Historical trends in the incidence of strandings of sperm whales (Physeter macrocephalus) on North Sea coasts: An association with positive temperature anomalies

Information on the migration patterns of sperm whales (Physeter macrocephalus) in the North Atlantic is preserved in historical strandings records, particularly for the North Sea, where sperm whale strandings have been documented since the 16th century, reflecting general public interest in large whales ashore. Most strandings in this area occur during or following the southward migration from the feeding grounds, when some animals enter the North Sea (in which they are thought to have difficulty navigating) instead of following their usual route through deep water to the west of the British Isles. There was much speculation about the causes of the high incidence of strandings on North Sea coasts in the 1990s, among which a recently published analysis of long-term trends in strandings indicated an effect of sunspot cycle length. We show that long-term interannual variation in the incidence of sperm whale strandings on North Sea coasts is related to positive temperature anomalies: the incidence of strandings was higher in warmer periods. The effect of temperature anomalies explains between 8 and 9% of variation in the strandings series. Inclusion of sunspot cycle length as an additional predictor did not significantly improve this model. It is suggested that this link with positive temperature anomalies may reflect changes in the distribution of the sperm whales’ main squid prey.