As Halloween draws closer and our thoughts turn to costumes and candy, this festive fall holiday also inspires us to dust the cobwebs off our spookiest stories.
Sevengill sharks, Notorynchus cepedianus, are named for the seven gills on each side of their pectoral fins. Most other sharks have only five gills. Locally, they reside in bays such as San Francisco, Monterey and Tomales, and are found in the South Atlantic, Pacific and Indian Oceans. Photo courtesy of Aquarium of the Bay
By Deb Self
Published: October, 2010
The leopard shark is the most common shark found in the Bay. One of the most colorful of all sharks, it is easily recognized by the brown-gray spots that led to its name. Leopard sharks grow up to six feet long and are harmless to humans, feeding mostly on benthic creatures that dwell on the floor of the Bay, such as crabs, shrimp, and herring. The second most abundant shark in the Bay is the smoothhound shark, which grows up to three feet long. This reddish-brown shark is a favorite prey of the California sea lion, and stays in shallow waters feeding on crabs, shrimp, worms and small fish. The Bay’s top aquatic predator is the sevengill shark, which gets its name from having seven gills on the sides of its head; it is black and gray, and can grow as long as ten feet and weigh over 250 pounds. The sevengill feeds on harbor seals and other sharks and is known to feed and pup in the Bay.
The spiny dogfish features a needle-sharp spine that protrudes from the front of its dorsal fin. This shark, one of the few to inhabit the Atlantic and Pacific, grows three feet long and feeds on crab, octopus, and fish. And finally, the soupfin shark, so named because its large fins were once highly valued and dried for use in soups (and in some places still are), is bluish gray and grows to five feet long. It possesses very sharp teeth for shredding herring, flounder, rockfish, and squid. Interestingly, when soupfin sharks are in open waters, the males remain in Northern California, while the females migrate to warmer southern waters. Other sharks visit the Bay, including the great white sharks who occasionally enter the Golden Gates but are most commonly seen near the Farallones islands during the fall months. (Sharktober!)
As top predators of the Bay, the health of the resident sharks is a good indicator of the health of the Bay’s ecosystem. However, to date, there has not been a comprehensive survey of shark populations in the Bay, and their total numbers remain largely unknown. (Current, ongoing studies by Sea Stewards with the California Academy of Sciences, the Monterey Bay Aquarium and by the Aquarium of the Bay will help elucidate the population size and the movements of local sharks.) Due to the Bay’s choppy, opaque waters, sharks are sometimes hard to spot, especially given their tendency to hang out near the floor of the Bay. Overfishing, pollution, dredging, and habitat degradation are just some of the threats these species encounter in the Bay’s urban setting.
Most of our knowledge of shark populations has come from fishing sharks. Shark fishing is a popular activity, and shark meat from Southern California can be found on the menus of local restaurants from a commercial fishery. There were once commercial shark fisheries in the Bay, but these collapsed due to overfishing. Today fisheries managers have a rough estimate of resident shark populations but it is uncertain what the recreational fishing pressure is having on the sharks. The most serious threat to sharks globally is the taking of sharks just for their fins to make shark fin soup. This practice, called shark finning, is estimated to take as many 73 million sharks per year. Although finning is illegal in waters of the United States and several other countries, there is no world ban and shark fin soup is commonly consumed in San Francisco from imported fins.
Another serious threat to local sharks comes from the Bay’s sediment pollution. The Bay’s sediment is contaminated with large amounts of heavy metals such as mercury and lead, as well as pollutants such as pesticides and PCBs. Because many sharks consume species located in the sediment, and because these predators are high on the food chain, sharks accumulate many of the Bay’s toxins in their muscles and fatty tissues. This pollution can cause population declines through developmental, neurological, and reproductive diseases. Toxic pulses of pollution can also decimate prey species, which the sharks rely on, thus leading to an inability to find sufficient nourishment in the Bay.
The Bay is also a very popular transit hub, with more than 7,000 container ships and deep-water tankers traversing the waters annually. This frequent ship movement requires constant dredging of the Bay’s bottom. Dredging is a highly disruptive activity that can result in the accidental killing of aquatic life, and more often redistributes buried pollutants to the sediment surface where the contaminants are readily accessible to marine life. Habitat degradation is also a common problem for sharks, as Bay fill and shoreline development reduce tidal marshland, salt ponds, and eelgrass beds that are breeding grounds and critical habitat for young sharks to grow.
Reducing our pollution to the Bay, making smart consumer choices at restaurants and grocery stores, and helping to restore the natural habitat around the shoreline can alleviate some of the pressure on local shark populations. San Francisco Baykeeper works to create a healthy, thriving Bay every day—and with your help, it may not be long before the Bay’s shark population is abundant. And if you should encounter sharks on your next Bay swim, statistics show that you most likely won’t be their next meal, but you’ll certainly have a thriller of your own to share next Halloween!
To learn more about Baykeeper’s work to keep pollution out of the Bay, visit baykeeper.org.
To learn more about local sharks attend some of the Sharktoberfest events in October and visit seastewards.org.
This article was written with the assistance of David McGuire, Director of Sea Stewards and Filmmaker, “City of the Shark,” and also draws on a previous Bay Crossings article by Teri Shore.
Deb Self is Executive Director of San Francisco Baykeeper, which uses science and advocacy to enforce clean water laws and hold polluters accountable. Deb has 25 years of experience in environmental advocacy and non-profit management, and enjoys paddling the Bay and walking its shorelines.
