The shark was reported around 1:15 p.m. Saturday to the harbormaster.
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The report said the shark was around one quarter of a mile south of Pillar Point Harbor.
No other details were given.
There have been several shark encounters in the Bay Area in the last few months, including fishermen who reeled in a shark near Alcatraz Island in July.
The San Mateo County Sheriff's Office is asking that people use caution if they enter the water near Half Moon Bay.