HONOLULU (AP) — Hawaii lifeguards rescued 13 people as powerful waves pounded the northern shores of the island of Oahu, kicking off what officials say is the most dangerous time of the year.

The Honolulu Star-Advertiser reports that emergency services officials say the lifeguards took about 2,800 preventative actions, such as stopping unprepared people from going into the ocean, in addition to the rescues on Friday.

Authorities issued high-surf warnings for northern and western beaches, advising people of strong currents and dangerous shore breaks.

Lifeguard Jeff Okuyama says the biggest swells occur during the winter months, so swimming and snorkeling is not an option for the unexperienced.

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Information from: Honolulu Star-Advertiser, http://www.staradvertiser.com