**This blog entry orginally appeared on the website oceanspaces.org.**/p>
As the largest estuary on the west coast of North America, the San Francisco Bay hosts many species of elasmobranchs, the taxonomic group of cartilaginous fishes we know as sharks, skates and rays. Although historically impacted by mining, pollution and habitat loss, the Bay remains an important nursery and feeding ground for commercial fisheries like Dungeness crab and herring but also for numerous species of sharks and rays. Several large sharks inhabit the front of the Bay, including sixgill, Broadnose sevengill and soupfin (also known as tope sharks). Leopard sharks, smooth-hound sharks and spiny dogfish also inhabit the bay, while other migratory species like the sixgill, salmon sharks and yes, great white sharks occasionally pay a visit. The leopard shark (a gentle spotted and barred shark- no relative to the sub tropical tiger shark) is widely distributed inside the Bay and gives birth to live pups along the margins in remaining wetlands and eelgrass beds. This species feeds along the bottom on fish eggs, worms, clams and other invertebrates, and range along the coastline British Columbia to the Sea of Cortez, Baja California Mexico.
The larger sharks like sevengills eat smaller sharks like spiny dogfish and the hound sharks. Avoiding predation, and even cannibalism by their own species, is likely why female sharks move into the shallowest parts of the estuary to give birth. We have recorded observations of large aggregations of pregnant leopard shark females and pupping in Richardson’s Bay, San Pablo Bay and in Alameda. Bat rays share the habitat and diet with leopard sharks and are commonly seen flapping in the shadows foraging in the mud for invertebrates. Recent studies by UC Davis biologists during the wetland restoration project in the south bay salt ponds have documented large amounts of rays and leopard sharks in that part of the Bay. These and other studies like our’s may reveal a recovery of large ocean predators that could provide increased hope to the health and balance of the San Francisco Bay.
In a joint study with the Monterey Bay Aquarium and the California Academy of Sciences, we have been tagging sharks in the Bay with the help of local fishermen for over 5 years, examining growth and movements of several species of sharks. Evidence suggests that the large Sevengill sharks are breeding in the deeper waters inside the Golden Gate. Tag returns indicate the Sevengill sharks are moving as far north as British Columbia and as far south as Baja California.
These sharks were once targeted commercially in the 1940s, primarily for the vitamin A in their livers. Along with the soupfins, sevengill sharks were fished until the population crashed and the fishery disbanded. In the 1970s and 1980s shark derbies caused further impacts on these and other large sharks in California, including white sharks. Since then, these populations appear to have recovered locally, yet the population size of sevengills and soupfin sharks is still unknown. Since an evaluation performed by Ebert in 2001 there has not been a population study on these sharks until recently. These sharks are consumed, primarily by recreational anglers and are also caught in the swordfish/shark driftnet fishery off Southern California. Known by fishermen as the “swordfish of the Bay”, the firm pale flesh of the sevengill shark is possibly sold under different acronyms along the southern coastline, as generic shark or as fish and chips. Samples from other species analyzed for mercury suggest that mercury in large Sevengill sharks exceeds health advisory guidelines.
Part of this study is a citizen science program called Shark Watch. Funded in part by the California Coastal Conservancy and Patagonia, students from the University of San Francisco and members of the community are “collecting” observations of sharks and rays in the wild or by observing fishermen on boats and at public wharves and adding the observation using the App iNaturalist.
Smart phone technology combined with the App provides an interactive educational opportunity providing a greater appreciation for sharks and rays, and adds useful scientific information for future management
So far Shark Watch has hundreds of observations from all over the world. Now sponsored by the California Academy of Sciences, we hope to increase observations by scientists on expeditions such as the Philippines Biodiversity Expedition, and by the public at large right here in our home waters.