Appendix A. A description of turtle tracking and diving behavior.
Two turtles were tracked after completing nesting in 2002 and a further seven turtles were tracked after nesting in 2003. Argos assigns a level of accuracy to each location. Based on previous trials of location accuracy (Hays et al. 2001) we generally used only locations of the best four classes (location classes 1,2,3, and A). The exception to this rule was on rare occasions when there was >7 days between these good quality locations and in that case we used locations of class 0 and B. Locations were then filtered to remove those locations which necessitated a speed of travel of >9 km/h between consecutive locations, i.e., > 30 km apart. This crude filter is suitable for analyzing the broadscale movements. Dates were expressed as “day of the year” where day 1 was January 1 in the year that the transmitter was attached to each individual.
A random sample of individual dive profiles for dives deeper than 10 m were relayed via Argos, with each dive time stamped in Greenwich Mean Time (GMT). By interpolation between Argos locations that straddled the time of each dive, we determined the location of each dive profile and hence converted all dive times to local time (15° longitude corresponds with a 1h time difference between local time and GMT). In addition, a random sample of 6-h summary statistics was obtained for the time spent diving to > 10 m, the mean depth of these dives, and their mean duration. To investigate the details of the pattern in diving over the 24-h cycle, we used the number of dives in each 1-h interval of the 24-h cycle multiplied by their mean duration as a relative measure of diving. However since not all dive profiles were received this approach did not provide an absolute measure of time spent diving. We therefore converted these relative measures to absolute units by using the 6-h summary data sets which showed absolute proportion of time spent diving. In this way we determined t he mean percentage of time spent diving to >10m over the 24-h cycle at different latitudinal bands.
LITERATURE CITED
Hays, G. C., S. Åkesson, B. J. Godley, P. Luschi, and P. Santidrian. 2001. The implications of location accuracy for the interpretation of satellite tracking data. Animal Behaviour 61:1035–1040.