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

et al. (2013) Possible Causes of a Harbour Porpoise Mass Stranding in Danish Waters in 2005. PLoS ONE 8(2): e55553. doi:10.1371/journal.pone.0055553 Possible Causes of a Harbour Porpoise Mass Stranding in Danish Waters in 2005 Andrew J. Wright 0 Marie Maar 0 Christian Mohn 0 Jacob Nabe-Nielsen 0 Ursula Siebert 0 Lasse Fast Jensen 0 Hans J. Baage 0 Jonas Teilmann 0 Christopher Fulton, The Australian National University, Australia 0 1 Department of Bioscience, Aarhus University , Roskilde , Denmark , 2 Institute for Terrestrial and Aquatic Wildlife Research, University of Veterinary Medicine , Foundation, Hannover, Bu sum, Germany, 3 Fisheries and Maritime Museum , Esbjerg V, Denmark , 4 Zoological Museum , The Natural History Museum of Denmark , University of Copenhagen, Copenhagen , Denmark 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. - Funding: This work formed part of the lead authors PhD, which was funded by the Aarhus University (http://www.au.dk/), the Danish research school SNAK (http://www.snak.biology.sdu.dk/) and the Carlsberg Foundation (http://www.carlsbergfondet.dk/). The funders had no role in study design, data collection and analysis, decision to publish or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. Over the years, a number of very disparate causal factors have been assigned to cetacean strandings. These include, but are not limited to: behavioural errors, such as failure of navigation related to the use of the Earths geomagnetic field [1]; atmosphericoceanic events, such as hurricanes [2]; compromised health status caused by infectious diseases or effects of anthropogenic activities [36]; and other issues of anthropogenic origin, such as contaminants loads [710]. More recently, growing evidence also implicates a more direct role of human activities (particularly military exercises) in causing cetacean strandings through exposure to noise ([1113] and review [14]). For example, mass strandings in beaked whales have been suggested to result from either acoustic trauma [15] or behavioural responses [1619] following exposure to navy sonar. Exposure to navy sonar has also been implicated in strandings of other species [14], including harbour porpoise (Phocoena phocoena) [20] and, most recently, common dolphin (Delphinus delphis) [21]. A wider suggestion is that the animals are following basic, innate behaviours during times of extreme stress responses, such as seeking the ancestral safety of land (see [22] pages 8283 and [23]). In the majority of cases it is perhaps most likely that a combination of factors are involved [2,2426]. To that extent, lunar and solar cycles have also been noted to correlate with, and potentially influence, stranding rates [2729]. Finally oceanographic currents and wind will also play a role in determining if a stranding will occur at all [30].In addition to being directly implicated in causing strandings, as mentioned for beaked whales above, behavioural responses to acoustic exposures have also been seen to more generally increase the risk of detrimental interactions with further human activities in other cetacean species. For example, North Atlantic right whales (Eubalaena glacialis) responded to novel alarm signals by coming near, but not actually to, the surface, placing them at the highest risk of being struck by ships [31]. Similarly, higher entanglement rates were reported for humpback whales (Megaptera novaeangliae) that were exposed to underwater explosions [32]. Although the mechanism was not identified, three possibilities were suggested by the authors. Firstly, the acoustic trauma associated with the explosions could have disorientated the whales. Secondly, the ability of the whales to detect the nets acoustically may have been compromised as a consequence of temporary threshold shifts in hearing. Finally, it is quite possible that the whales were responding behaviourally to the explosions. With regards to harbour porpoise, bycatch is typically the most commonly declared cause for strandings [3335], although disease, contaminants [8] and lethal interactions with bottlenose dolphin (Tursiops truncatus) [36] have also been receiving increased attention over the last decade or so. Bycaught animals may become stranded as many cetaceans are simply thrown back into the sea or fall out of the nets before being hauled on board. Set gillnets are especially problematic for this species [35]. However, while mass strandings (typically defined as two or more individuals stranding in the same location, but not a mother and calf) are not uncommon in some species (e.g., pilot whales; Globicephala spp.) they are rare for harbour porpoise [23]. Generally, only single animals will strand at any given time [33]. Accordingly, unusual mortality events (UMEs) for porpoises are generally characterised by a substantial increase in the rate of strandings, rather than the presence of a typical mass stranding. For example, 15 porpoises that stranded in one month (compared with six per year) were declared to cons (...truncated)