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. 2013 Apr 22;8(4):e62180.
doi: 10.1371/journal.pone.0062180. Print 2013.

The stranding anomaly as population indicator: the case of harbour porpoise Phocoena phocoena in North-Western Europe

Helene Peltier  1 , Hans J Baagøe, Kees C J Camphuysen, Richard Czeck, Willy Dabin, Pierre Daniel, Rob Deaville, Jan Haelters, Thierry Jauniaux, Lasse F Jensen, Paul D Jepson, Guido O Keijl, Ursula Siebert, Olivier Van Canneyt, Vincent Ridoux
Affiliations

The stranding anomaly as population indicator: the case of harbour porpoise Phocoena phocoena in North-Western Europe

Helene Peltier et al. PLoS One. .

Erratum in

  • PLoS One. 2014;9(1). doi:10.1371/annotation/1ce2f54e-1eea-4dba-8742-05db1017b44f

Abstract

Ecological indicators for monitoring strategies are expected to combine three major characteristics: ecological significance, statistical credibility, and cost-effectiveness. Strategies based on stranding networks rank highly in cost-effectiveness, but their ecological significance and statistical credibility are disputed. Our present goal is to improve the value of stranding data as population indicator as part of monitoring strategies by constructing the spatial and temporal null hypothesis for strandings. The null hypothesis is defined as: small cetacean distribution and mortality are uniform in space and constant in time. We used a drift model to map stranding probabilities and predict stranding patterns of cetacean carcasses under H0 across the North Sea, the Channel and the Bay of Biscay, for the period 1990-2009. As the most common cetacean occurring in this area, we chose the harbour porpoise Phocoena phocoena for our modelling. The difference between these strandings expected under H0 and observed strandings is defined as the stranding anomaly. It constituted the stranding data series corrected for drift conditions. Seasonal decomposition of stranding anomaly suggested that drift conditions did not explain observed seasonal variations of porpoise strandings. Long-term stranding anomalies increased first in the southern North Sea, the Channel and Bay of Biscay coasts, and finally the eastern North Sea. The hypothesis of changes in porpoise distribution was consistent with local visual surveys, mostly SCANS surveys (1994 and 2005). This new indicator could be applied to cetacean populations across the world and more widely to marine megafauna.

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Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Theoretical scheme of the experiment.
Figure 2
Figure 2. Seasonal maps of stranding probability in the study area from 1990 to 2009.
The darker the colour the higher the probability that animals dying in the corresponding cell would reach the coast.
Figure 3
Figure 3. Relative numbers of expected strandings along the coasts of seven large subareas from 1990 to 2009 (stranding.km −1.year−1).
BB: Bay of Biscay, WC: western Channel, EC: eastern Channel, SWNS: south-western North Sea, NWNS: north-western North Sea, SENS: south-eastern North Sea, MENS: mid-eastern North Sea, NENS: north-eastern North Sea.
Figure 4
Figure 4. Average monthly distribution of observed strandings (black bars), expected strandings (grey bars) and stranding anomaly (white bars) (n) from 1990 to 2009.
Figure 5
Figure 5. Harbour porpoise strandings collected by European stranding schemes (n) from 1990 to 2009.
Figure 6
Figure 6. Annual numbers of observed harbour porpoise strandings (n) from 1990 to 2009.
Figure 7
Figure 7. Long term harbour porpoise stranding anomalies in large subareas (n) from 1990 to 2009.
Black arrows represent detected breakpoints in time series.
Figure 8
Figure 8. Correlograms of harbour porpoise stranding anomaly in large subareas from 1990 to 2009.
If autocorrelation falls outside the dotted lines, it is considered to be significant 0 at a 5% probability level.
See this image and copyright information in PMC

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