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Swim speed and movement patterns of gravid leatherback sea turtles (Dermochelys coriacea) at St Croix, US Virgin Islands

Scott A. Eckert

Hubbs Sea World Research Institute, 2595 Ingraham Street, San Diego, CA 92109, USA

View larger version (32K): [in a new window]   Fig. 1. Frequency distribution of swim speed for five leatherback sea turtles during one internesting interval at St Croix in the US Virgin Islands. The distribution is slightly bimodal for most turtles, with peaks at 0.2-0.4 m s-1 and 0.5-0.8 m s-1. The duration of each sampling interval was 20 s.

View larger version (26K): [in a new window]   Fig. 2. Two sections of the swim-speed record from leatherback 2, illustrating type-1 and type-2 swim behavior. Type-1 is characterized by speed slowing then increasing over a 5-15 min period, bordered by intervals of very low or zero speed. Type-2 speed behavior is characterized by periods of continued high speed swimming without abrupt speed drops. Type-1 behavior is most common for all turtles, but the proportion of type-2 swimming increased during the last few days of each internesting interval.

View larger version (47K): [in a new window]   Fig. 3. A 24 h section of the simultaneous swim-speed and dive record for leatherback 5, illustrating the relationship between dive profile and swim speed. Dive behavior is similar to that recorded in previous dive studies at St Croix, with shallower and more frequent diving at night (19:00 h-05:29 h) than during the day (05:30 h-18:59 h).

View larger version (32K): [in a new window]   Fig. 4. Two sections of the swim-speed and dive records of leatherback 6, illustrating how type-1 speed records follow the profile of dive depth, while type-2 speed records occur during shallow swimming by the turtle when the TDR may be registering a depth of 0 m.

View larger version (18K): [in a new window]   Fig. 5. Daily locations of seven leatherbacks during a single internesting interval around the island of St Croix, US Virgin Islands (in black). Locations were established using directional VHF radio telemetry, in which turtle location was determined relative to the location of a shore-based VHF radio receiver and recorded as a compass bearing. Since each bearing has an inherent error, the probable location was expressed as a triangular shaped polygon. Where triangulation occurred, because a turtle was located from a number of shore-based locations, only the area where the polygons overlapped is shown. The primary nesting area on St Croix is Sandy Point National Wildlife Refuge on the southwest tip of the island. Dates are expressed as month/day.