Possible Causes of a Harbour Porpoise Mass Stranding in Danish Waters in 2005

Abstract

An unprecedented 85 harbour porpoises stranded freshly dead along approximately 100 km of Danish coastline from 7–15 April, 2005. This total is considerably above the mean weekly stranding rate for the whole of Denmark, both for any time of year, 1.23 animals/week (ranging from 0 to 20 during 2003–2008, excluding April 2005), and specifically in April, 0.65 animals/week (0 to 4, same period). Bycatch was established as the cause of death for most of the individuals through typical indications of fisheries interactions, including net markings in the skin and around the flippers, and loss of tail flukes. Local fishermen confirmed unusually large porpoise bycatch in nets set for lumpfish (Cyclopterus lumpus) and the strandings were attributed to an early lumpfish season. However, lumpfish catches for 2005 were not unusual in terms of season onset, peak or total catch, when compared to 2003–2008. Consequently, human activity was combined with environmental factors and the variation in Danish fisheries landings (determined through a principal component analysis) in a two-part statistical model to assess the correlation of these factors with both the presence of fresh strandings and the numbers of strandings on the Danish west coast. The final statistical model (which was forward selected using Akaike information criterion; AIC) indicated that naval presence is correlated with higher rates of porpoise strandings, particularly in combination with certain fisheries, although it is not correlated with the actual presence of strandings. Military vessels from various countries were confirmed in the area from the 7th April, en route to the largest naval exercise in Danish waters to date (Loyal Mariner 2005, 11–28 April). Although sonar usage cannot be confirmed, it is likely that ships were testing various equipment prior to the main exercise. Thus naval activity cannot be ruled out as a possible contributing factor.

Spatially Explicit Analysis of Genome-Wide SNPs Detects Subtle Population Structure in a Mobile Marine Mammal, the Harbor Porpoise

“…An important issue to consider, however, is the quality of genomic DNA, as ddRAD-seq can be limiting in this respect–it requires high-quality genomic DNA [88]. Specifically with regard to harbor porpoise tissue samples it is difficult to acquire or expect ‘fresh’ samples, since most are collected from stranded individuals and a smaller number are from by-catch [89]. Particularly in the case of strandings, tissue is collected from animals in various stages of decomposition with concomitant decreases in DNA quality.…”

Increased harbor porpoise mortality in the Pacific Northwest, USA: understanding when higher levels may be normal

“…Cause of death could only be determined for 5 of 11 porpoises examined as part of that investigation; 2 were attributed to traumatic injury and the remaining 3 were the result of infectious disease processes (salmonellosis, pneumonia, and peritonitis). Although a large number of animals had primary or secondary diagnoses of infection or inflammation during the UME (nearly half of these due to respiratory illnesses), this proportion would be anticipated with wild stranded individuals and has been observed in stranded porpoises in other regions (Baker & Martin 1992, Siebert et al 2001, Jauniaux et al 2002, Bogomolni et al 2010, Wright et al 2013). The spatio-temporal cluster of mortality involving respiratory disease in the eastern end of the Strait of Juan de Fuca is interesting and may have been a consequence of local factors, such as increased exposure to respiratory diseases or increased harbor porpoise susceptibility in this particular area.…”

Causes of mortality of harbor porpoises Phocoena phocoena along the Atlantic and Pacific coasts of Canada

“…Parasitism was commonly observed but was considered a secondary diagnosis in most cases, suggesting caution in interpreting parasitic infection as a significant cause of death. Previous reports of harbor porpoise mortality often indicate that the top 3 causes of death are emaciation, negative fisheries interactions, and infectious disease (Trippel et al 1996, Jauniaux et al 2002, Wright et al 2013, Huggins et al 2015.…”