Abstract

Due to the distinctive anatomy of the upper respiratory tract of cetaceans, with the nares being located on top of the head rather than in the front, an articulation of the larynx and the vertical nasal passages is necessary for the physiological respiration of cetaceans.¹ However, a separation of the airways for intake of larger food items seems possible.² In the event of obstruction and impeded re-articulation of the larynx, the connection between the blowhole and the lungs cannot be obtained, which ultimately leads to lethal hypoxia and suffocation. Only a few cases of fatal asphyxiation in odontocetes with laryngeal displacement and airway obstruction caused by different prey fish species have been reported in the literature. These include single case reports^2,3,4 and one retrospective evaluation in a bottlenose dolphin (Tursiops truncatus) population⁵. As part of the German stranding network of Schleswig-Holstein, stranded marine mammals are regularly necropsied at the Institute for Terrestrial and Aquatic Wildlife Research (ITAW). During the post-mortem examinations of harbor porpoises (Phocoena phocoena) in 2016, eleven animals were found with signs of fatal asphyxiation due to obstruction of the upper respiratory tract by ingested flatfish. In some cases the fish were even curled up into the nasal passages, completely obstructing the lumen. Nine of the eleven animals stranded on the German North Sea island of Sylt between April and July. In the time period between 1990 and 2015, lethal asphyxiation caused by flatfish was found to be the cause of death in a total of 31 harbor porpoises. Here again, the majority (71%) of these animals stranded on the Island of Sylt during the summer months. To the authors' knowledge, no similar cases for harbor porpoises have been reported from other bordering countries and North Sea stranding networks in 2016. As all observed cases were caused by flatfish, these prey fish species seem to have a high potential to cause lethal asphyxiation. The cause for the extremely increased local occurrence of fatal asphyxiation, especially in 2016, is unknown. Two possible explanations might be an unusual high population density of flatfish or a steep decline in other, regular prey fish around the Island of Sylt and thereby an increased consumption of flatfish by harbor porpoises.

Acknowledgements

The authors would like to thank all dedicated individuals of the marine mammal stranding response network and the Ministry of Energy, Agriculture, the Environment and Rural Areas (MELUR) who make the Schleswig-Holstein stranding response scheme possible, as well as all ITAW colleagues for their support during necropsies and sample collection.

* Presenting author

Literature Cited

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