190 



Fishery Bulletin 101(1) 



170°E 180 170°W 160°W 150°W 140°W 130°W 120''W 



45°N 



40°N - 



35°N 



30°N - 



25°N - 



20°N - 



15°N- 



10°N 



Loggerhead 22534 



Loggerhead 24747 



S 082300 



Ohve Ridley 22533 



S 081799 



: 091299 

 Olive Ridley 22532 



T 



20°N 

 15°N 



10°N 



170°E 180 170°W 160°W 150°W 140°W 130°W 120°W 110°W 



Figure 1 



The start (S) and end (E) dates and track lines for the four turtles with satellite-linked dive recorders. 



Data on daily location of the turtles were estimated from 

 the signals received by the Argos receiver on a NOAA satel- 

 lite. The position data were edited, and only the single most 

 accurate daily position was plotted. The accuracy of each 

 position was estimated by Argos as a function of the number 

 and configuration of satellites and the number of transmis- 

 sions received. Data on the dive behavior transmitted by 

 the Argos receiver were not individual dive profiles but 

 rather frequency distributions of time at depth, dive du- 

 ration, and maximum dive depth, aggregated over four 

 6-hour periods and binned in specific depth or time inter- 

 vals. The lower range of the depth bins (in meters) for the 

 time-at-depth distributions were 1, 3, 5, 10, 15, 25, 35, 50, 

 60, 75, 100, 125, 150, 150-I-. Each time the turtle descended 

 below 2 m, it was recorded as a dive. The lower range of 

 the depth bins (in meters) for the dive-depth distributions 

 were 5, 10, 15, 20, 25, 30, 40, 50, 60, 70, 80, 100, 150, 150-h. 

 The 6-hour periods over which the time-at-depth and 

 dive-depth data were pooled were programed in Hawaii 

 standard time as 2100-0300, 0300-0900, 0900-1500, and 

 1500-2100 h. One period was night, another mid-day; one 

 included dawn, the other dusk. Mean time-at-depth and 

 dive-depth distributions for each turtle in each of the four 

 time periods were computed as the average of all frequency 

 distributions for each 6-hour period. Mean time-at-depth 

 and dive-depth distribution for the combined four time pe- 

 riods for each species were computed as the average of the 

 four mean time-at-depth and dive depth distributions for 

 each turtle, then averaged by species. 



Finally, after every 20 transmissions a special status 

 message that contained technical data about the operation 

 of the transmitter and the maximum dive depth of that 

 day was transmitted. Both the loggerhead and the olive 

 ridley sea turtles made some dives below 150 m; however, 



the histogram data did not indicate how much deeper than 

 150 m these animals dived. The maximum dives sent in 

 the status messages were used to obtain some data on the 

 deep dives. 



Results 



The positions of the four turtles showed that the turtles 

 were occupying the characteristic habitats for each spe- 

 cies: the loggerhead sea turtles were found in the northern 

 portion of the subtropical gyre, and the olive ridley sea 

 turtles were found farther south, well within the center of 

 the subtropical gyre (Fig. 1). Loggerhead no. 24747, which 

 was released with the hook removed, measured 83 cm 

 (straight carapace length (SCL)) and transmitted data for 

 5.4 months. Loggerhead no. 22534, released with the hook 

 deeply ingested, measured 61 cm SCL and transmitted 

 data for 5.2 months. Olive ridley no. 22533, released with 

 the hook deeply ingested, measured 57 cm SCL and trans- 

 mitted data for 3.4 months. Olive ridley no. 22532, which 

 was released after a hook was removed, measured 58 cm 

 SCL and transmitted data for 0.8 months. 



The time-at-depth frequency distributions for day and 

 night periods for each of the sea turtles showed consistent 

 diurnal and species differences in their dive-depth distri- 

 butions (Fig. 2). The turtles spent more time at the surface 

 during the day than at night and also dived deeper during 

 the day (Fig. 2). We do not show the div(>-depth distribu- 

 tion for the dawn and dusk periods, but these frequency 

 distributions fell between the distribution for day and 

 night periods. 



Because it can often take as long as 20 hours to com- 

 pletely set and retrieve a longline, we examined timo-at- 



