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Bay Crossings Bay Environment

Sharks First! In San Francisco Bay!

By Teri Shore Bluewater Network

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 ( 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 water pollution.

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 bay.

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 schools.

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 first!

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!