Genetic analysis in aid of delineation of Management Units for a wide-ranging marine mammal, the grey seal (Halichoerus grypus), in European waters

Abstract The present study aims at filling genetic structure gaps of grey seals (Halichoerus grypus) in the Northeast Atlantic, where effective Management Units (MUs) must be established to fulfil international obligations set by OSPAR and the EU’s MSFD. Mitochondrial and nuclear genetic markers were analysed for seals from the island of Ireland, southwest England and the German/ Danish North Sea coasts, whereby the integration of previously published data led to the largest genetic dataset analysed for this species to date. Results revealed that individuals from the island of Ireland are part of a single interbreeding population, with Southwest England being a source of migrants to the island of Ireland, and the southern North Sea (Germany, Denmark) being either a source or sharing a common source of migrants to the island of Ireland. Based on observed genetic structure within the Northeast Atlantic, two MUs are proposed: (i) the Faroe Islands, Scotland and the North Sea and (ii) the island of Ireland, southwestern UK (Cornwall) and France. Additionally, Northwest Scotland and the English Channel/Dutch North Sea are proposed as transition zones. Given the species’ high mobility, an adaptive management plan based on an ongoing regional/ European scale monitoring programme is recommended.