MARINE MAMMAL SCIENCE, 23(2): 445–452 (April 2007) C  2007 by the Society for Marine Mammalogy No claim to original US government works DOI: 10.1111/j.1748-7692.2007.00104.x ULTRASONOGRAPHIC CHARACTERIZATION OF REPRODUCTIVE ANATOMY AND EARLY EMBRYONIC DETECTION IN THE NORTHERN FUR SEAL (CALLORHINUS URSINUS) IN THE FIELD GREGG P. ADAMS Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5B4, Canada E-mail: gregg.adams@usask.ca J. WARD T ESTA Alaska Fisheries Science Center, National Marine Mammal Laboratory, Biological Sciences Department, University of Alaska Anchorage, 3211 Providence Drive, Anchorage, Alaska 99508, U.S.A. CAROLINE E. C. GOERTZ Alaska SeaLife Center, P. O. Box 1329, Seward, Alaska 99664, U.S.A. ROLF R. REAM JEREMY T. STERLING Alaska Fisheries Science Center, National Marine Mammal Laboratory, 7600 Sand Point Way NE, Seattle, Washington 98115, U.S.A. By the 1970s the population of northern fur seals (Callorhinus ursinus) on the Pribilof Islands had declined to one-third of their historic numbers. They were declared “depleted” under the Marine Mammal Protection Act in 1988 (Chapman 1961, Loughlin et al . 1994). After nearly two decades of relative stability in major rookeries, censuses of pups and breeding bulls indicate a new decline beginning in 2000 (Towell et al . 2006). The reason for the current decline is unknown but may be related to a decrease in reproductive rates. From pelagic collections taken when the seal population was near its peak, fecundity was estimated to be high; i.e., >80% of adult females between 7 and 16 yr of age were pregnant (Trites and York 1993). However, there are no current estimates of pregnancy or pupping rates that may be used to determine if reduced fecundity is contributing to the present population decline. The Conservation Plan for the Northern Fur Seal (National Marine Mammal Lab- oratory 1993) calls for monitoring of reproductive rates using nonlethal techniques. Diagnostic, B-mode ultrasonography has been used for more than two decades as a 445