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Peribullar sinuses of harbour porpoises Phocoena phocoena are parasitized with high prevalence by the nematode Stenurus minor. The effect of S. minor on the hearing ability of this species is still undetermined. Here, we review the occurrence of S. minor in the inner ear of harbour porpoises recovered from strandings in the North and Baltic Seas. In particular, we present the results from ears collected in German and Danish waters from 2002 to 2016 and from Dutch waters from 2010 to 2016. While the prevalence of S. minor in pterygoid and peribullar sinuses and tympanic cavity was high in harbour porpoises (66.67% in our cases), its prevalence in the cochlea was rare. Only 1 case out of 129 analysed by either histology, electron microscopy or immunofluorescence showed the presence of a nematode parasite morphologically consistent with S. minor at the most basal portion of the right cochlea. This individual also had severe haemorrhage along the right cochlear spiral, which was likely caused by ectopic S. minor migration. Although this animal might have had impaired hearing in the right ear, it was otherwise in good body condition with evidence of recent feeding. These findings highlight the need to study the effect of parasites on hearing, and other pathological changes that might impair appropriate processing of acoustic information.

In 2006, a marked increase in harbor porpoise Phocoena phocoena strandings were reported in the Pacific Northwest of the USA, resulting in the declaration of an unusual mortality event (UME) for Washington and Oregon to facilitate investigation into potential causes. The UME was in place during all of 2006 and 2007, and a total of 114 porpoises stranded during this period. Responders examined 95 porpoises; of these, detailed necropsies were conducted on 75 animals. Here we review the findings related to this event and how these compared to the years immediately before and after the UME. Relatively equal numbers among sexes and age classes were represented, and mortalities were attributed to a variety of specific causes, most of which were categorized as trauma or infectious disease. Continued monitoring of strandings during 4 yr following the UME showed no decrease in occurrence. The lack of a single major cause of mortality or evidence of a significant change or event, combined with high levels of strandings over several post-UME years, demonstrated that this was not an actual mortality event but was likely the result of a combination of factors, including: (1) a growing population of harbor porpoises; (2) expansion of harbor porpoises into previously sparsely populated areas in Washington's inland waters; and (3) a more well established stranding network that resulted in better reporting and response. This finding would not have been possible without the integrated response and investigation undertaken by the stranding network.