Marine megavertebrates adrift: A framework for the interpretation of stranding data in perspective of the European Marine Strategy Framework Directive and other regional agreements

For many species of marine megafauna, strandings remain the most important source of biological samples. Because of their opportunistic nature however, strandings data have long been under- or misused in the assessment of population conservation status. Even if many national and international regulations promote the use of strandings in monitoring strategies, the interpretation of strandings remains controversial. The aim of this study is to provide a context for the interpretation of marine megafauna stranding data, in order to assess the achievement of specific objectives against Good Environmental Status criteria in the context of the EU Marine Strategy Framework Directive or other regional agreements. The first step is to construct an a priori spatial distribution under a null hypothesis H0. The a priori spatial distribution of theoretical dead animals can either be set uniformly, consistent with current knowledge on abundance of marine vertebrates, or based on management objectives. The drift prediction of these theoretical carcasses would provide a time series of strandings expected under the null hypothesis. The reverse drift of observed strandings would highlight mortality areas of stranded animals. The correction of these areas by the probability of getting stranded according to drift conditions would provide an estimated distribution of dead animals inferred from strandings. The differences between expected and observed situations constitute anomalies and highlight cases where inferred distribution departs from the a priori spatial distribution. This work proposes several population indicators that can be used anywhere in the world and can be applied for all large marine vertebrates found stranded. The integration of these indicators in MSFD and various regional agreements could provide cost-effective and relevant information on protected species. In the context of impaired ecological situations, the complementary use of several population indicators could strengthen the diagnosis made regarding conservation status and hence conservation strategies.