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.