Biology Reference
In-Depth Information
except for the clams (which hail from Asia) and the introduced Striped
Bass. Many of the behavioral and physiological adaptations these species
have evolved to survive in the bay environment—from migration timing
to spawning strategies—are shared to some degree by myriad other organ-
isms that call the estuary home (see p. 183, “Mini-Guide: Species in Peril”).
Environment in Flux
San Francisco Bay is a place of constant change. Conditions in bay habitats
shift daily, seasonally, and annually, dictating which species are found
where. Most of that change is driven by water conditions. The temperature
of ocean waters, the strength of river flows, and whether the marine cur-
rents flow toward land or out to sea all affect how welcoming the bay may
be for different species.
Species found in bay waters one month may turn up again next year, in
10 years, or not for half a century. “It's always interesting to see which or-
ganisms do well under a particular set of circumstances,” says Randy Bax-
ter, a senior fisheries biologist in charge of the California Department of
Fish and Game's long-term monitoring program for fish and invertebrates.
“The environment changes offshore and changes onshore, and you get this
kaleidoscopic mix of different circumstances that favor one group of organ-
isms over another group, over different years. It seems like it's never end-
ing—there's always another combination that does something unexpected.”
Salinity is a strong structuring component for all communities of bay
organisms. How much salt a creature can handle dictates where it can live,
feed, and reproduce. Seawater is typically 35 parts per thousand (ppt) of
salt. But young Dungeness Crabs seek out areas with 15-25 ppt, making
the bay an attractive haven. Species that prefer saltier water predominate
in the Central and South Bay, whereas estuarine and freshwater species
live in the upper bays and rivers. Two-thirds of the fish species swimming
around the bay are marine.
The mix of salt and fresh water in the estuary can undergo dramatic
shifts. In areas such as Suisun Bay, the water can turn from drinkable to
tangy with salt within days. In deeper waters nearer the Golden Gate, con-
ditions remain more constant. However, swings in salinity are not nearly
as dramatic as they used to be in some areas; human management of the
fresh water draining into the bay has smoothed out the extremes. The re-
sult is a more inviting delta for many nonnative freshwater species. Twelve
of the 13 most commonly caught species in the bay are native; in the delta,
less than 20 percent can claim local origins.
Beyond salinity, a number of other physical factors influence what lives
