338
S. Alvarez-Borrego
Island and adjacent areas are also important for seabirds. The brown pelican (Pelecanus occidentalis) is the most common species during early winter and double-crested cormorants (Phalacrocorax auritus) are abundant
during both winter and spring. The delta supports only moderate numbers
of American white pelicans (Pelecanus erythrorhynchos). Gulls account for
23 o/o of the seabirds, the most common species are the ring-billed gull
(Larus delawarensis), while terns and skimmers represent 16 o/o of seabirds. The black skimmer (Rhynchops niger) and Caspian tern (Sterna
caspia) are most abundant. During late spring and summer they are less
obvious because they concentrate in breeding grounds but during courtship and pair-forming activities in early spring they are more conspicuous.
22.5 Human Impacts
Apart from San Felipe (pop. -15,000) and El Golfo de Santa Clara (pop.
-600), which are sustained by fishing and recreational activities, there are
no towns at the shores of the Upper Gulf. However, the Mexicali valley at
the extreme NE of the Baja California peninsula, north of the Upper Gulf,
is one of the most important agricultural regions in Mexico. The possible
transport of pesticide from a total area of 328,000 ha, of which 186,000 ha
are under irrigation, into the marine environment of the Upper Gulf has
been a concern since the early 1970s. In 1972-1973, the average total DDT
in the clam Chione californiensis was 0.067 ppm in specimens from Montague Island and 0.145 ppm in specimens from shores 30 km north of San
Felipe, opDDD being the most abundant metabolite (Guardado-Puentes et
al. 1973). However, about 10 years later the concentrations in Chione californiensis from beaches of El Golfo de Santa Clara had decreased
(0.005-0.011 ppm) and low levels (0.008-0.009 ppm) were found in specimens of the mussel Modiolus capax 20 km south of San Felipe. Current
levels are at least two orders of magnitude below the tolerance limits for
human consumption (US Food and Drug Administration) and well below
the action levels proposed by the US National Academy of Sciences as a
danger to the environment (Gutierrez-Galindo et al. 1988).
S. Alvarez-Borrego
Island and adjacent areas are also important for seabirds. The brown pelican (Pelecanus occidentalis) is the most common species during early winter and double-crested cormorants (Phalacrocorax auritus) are abundant
during both winter and spring. The delta supports only moderate numbers
of American white pelicans (Pelecanus erythrorhynchos). Gulls account for
23 o/o of the seabirds, the most common species are the ring-billed gull
(Larus delawarensis), while terns and skimmers represent 16 o/o of seabirds. The black skimmer (Rhynchops niger) and Caspian tern (Sterna
caspia) are most abundant. During late spring and summer they are less
obvious because they concentrate in breeding grounds but during courtship and pair-forming activities in early spring they are more conspicuous.
22.5 Human Impacts
Apart from San Felipe (pop. -15,000) and El Golfo de Santa Clara (pop.
-600), which are sustained by fishing and recreational activities, there are
no towns at the shores of the Upper Gulf. However, the Mexicali valley at
the extreme NE of the Baja California peninsula, north of the Upper Gulf,
is one of the most important agricultural regions in Mexico. The possible
transport of pesticide from a total area of 328,000 ha, of which 186,000 ha
are under irrigation, into the marine environment of the Upper Gulf has
been a concern since the early 1970s. In 1972-1973, the average total DDT
in the clam Chione californiensis was 0.067 ppm in specimens from Montague Island and 0.145 ppm in specimens from shores 30 km north of San
Felipe, opDDD being the most abundant metabolite (Guardado-Puentes et
al. 1973). However, about 10 years later the concentrations in Chione californiensis from beaches of El Golfo de Santa Clara had decreased
(0.005-0.011 ppm) and low levels (0.008-0.009 ppm) were found in specimens of the mussel Modiolus capax 20 km south of San Felipe. Current
levels are at least two orders of magnitude below the tolerance limits for
human consumption (US Food and Drug Administration) and well below
the action levels proposed by the US National Academy of Sciences as a
danger to the environment (Gutierrez-Galindo et al. 1988).