Francisco to the Russian River and that eastern Pacific coastal populations rarely migrate far, unlike western Atlantic Harbor porpoise. The common bottlenose dolphin (Tursiops truncatus) has been extending its current range northwards from the Southern California Bight. The first coastal bottlenose dolphin in the San Francisco Bay Area in recent times was spotted in 1983 off the San Mateo County coast in 1983. In 2001 bottlenose dolphins were first spotted east of the Golden Gate Bridge and confirmed by photographic evidence in 2007. Zooarcheological remains of bottlenose dolphins indicated that bottlenose dolphins inhabited San Francisco Bay in prehistoric times until at least 700 years before present, and dolphin skulls dredged from the Bay suggest occasional visitors in historic times. Pollution Industrial, mining, and other uses of mercury have resulted in a widespread distribution in the bay, with uptake in the bay's phytoplankton and contamination of its sportfish. In January 1971, two Standard Oil tankers collided in the bay, creating an oil spill disaster, which spurred environmental protection of the bay. In November 2007, a ship named COSCO Busan collided with the San Francisco–Oakland Bay Bridge and spilled over of bunker fuel, creating the largest oil spill in the region since 1996. The Bay was once considered a hotspot for polybrominated diphenyl ether (PBDE) flame retardants used to make upholstered furniture and infant care items less flammable. PBDEs have been largely phased out and replaced with alternative phosphate flame retardants. A 2019 San Francisco Estuary Institute (SFEI) study assayed a wide range of these newer flame retardant chemicals in Bay waters, bivalve California mussels (Mytilus californianus), and harbor seals (Phoca vitulina) which haul out in Corkscrew Slough on Bair Island in San Mateo County, with phosphate flame retardant contaminants such as tris(1,3-dichloro-2-propyl)phosphate (TDCPP) and triphenyl phosphate (TPhP) found at levels comparable to thresholds for aquatic toxicity. Bay fill and depth profile San Francisco Bay's profile changed dramatically in the late 19th century and again with the initiation of dredging by the US Army Corps of Engineers in the 20th century. Before about 1860, most bay shores (with the exception of rocky shores, such as those in Carquinez Strait; along Marin shoreline; Point Richmond; Golden Gate area) contained extensive wetlands that graded nearly invisibly from freshwater wetlands to salt marsh and then tidal mudflat. A deep channel ran through the center of the bay, following the ancient drowned river valley. In the