The incident was reported off the northwest side of Santa Rosa Island in Ventura County around 3:15 p.m.
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The U.S. Coast Guard says a good Samaritan notified Coast Guard watchstanders of a 37-year-old man who had been bitten by a shark on his right leg.
The Coast Guard says they were able to quickly find him because he was with a friend who was able to tell rescue crews where he was located.
A tourniquet was placed on the man's leg to prevent further blood loss, and he was later taken to an hospital where he was reportedly in stable condition.
Officials are unsure of what type of shark attacked the man.