Day and night-time diving behavior of fin whales in the Western Ligurian Sea

Mediterranean fin whales concentrate in the Ligurian Sea during the summer to feed on large swarms of the euphausiid Meganyctiphanes norvegica, known as one of their principal prey in the Northern Hemisphere. The diving behavior of 15 whales was studied, within a long-term study on the habitat use of this species, between 1998-2002 by using suction-cup attached tags, equipped with velocity-time-depth-recorders. Here we report the results of three successful taggings, during which the tag remained on the whales for a minimum of 6 hours (mean time = 8h 9min), and during which the whales reached depths of at least 468 m (estimated maximum depth 565 m). The diving behavior performed by the 3 tagged individuals differed considerably. Two whales started to perform deep dives (>400 m) after 5 PM, in coincidence with the presumed upward migration of the deep scattering layer, while until 5 PM they remained close to the surface performing 45 dives shallower than 100 m (mean depth = 27.8 m, SD = 19.0). The third whale was tagged at 11:50 AM and till 6 PM performed a series of 19 deep dives (mean depth = 449.0 m, SD = 22.5). One of the deployed tags remained on the whale till 10 PM, and showed a steady decrease in diving depth, dropping from 400 m at 7:40 PM to less than 50 m at 9:50 PM. The data presented here suggest that Mediterranean fin whales are unlikely to dive deeper than 600 m. The different depths reached by the tagged whales at the same time of day in different days indicate that prey distribution throughout the water column changes from time to time. Moreover, the diving behavior performed by the second whale suggests that fin whales in the Ligurian Sea follow the deep scattering layer's diel upward migration.