Ultrasonographic detection of early embryonic development in the northern fur seal (Callorhinus ursinus) in the field

(Manuscript submitted to Marine Mammal Science – abbreviated version follows)


Gregg P. Adams1, J. Ward Testa2, Caroline E.C. Goertz3, Rolf R. Ream4, and Jeremy T. Sterling4


After more than 40 years of managed growth under the North Pacific Fur Seal Convention of 1911, the population of northern fur seals (Callorhinus ursinus) on the Pribilof Islands reached a peak of ~ 2 million animals, but was reduced sharply by intensive female harvest in the late 1950 and 60’s. Following cessation of the female harvest, the population rebounded briefly before plummeting sharply and leveling off in 1980 at roughly one third of their peak numbers. The species was declared “depleted” under the Marine Mammal Protection Act in 1988. After nearly 2 decades of population stability or continued slight decline in major rookeries on the Pribilof Islands, censuses of pups and breeding bulls have revealed a renewed decline beginning in 2000. 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 years of age were pregnant. 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 calls for monitoring of reproductive rates, and cites the need for new techniques that don’t require lethal collections.

To test the effectiveness of ultrasonography for determining reproductive status of pregnant fur seals, ten female northern fur seals of adult size were captured and restrained using conventional methods (Gentry and Holt 1982) on St Paul Island, Alaska on November 13‑14, 2005. Images of the uterus and ovaries were obtained via transrectal ultrasonography and recorded as digital video. Ultrasonographic identification of ovarian and uterine structures was based on previous experience with reproductive ultrasonography in other species.. Although we were prepared to induce general anesthesia, ultrasonographic examinations were well-tolerated and neither anesthesia nor tranquilization was required.

Ovarian follicles as small as 2 mm and as large as 15 mm in diameter were detected.  At least one corpus luteum (CL) was detected in all 10 seals . A single CL was detected in 8 seals, and 2 CL were detected in 2 seals. The diameter of the CL ranged from 16 to 25 mm (20.7±2.8 mm) and they were characterized as well-circumscribed, roughly spherical, relatively hypoechogenic structures within the ovary. A relatively small (10 mm) brightly echogenic structure was interpreted as a corpus ablicans in 3 ovaries.  In 9 of the 10 seals, a clearly circumscribed accumulation of fluid (black structure) was detected within the uterine lumen, and was interpreted as an embryonic vesicle. The vesicles ranged from 2 to 17 mm in diameter (8.7±5.1 mm, using 2 measurements taken perpendicular to each other), and were spherical or “guitar-pick” in shape. The apex of the “pick”-shaped vesicles appeared to be bordered on either side by thickened portions of the dorsal uterine wall.

Transabdominal (external) ultrasonography is now commonly performed on many delphinids for pregnancy diagnosis (Stone 1990, Williamson et al. 1990, Brook 2001, Brook et al. 2004, Kellar et al. 2006), and has been reported in one study on pinnipeds (Young and Grantmyre 1992). We demonstrated that transrectal ultrasonography is a safe approach and one that can provide superior imaging of the reproductive organs due to tissue proximity and minimal signal attenuation and artifact. The transrectal approach was rapid, well-tolerated and obviated the need to shave external pelage in this wild-caught pinniped. Portable diagnostic ultrasound equipment for field applications by qualified veterinarians is now commercially available, and can provide immediate visual confirmation of pregnancy and ovarian status.