Crossings Bay Environment
Sharks First! In San Francisco
By Teri Shore
The water is too darned cold and
choppy, but here’s another reason to think twice before swimming in
San Francisco Bay: sharks! Hundreds of thousands of sharks cruise the
deep channels and shallow estuaries of the greater Bay waters. These
are not the great whites or makos that swim the cold Pacific Ocean
(and occasionally snack on surfers), but five other interesting and
mysterious sharks that prefer the relatively warm (to a shark) waters
inside the Golden Gate. In fact, the Bay is a major shark nursery.
This was all news to me until I
read Ron Russo’s fascinating story about sharks in the April-June
2001 issue of Bay Nature (www.baynature.com). Russo is one of the few
people to study sharks in San Francisco Bay. The chief naturalist at
the East Bay Regional Park District became interested in the secrets
of the Bay’s sharks thirty years ago when more than 1,000 of them
washed up dead or dying along the Alameda shoreline. No one could
determine exactly why so many died at once, but researchers suspected
that the high levels of toxins (heavy metals and chlorinated
hydrocarbons) found in the sharks’ tissues may have been the cause.
"My curiosity piqued, I
decided to undertake a research project focusing on sharks in San
Francisco Bay," said Russo. Since that time, he and other
scientists have examined more than 4,000 sharks captured in various
locations around the Bay. Russo gave me the green light to summarize
some of the observations about Bay sharks from his Bay Nature story
for Bay Crossings readers.
The local shark population consists
of five species: leopard shark, sevengill cowshark, brown smoothhound,
spiny dogfish and soupfin shark. All five breed and give birth in the
Bay. Most are year-round residents. The shark populations appear
healthy, though little is known about their life cycles. Researchers
believe the sharks may be vulnerable to overfishing, dredging and
The leopard shark is the most
common shark in California’s bays and estuaries. The year-round
resident is seen most often navigating around piers and jetties. Both
recreational and commercial fishers seek the leopard shark for sport
and eating. Striking charcoal-gray spots and bars adorn the shark’s
off-white skin, making it among the most vibrantly colored of all
Pacific sharks. Unlike other species, the leopard shark spends much of
its time resting just below the surface of bottom mud. To eat, the
shark thrusts its face into the mud to feed on clams, crabs and worms,
perhaps exposing it to toxins that accumulate in the bottom of the
Leopard shark breeding season
occurs in June and July when the sharks congregate around tidal salt
marshes. The shallow waters of the South Bay constitute a major
nursery area for the species. Young leopard sharks rely on patches of
eelgrass beds to survive and grow.
The slow-moving, powerful sevengill
cowshark is the bay’s top marine predator. Growing as long as 10
feet and weighing as much as 250 pounds, the cowshark feeds on harbor
seals and other sharks. It gets its name from the seven gills on each
side of its head, two more than found on most sharks. The
black-speckled, gray-bodied adults venture into the deeper channels of
the bay and out into the Pacific, while newborns and juveniles find
refuge in the shallows of the Bay.
The brown smoothhound shark is much
smaller than the cowshark, growing to a maximum of 36 inches long. The
bronze-to-reddish brown shark is the second most abundant in the Bay.
Although its skin appears to be smooth, like other sharks, the skin is
really made of miniature teethlike structures called denticles that
face toward the tail. This shark is a favorite prey of the California
sea lion. The brown smoothhound frequents only shallow, inshore waters
feeding on crabs, shrimp, worms and small fish. Only during breeding
season in June and July does the smoothhound move into more open
waters near shore.
Next in line is the spiny dogfish,
one of the few shark species found in both the Atlantic and Pacific.
Its distinguishing feature is a single, needle-sharp calcified spine
that protrudes from the front of each dorsal fin. This long-lived
shark does not reach sexual maturity until 20 to 25 years of age. It
uses the Bay primarily as a nursery during spring, giving birth after
a lengthy 24 months of gestation. Growing to three feet in length, it
dines on crab, octopus, fish and fish eggs from the resident herring
Lastly, the soupfin shark enters
the Bay only in spring for breeding and birthing. Its large fins were
once prized and dried for use in soups. The blue-to-dark-grey 60-inch
sharks possess extremely sharp teeth for shredding herring, flounder,
rockfish, mackerel and squid. Interestingly, when they return to sea,
the males reside in Northern California, while the female head for the
southern end of the state.
Russo said that overfishing and
pollution are probably the biggest human-caused threats to Bay sharks.
While the movie Jaws elevated sharks to stardom, it also spawned a new
shark sportfishing industry. In the Bay, shark fishing has become an
alternative to salmon, halibut and rockfish fishing when conditions
outside the Golden Gate are too rough. Because sharks are now often
found on the menu of restaurants and on dinner tables, researchers are
concerned about overfishing as the demand for shark meat increases.
Pollution is another possible
threat to Bay sharks. Run-off from homes, roads and agriculture
pollutes the Bay. Dredging for ports, marinas and shipping channels
stirs up bottom sediment. This lets loose toxins buried in the mud and
exposes sharks and other marine life to higher levels of pollution.
From a ferry rider and
environmentalist’s point of view, I wonder how the building of new
ferry terminals and the operation of high-speed ferries in shallow Bay
waters might affect the Bay sharks. Or how the proposed new airport
runways for SFO might impact shark nurseries? Dredging and filling the
Bay certainly doesn’t expand or improve the underwater environment.
Only 316 acres of eelgrass beds remain in the Bay to provide nursery
areas for leopard sharks. Maybe it’s time to think about sharks
In any case, one thing the sharks
won’t have to worry about, and that’s me swimming among their many
spiny, tooth-skinned numbers!