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

<div><p>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 (<i>Cyclopterus lumpus</i>) 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.</p> </div

Temperature anomalies of Skagerrak and Danish North Sea bottom waters in March and April 2005.

<p>Each image represents the difference in DMI-BSHcmod modelled bottom temperatures across 5-day periods relative to the 2003–2008 mean bottom temperatures from the same periods. Each pixel represents one 6 nm² model area. Note the movement of unusually warm bottom waters into the stranding area between 6^th and 15^th April. Black rectangles indicate the location of the Western and Eastern box respectively.</p

Lumpfish landings from the Skagerrak and Central North Sea, 2003–2008.

<p>The data is aggregated in 5-day periods and the time of the 2005 UME is highlighted in red. The mean line and standard deviation (SD) were calculated from values in the same 5-day periods across the entire 2003–2008 period, including 2005.</p

Harbour porpoise strandings on the Danish west coast, 2003–2008 (N = 438).

<p>The data is aggregated in 5-day periods and the time of the 2005 UME is highlighted in red. The mean line and standard deviation (SD) were calculated from values in the same 5-day periods across the entire 2003–2008 period, including 2005.</p

Locations of harbour porpoise strandings on the west coast of Denmark, 2003–2008 (N = 438).

<p>Circled crosses represent strandings in the April 2005 UME, with date (in April) given at each location, with the number of porpoises found in parenthesis. The location of the additional stranding on the 18^th is also noted.</p

The relative importance of independent variables in the final binomial model.

<p>The sum of squares for each variable are reported for the final model. The variables were added one at a time until all were included, using the lowest AIC so that the best model was maintained at all times. Variables that reduced the model by at least 2 AIC (in addition to the first variable added) are highlighted in bold. No interactions were found to improve the full model through forward selection. Each variable has one degree of freedom. The final model had residual 415 degrees of freedom. BW Temp is the 25-day (five 5-day periods) running mean in the bottom temperature of the Western box (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055553#pone-0055553-g003" target="_blank">Figure 3</a>). W-E Temp is the difference in the temperature of the bottom waters between the Western and Easter boxes (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0055553#pone-0055553-g003" target="_blank">Figure 3</a>). Navy is the presence-absence of naval activity, with “No Known Activity” as the base category.</p

Summary of both the exponential and density-dependent model fitted to the moult count (top) and pup counts (bottom).

<p>Variables shown are the degrees of freedom (df), log-likelihood (logLik) and AIC.</p

Summary of the exponential and density-dependent models fitted to the moult count (top) for all regions combined and pup counts (bottom).

<p>Variables shown are the degrees of freedom (df), log-likelihood (loglik) and AIC.</p

Examples of tracks of six adult females tagged in the Netherlands (black circles), migrating towards Germany during the pupping season (pink circles)[72–74].

<p>Examples of tracks of six adult females tagged in the Netherlands (black circles), migrating towards Germany during the pupping season (pink circles)[<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0189674#pone.0189674.ref072" target="_blank">72</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0189674#pone.0189674.ref074" target="_blank">74</a>].</p

Overview of differences between regions in the Wadden Sea, including area sizes and seal densities at the beginning and end of the study period, timing of stranding expressed as percentage of the total number of animals found dead during the PDV epizootics of 1988 and 2002.

<p>Adopted after [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0189674#pone.0189674.ref040" target="_blank">40</a>], hunting regulations and rehabilitation.</p