Analysis of historical and recent diet and strandings of sperm whales (Physeter macrocephalus) in the North Sea

摘要

The increasing frequency of sperm whale (Physeter macrocephalus) strandings during the winter in the North Sea has resulted in many theories about why this phenomenon occurs. Using a newly updated catalogue of North Sea sperm whale strandings, the possible roles of environmental drivers, which might affect the entry of migrating sperm whales into the North Sea and/or their stranding, were investigated using generalised additive mixed models. Little or no evidence was found of effects of sunspot activity, the North Atlantic Oscillation (NAO) Index or sea surface pressure around Iceland (a component of the NAO) on the occurrence of strandings. Several sea and land surface temperature indices were positively correlated with the occurrence of strandings. There is evidence of changing relationships between strandings and environmental variables during the last three decades and, given the absence of an obvious mechanism by which the temperature-strandings link might operate, it is important to recognisethat several different processes may contribute to the strandings, including the recovery of the sperm whale population following the cessation of commercial whaling in the late 20th century. In addition, and since temperature could also be affecting the whales' prey and changes in prey distribution could explain whale stranding patterns, this paper updates previous studies of diet, confirming the continued dominance of the Boreoatlantic armhook squid Gonatusfabricii in stomach contents of sperm whalesstranded on North Sea coasts, although remains of small numbers of North Sea species were also found, suggesting some feeding within the North Sea.