South San Francisco Bay National Wildlife Refuge - Alviso, Fremont, Mowry Slough Units
Map SatelliteFrom north to south these three units, which lie on the eastern and southern portions of the Bay, are Fremont, Mowry Slough and Alviso. Various habitats by unit are as follows:
Alviso – salt pond 982 ha. (2,425 a.), mudflat 57 ha. (140 a.), salt marsh 174 ha. (430 a.), open water 34 ha. (85 ha.).
Fremont – salt pond 1,156 ha. (2,857 a.), mudflat 718 ha. (1,775 a.), salt marsh 57 ha. (141 a.), open water 290 ha. (717 ha.), upland 12 ha. (30 a.). Mowry Slough – salt pond 1,395 ha. (3,445 a.), mudflat 666 ha. (1,646 a.), salt marsh 652 ha. (1,611 a.), open water 191 ha. (473 ha.).
See here for the general description and other comments. The rare plant Cordylanthus maritimus ssp. maritimus occurs in this portion of the Refuge.
Though primarily salt ponds, the Alviso is a breeding area for the endangered California clapper rail, Rallus longirostris obsoletus, and the salt-marsh harvest mouse, Reithrodontomys raviventris; the latter is found in other units also. This and the adjacent Mowry Slough is an important nursery for the striped bass, Roccus saxatilis, and white sturgeon, Acipenser transrnontanus.
In Mowry Slough there is a hauling ground for a colony of some 400 harbor seals, Phoca vitulina (see Richardson Bay and Lower Tubbs Island).
The Fremont unit, primarily salt ponds, is heavily used by greater and lesser scaup, Aythya marila and Aythya affinis; indeed, the bulk of the Pacific Flyway scaup utilizes this Refuge.
December 1975
Inventory of California Natural Areas
Revision © 2009 Steven Louis Hartman