Sometimes Sperm Whales Cannot Find Their Way Back to the High Seas

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Sperm whale mass strandings are mysterious events, which raise the concern and curiosity of the public opinion. The causes remain largely unknown, although many hypotheses have been considered and analyzed, including natural factors, such as biologic disease agents ; impairment of the navigation and echo-location systems due to bathymetric features, acoustic dead zones or anomalies of the Earth’s geomagnetic field due to solar activity , the effects of lunar cycles meteorological and oceanographic factors like local disturbances or basin-related temperature variations influencing prey distribution and large-scale climatic events. Furthermore, anthropogenic factors like noise pollution  or environmental contaminants have been also proposed as possible causes of strandings.

Mass strandings of sperm whales (Physeter macrocephalus) remain peculiar and rather unexplained events, which rarely occur in the Mediterranean Sea.
Mass strandings of sperm whales (Physeter macrocephalus) remain peculiar and rather unexplained events, which rarely occur in the Mediterranean Sea.

A strong social component, which may prompt healthy animals to follow sick or disordered members of a pod, has been also considered as an additional relevant feature to be pondered in investigating the causes of mass strandings Mass mortalities involving sperm whales are usually clustered in determined geographical areas, such as the North Sea  and in the Southern Australian and New Zealand

The seven sperm whales took the same “wrong way” into the Adriatic Sea, a potentially dangerous trap for Mediterranean sperm whales.

 

The seven sperm whales took the same “wrong way” into the Adriatic Sea, a potentially dangerous trap for Mediterranean sperm whales.
The seven sperm whales took the same “wrong way” into the Adriatic Sea, a potentially dangerous trap for Mediterranean sperm whales.

Although in the Mediterranean Sea the sperm whale is one of eight cetacean species considered to be regular inhabitants, mass strandings are rarely reported. In December 2009, a pod of seven sperm whales stranded along the coastline of the Gargano Promontory (Italy), in the Southern Adriatic Sea. Three animals were still alive and died within 48 hours after stranding.

Sperm whales are considered to be vagrant or absent in the waters surrounding the stranding place, and particularly in the Central and Northern areas of the Adriatic Sea, where the habitat is not proper to this deep-diving species. Sperm whales in the Mediterranean Sea occur preferentially in deep continental slope waters where mesopelagic cephalopods are most abundant .In the Adriatic Sea, sperm whale mass strandings have occurred five times since historical times, with the oldest known instance dating back to 1584.sperm whale standing ancient

In addition, some reports of single individuals stranded dead or alive included mention to one or more other sperm whales sighted at sea in the close proximity to the stranding location, sometimes for several days . Groups stranded on the Adriatic Sea coasts (range 3–8 individuals) are smaller compared to the mean size of groups stranded outside of the Mediterranean Sea .

 

Nocturnal necropsy and gas bubbles. Fig. A shows a scene of the nocturnal necropsy on animal no. 7. In Fig. B and C gas bubbles in the heart veins (black arrows) and in an intracardiac clot of animal no. 5 are shown.
Nocturnal necropsy and gas bubbles.
Fig. A shows a scene of the nocturnal necropsy on animal no. 7. In Fig. B and C gas bubbles in the heart veins (black arrows) and in an intra cardiac clot of animal no. 5 are shown.

Despite all these observations, we were not able to confirm that these stranded sperm whales formed a single stable group with asocial hierarchy, although we would rather suggest that more than one loose male aggregation and/or several solitary individuals could have coalesced in a limited sea area, most likely in the Ionian Sea, between summer and fall. From there they subsequently entered the Adriatic Sea for unknown reasons.

To the best of our knowledge, no relevant unusual natural events (i.e. seaquake or weather storms) or noxious anthropogenic activities (military drills using sonar) that could have caused an avoidance behaviour occurred temporally and spatially associated with the event.

The only relevant anomaly reported by the marine data archives was the increased sea superficial temperature in November and December along the Hellenic Trench and Eastern part of the Adriatic Sea, possibly constituting a thermal front in which upwelling and/or down welling could have been favourable to the development of cephalopod populations.

 

 

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