Mowry Slough is located in the South Bay, on Don Edwards San Francisco Bay National Wildlife Refuge Land. Mowry is one of the larger sloughs in south San Francisco Bay (Fancher and Alcorn, 1982), and is surrounded by saltmarsh habitat .
Mowry Slough is our third study site, and is considered the primary pupping site for harbor seals in the South Bay. Extensive rookeries in the Mowry area were reported as early as 1928 (Bonnot, 1928). The seal population at Mowry fluctuates seasonally, with highest numbers seen during the pupping season, in April and May (Fancher and Alcorn, 1982). In 1992, Kopec and Harvey (1995) counted 67 pups during the pupping season at Mowry Slough, and note that between 44% and 65% of all pups observed in San Francisco Bay were recorded at Mowry Slough.
Fecal sample analysis done in part at Mowry Slough indicated that Bay seals feed on a number of fish species, including the plainfin midshipman (Porichthys notatus), the northern anchovy (Engraulis mordax), the Pacific staghorn sculpin (Leptocottus armatus), and the yellowfin goby (Acanthogobius flavimanus). The yellowfin goby is an introduced species and constituted greater than 54% of the total number of prey items found (Torok, 1994).
Seals at Mowry Slough appear to be subject to few disturbances from humans, due to the remoteness of the haul out sites there. Probable exceptions to this general lack of disturbance may be due to duck hunting season, in the fall/winter, and periodic work on the levees that crisscross the marsh area. Boat traffic is prohibited within Mowry Slough during the harbor seal pupping season (Kopec and Harvey, 1995).
Mowry Slough is largely inaccessible without
permits from the Refuge and from private landowners. However, parts
of the Refuge can be toured
on foot, and this is a great place to visit and watch for wildlife and
wetland birds. For more information, contact the Refuge
or stop by their visitor's center, located very near the eastern end of
the Dumbarton Bridge.
Bonnot, P. (1928) Report on the seals and sea lions of California. Calif. Dep. Fish and Game, Fish Bull., 64 (2): 127-132.
Fancher, L. and Alcorn, D. (1982) Harbor seal census in south San Francisco Bay (1972-1977 and 1979-1980). Calif. Fish and Game 68(2): 118-121.
Kopec, D. and Harvey, J. (1995) Toxic pollutants, health indices, and population dynamics of harbor seals in San Francisco Bay, 1989-91: a final report. Technical publication. Moss Landing, CA: Moss Landing Marine Labs.
Torok, M.L. (1994) Movements,
daily activity patterns, dive behavior, and food habits of the harbor seals
(Phoca vitulina richardsii) in the San Francisco Bay, California.
M.S. Thesis, California State University, Stanislaus. 88 pp.
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