Cause of sperm whale strandings is still unclear

29 stranded sperm whales have been found dead or in severe condition in North Sea waters since the beginning of January 2016. Nineteen carcasses were recovered in the Wadden Sea and two near the German island of Helgoland. Furthermore, six sperm whales were found in the Wash area (in the UK), and another one on the French Coast near Calais (see map below). Whale strandings have been reported in the North Sea since the 16th century. In recent years they have mostly occurred in Denmark, usually with one or two dead animals per year. “The current frequency of whale strandings is highly unusual, and has not been observed since 1996 and 1997, when two groups of sperm whales, of 16 and 13 animals respectively, stranded on the Danish island of Rømø”, states Lasse Fast Jensen, curator at the Fisheries and Maritime Museum in Esbjerg and chair of the Trilateral Seal Expert Group (TSEG).

“The sperm whales are believed to belong to one group, on its way from the rich feeding grounds off the Norwegian coast in the North Atlantic Ocean, southwards to the Azores,” notes Sascha Klöpper, Deputy Secretary, Trilateral Monitoring and Assessment Programme at the Common Wadden Sea Secretariat. “Instead of keeping west of the British Isles, they strayed into the North Sea. Sperm whales are often found in the Atlantic Ocean and their diet consists mainly of Atlantic squid. In shallow waters such as the North Sea they usually do not find enough prey.”

Prof. Dr. Ursula Siebert from the Institute for Terrestrial and Aquatic Wildlife Research (ITAW) / University of Veterinary Medicine Hanover – Foundation (TiHo), involved in the recovery and investigation of sperm whales in Lower Saxony and Schleswig-Holstein, states that “the exact reasons why the animals end up in the North Sea remain unclear and will be further investigated by pathological and biological investigations of the recently recovered cadavers”. According to Ms. Siebert, the migration route of the sperm whales could have been altered by human activities, a regional change in prey organisms, or other environmental factors. Temperature shifts in seawater might be the root cause; as such changes can alter the migration pattern among prey populations, attracting the sperm whales into the North Sea.

All animals were males, of roughly the same age, size and weight, which further underpins the hypothesis that they belonged to the same group. According to Abbo van Neer of ITAW / Tiho, “they seemed not to suffer from acute food shortage, since some of the investigated stomachs contained large numbers of squid beaks, and their nutritional status was good”. He added that “apart from natural food, first investigations of their stomachs revealed big plastic waste items, such as parts of buckets and large parts of fishing gear. However, the actual cause of death is probably a collapse of the respiratory tracts, as well as an interruption of the blood flow, since the enormous weight of the bodies suppresses the breathing and further body functions. Their lateral stranding position could also have submerged their blow wholes.”

Removing the cadaver of a stranded whale is a challenging task, and can even be dangerous. As Mr. van Neer explains, “due to their insolating fatty layer, bacterial activities cause temperatures of up to 50°C within the bodies. Within days, the corpses decay significantly and might even explode as a result of gases that increase the inner body pressure.”

Map of the southern North Sea showing sperm whales stranding sites and numbers © Klöpper CWSS