Maybe its Shark Week but a recent tweeting at Pier 39 San Francisco has been raising questions about the identification of a local shark. The shark in question was sighted swimming in the Bay and has been tweeted as a Great White Shark. In a film we produced for the Aquarium of the Bay in 2015, City of the Shark we discuss white sharks in the Bay and again in our new production Swim for Sharks. Great White Sharks do indeed visit the Bay. (henceforth called white sharks- because as Dr. John McCosker observes, they need no superlatives) Stanford’s Tagging of Pacific Predators (TOPP) have recorded four individual tagged adult white sharks inside the San Francisco Bay in 2010, with one shark returning the following year. The tags send out a signal that is recorded on receivers located around the Bay to detect salmon, sturgeon and other tagged fish such as sharks who happen to swim past. The white sharks in question stayed around the front of the Bay and exited without significant duration. Dolphin Club swimmers frequently ask about sharks and with good reason since we swim these waters year round. Although we share these waters with the occasional white shark, there are many other species that are far more common.
The shark in question is far more likely to be a Salmon shark, (Lamna ditropis). Adult salmon sharks resemble juvenile white sharks and are frequently mistaken for white sharks.
Both have the cryptic counter coloration with a darker color on the dorsal surface and lighter coloration on the bottom. This adaptation helps the predators blend in with the darker water beneath when looking down, and with the surface when looking upwards.
Adult salmon sharks are medium grey to black over most of the body, with a white underside with darker blotches. Juveniles are similar in appearance, but generally lack blotches. The snout is short and cone-shaped, and the overall appearance is similar to a small great white shark. Occasionally a reddish pink hue is detectable along the white margins.
The great white shark has a robust, large, conical snout. The upper and lower lobes on the tail fin are approximately the same size which is similar to some mackerel sharks. Like the salmon shark the white shark also displays counter shading, by having a white underside and a grey dorsal area (sometimes it is brown or blue) that gives an overall mottled appearance.A distinguishing feature of white sharks is the black margins on the underside of the pectoral fins.
Although we cant see the second dorsal or beneath the pectorals which are a sure giveaway- nor do we have anything to scale the picture it is unlikely to be a juvenile white shark. Juvenile white sharks are born and develop in Southern California, and to my knowledge there has never been a juvenile white shark identified in the Bay. Baby white and young white sharks feed on fish and develop in the Southern California Bight off Los Angeles. After a few years the older sharks venture north and adjust their diet from fish to marine mammals.
The sharks off the San Francisco Coastline are adult and sub adult sharks. Currently, the adult white sharks are beginning to make their return to feed off the Farallon Islands after an incredible migration of over two thousand miles offshore.
Given that the Bay is well within the range of salmon sharks, it is highly probable that this a salmon shark.
The great thing is that people are excited to see a living shark here in the San Francisco Bay without mention of jaws or man eating beasts. Lets make Shark Week Every week by protecting these magnificent predators.