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van Neer, Abbo; Jensen, Lasse F.; Siebert, Ursula (2015). «Grey seal (Halichoerus grypus) predation on harbour seals (Phoca vitulina) on the island of Helgoland, Germany». Journal of Sea Research97: 1-4. doi:10.1016/j.seares.2014.11.006. Bibcode: 2015JSR....97....1V.
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