Massed strandings of whales and dolphins – effects of wind, waves and tides.

Abstract We have examined125 mass-stranding events of cetaceans ( > = 10 individuals) on New Zealand shores over the past 40 years, with a view to understanding the causes of this longstanding puzzling phenomenon. The wind, waves, wave refraction, shore slopes and tides at the dates and locations of these events were considered. The mass-strandings involved 10 different species, but by far the most common involved the Long-finned Pilot Whale, Globicephala melas edwardii. We find that the data appear to be consistent with a three-stage process. The first stage is that when an animal becomes ill, its body may become bloated and float on the surface, and the wind and waves may drive it onshore. The second stage is that pod members may accompany a dying (incapacitated) or dead body as a result of strong social bonds, in species that live in pods. The third stage involves the tides and the beach slope. If these are of sufficient amplitude, the nearby attendees will quickly become stranded in the intertidal region of a gently sloping beach, as the water level falls. We have evaluated the evidence we have available for the first and third stages, omitting strandings inside estuaries. In the overwhelming majority (91%) of these mass-strandings, the data indicate that wind and waves would have driven floating bodies toward the stranding site. Examination of the nearshore slopes and the tide ranges at the stranding sites showed that the vast majority were on slowly shelving beaches with large tidal ranges, so that the water level would retreat rapidly over 10s of metres. These 2 results are even more pronounced if only mass strandings of Long-fin Pilot Whales are considered.