The Colorado River Estuary and Upper Gulf of California, Baja, Mexico
337
studies, the Mexican Federal Government recognized the importance of
the Upper Gulf of California as a reproduction and nursery area for fish
and closed the area adjacent to the Colorado River mouth for fishing in
1955.
Among the invertebrates, including Cnidaria, Mollusca, Arthropoda,
and Echinodermata, 17 of the most abundant species were reported
around Montague Island and 48 from the oceanic region. Shrimp trawling
before the decree of the Reserve of the Biosphere in 1993 impacted much of
the fish and invertebrate fauna. As for fish, the area north of El Golfo de
Santa Clara and San Felipe appears to be important for juveniles of the blue
shrimp (Penaeus stylirostris; Felix-Pico and Mathews 1975). The landing of
shrimp catches in San Felipe (>90% P. stylirostris) and freshwater flow of
previous years from the Colorado River into the Upper Gulf correlate significantly. While landings are approximately 200 tons year- 1 in the absence
of freshwater flow, they increase to more than 600 tons year- 1 in years following freshwater input (S. Galindo-Bect and M. Page, pers. comm.).
Marine mammals are common in the Upper Gulf of California and adjacent areas. A colony of the abundant sea lion Zalophus californiensis
exists off San Felipe and seven species of cetacean have been recorded after
a period (1986-1987) of freshwater release (Silber et al. 1994). Bottlenose
dolphins (Tursiops truncatus) are among the most frequently encountered
groups (1,416 ind.) and are the only marine mammals in the Colorado
River and in shallow waters of less than 2.5 m depth. The Upper Gulf is part
of the habitat of an endangered endemic species, the small dolphin commonly known as "vaquita" (Phocoena sinus; 110), which occurs in small
groups. The common dolphin Delphinus delphis (14239) is numerically the
dominant cetacean, while fin whales, i.e., Balaenoptera physalus (215),
Bryde's whales, i.e., Balaenoptera edeni (7), killer whales, i.e., Orcinus orca
(17), and gray whales, i.e., Eschrichtius robustus (3) are less common.
Prior to changes in freshwater flux, the Colorado River delta was important for water birds and today it still offers habitat for 36 species
(> 100,000 ind.) in winter, though the habitat is less important during the
summer season ( -500 ind. with 15 species; Mellink et al. 1997). During the
winter, Charadriiformes dominate the avifauna which rest and feed in the
Colorado River delta. The vast majority (80%) of shorebirds (163,744) in
January are sandpipers (mainly Calidris mauri) together with more than
9000 American avocets and nearly 8000 willets (Morrison et al. 1992).
Smaller shorebirds occur mainly on soft mudflats at the southern end of
Montague and Pelicano islands while medium and larger birds visit harder
mudflats at the western margin of the delta. The delta was one of the first
Mexican sites to be incorporated into the Western Hemisphere Shorebird
Reserve Network because of its importance for shorebirds. Montague
337
studies, the Mexican Federal Government recognized the importance of
the Upper Gulf of California as a reproduction and nursery area for fish
and closed the area adjacent to the Colorado River mouth for fishing in
1955.
Among the invertebrates, including Cnidaria, Mollusca, Arthropoda,
and Echinodermata, 17 of the most abundant species were reported
around Montague Island and 48 from the oceanic region. Shrimp trawling
before the decree of the Reserve of the Biosphere in 1993 impacted much of
the fish and invertebrate fauna. As for fish, the area north of El Golfo de
Santa Clara and San Felipe appears to be important for juveniles of the blue
shrimp (Penaeus stylirostris; Felix-Pico and Mathews 1975). The landing of
shrimp catches in San Felipe (>90% P. stylirostris) and freshwater flow of
previous years from the Colorado River into the Upper Gulf correlate significantly. While landings are approximately 200 tons year- 1 in the absence
of freshwater flow, they increase to more than 600 tons year- 1 in years following freshwater input (S. Galindo-Bect and M. Page, pers. comm.).
Marine mammals are common in the Upper Gulf of California and adjacent areas. A colony of the abundant sea lion Zalophus californiensis
exists off San Felipe and seven species of cetacean have been recorded after
a period (1986-1987) of freshwater release (Silber et al. 1994). Bottlenose
dolphins (Tursiops truncatus) are among the most frequently encountered
groups (1,416 ind.) and are the only marine mammals in the Colorado
River and in shallow waters of less than 2.5 m depth. The Upper Gulf is part
of the habitat of an endangered endemic species, the small dolphin commonly known as "vaquita" (Phocoena sinus; 110), which occurs in small
groups. The common dolphin Delphinus delphis (14239) is numerically the
dominant cetacean, while fin whales, i.e., Balaenoptera physalus (215),
Bryde's whales, i.e., Balaenoptera edeni (7), killer whales, i.e., Orcinus orca
(17), and gray whales, i.e., Eschrichtius robustus (3) are less common.
Prior to changes in freshwater flux, the Colorado River delta was important for water birds and today it still offers habitat for 36 species
(> 100,000 ind.) in winter, though the habitat is less important during the
summer season ( -500 ind. with 15 species; Mellink et al. 1997). During the
winter, Charadriiformes dominate the avifauna which rest and feed in the
Colorado River delta. The vast majority (80%) of shorebirds (163,744) in
January are sandpipers (mainly Calidris mauri) together with more than
9000 American avocets and nearly 8000 willets (Morrison et al. 1992).
Smaller shorebirds occur mainly on soft mudflats at the southern end of
Montague and Pelicano islands while medium and larger birds visit harder
mudflats at the western margin of the delta. The delta was one of the first
Mexican sites to be incorporated into the Western Hemisphere Shorebird
Reserve Network because of its importance for shorebirds. Montague