Sipilä, Tero, and Heikki Hyvärinen. “Status and Biology of Saimaa (Phoca Hispida Saimensis) and Ladoga (Phoca Hispida Ladogensis) Ringed Seals”. NAMMCO Scientific Publications, vol. 1, June 1998, pp. 83-99, https://doi.org/10.7557/3.2982.