Diel Oscillations in Whale Shark Vertical Movements Associated with Meso- and Bathypelagic Diving
Juerg M. Brunnschweiler and David W. Sims
Abstract.—A female whale shark Rhincodon typus satellite tagged off the coast of Mozambique and moving to the east coast of Madagascar where the tag popped off after 87 d showed increased meso- and bathypelagic diving while crossing the bathymetrically nonconstraining southern part of the Mozambique Channel. After three weeks with daily maximum diving depths in the epipelagic zone at the beginning of the track, when the whale shark stayed close to the Mozambique coast, 44 and 19 d with maximum diving depths in the meso- and bathypelagic zones, respectively, were recorded. On a total of 10 d the whale shark dived to the maximum depth (1,285.7 m) the pop-off tag could record. Quantitative analysis of the diving time series using Fast Fourier Transform identified a diel cycle of vertical movements, with deeper depths occupied during the day and shallower depths during daytime. Using a single bandpass filter to identify where in the time series the diel oscillations were most pronounced showed that they were mostly associated with periods of meso- and bathypelagic diving during the oceanic phase of the long-distance movement. These data indicate that meso- and bathypelagic diving is common for whale sharks in bathymetrically nonconstraining habitat, expanding the habitat of this circumglobal species into the bathypelagic realm, and support the hypothesis that these were dives related to foraging behavior when crossing less productive deep oceanic waters.