A. Tripathy for assistance with the spectrofluorometer.  This research was supported by funding from the National Marine Fisheries Service.
 

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