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Blue whales, Balaenoptera musculus , in northeast Pacific waters often produce a two-part call. The first part, A, is amplitude-modulated, lasting about 17 s, and the second part, B, is frequency-modulated, lasting about 19 s. Type B calls are more easily detected in recordings and are often the signal tracked in long-term monitoring. Automatic detection methods applied to data from a hydrophone moored at 8°N, 95°W in the eastern tropical Pacific (ETP) from 15 May 1996 to 15 May 1997 showed seasonal variability in call detection. Calls were detected most often from February to June. Blue whales are believed to use the ETP region as a wintering ground but probably also feed there. Blue whales forage primarily on krill that undertake diel vertical migrations from several hundred metres depth during the day to near the surface at night. We tested the null hypothesis that blue whale calling behaviour does not show significant diel variation in order to determine whether variability in calling could provide an indication as to the behavioural function of B calls. The hourly rate of B calls showed a strong diel pattern. There were significantly more calls during dark and twilight periods than during light periods ( P |
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