Reference

Peter J. H. van Bree, L. 'T Hart, Albert Moesker, L. Vedder (1988): On the pupping period of Grey Seals, Halichoerus grypus (Fabricius, 1791), reproducting on a shoal near the Island of Terschelling, the Netherlands – Mammalian Biology (früher Zeitschrift für Säugetierkunde) – 53: 59 - 60.