The fall is prime time for sharks off the California coast and a couple have made some early appearances in the waters off Stinson Beach.
"When many sharks migrate or move from one point to another they migrate at the surface," said Peter Kimley, Ph.D., a shark expert from U.C. Davis.
Kimley believes the shark seen Thursday evening and Sunday night, possibly great whites, were looking for food at nearby Harbor Point.
"There are sharks that feed on harbor seals there. That's the objective of these sharks," said Kimley.
A shark sighting prompts an automatic three-day water closure, but this time officials are being cautious and adding an extra two days over the Labor Day weekend. The warning signs didn't scare off a family of four from San Rafael. The two young boys even body surfed in the shallow water.
"We weren't out far enough to be in any shark-infested waters we feel. We think it is safe enough to play," said Tim Sumner, from San Rafael.
They say sandy beaches like Stinson are safer because sharks don't have a way to hide from their pray. There are two factors that make an area of water dangerous for swimmers.
"One there are seals there. They feed on seals. Second, there is a rocky bottom over which they can ambush those seals," said Kimley.
The key this weekend for people at Stinson Beach is to not look like a seal in the water.