The diving behaviour of green turtles undertaking oceanic migration to and from Ascension Island: dive durations, dive profiles and depth distribution
G. C. Hays1,*,
S. Åkesson2,
A. C. Broderick1,
F. Glen1,
B. J. Godley1,
P. Luschi3,
C. Martin1,
J. D. Metcalfe4 and
F. Papi3
1 School of Biological Sciences, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK,
2 Department of Animal Ecology, Lund University, Ecology Building, SE-223 62 Lund, Sweden,
3 Dipartimento di Etologia, Ecologia, Evoluzione, University of Pisa, Via A. Volta 6, I-56126 Pisa, Italy and
4 The Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Lowestoft NR33 0HT, UK
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Fig. 2. For time/depth-recorder-equipped turtles, the depth distribution recorded as individuals travelled back to Ascension Island. (A–E) The percentage of time spent in each 1 m depth increment, revealing a bimodal pattern of depth utilisation, with peaks near the surface and between approximately 10 and 20 m. (F–J) The near-surface depth distribution in more detail, showing the percentage of time spent in 20 cm depth increments excluding dives deeper than 5 m.
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Fig. 3. Examples of (A) type 1 and (B) type 2 dives. In both dive types, most of the dive was spent in a period of gradual ascent, with this phase being preceded by a short period of more rapid ascent for type 2 dives. Time is shown as GMT, which equals local time at Ascension Island. For each of five turtles migrating back to Ascension Island, 99.4 %, 92.5 %, 100 %, 99.1 % and 96.7 % of dives more than 5 min in duration were either type 1 or type 2.
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Fig. 4. For five turtles migrating back to Ascension Island, the proportion of time each hour spent conducting type 1 and 2 dives. For all individuals, there was a nocturnal peak in the occurrence of these midwater resting dives. Times are GMT, with sunrise and sunset (calculated at the midpoint between Ascension Island and the point of release) shown by the dashed vertical lines.
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© The Company of Biologists Ltd 2001
, night; 
, day; +, 6 h periods that straddle dusk and dawn. Four periods of long submergences (between 55 and 67 min) in F are plotted as 50 min to allow equal scaling of the y-axis in the different plots while maintaining easy visual comparison between the different parts of the figure.