The gargantuan surf that pummeled Oahu’s north and west shores Saturday proved dangerous for many and deadly for one West Side family when a 64-year-old man died after he and his 16-year-old grandson were swept into the ocean while fishing at Nanakuli.

With 50-foot waves on the North Shore’s outer reefs and some 50-foot closeout sets at Waimea Bay, “it’s definitely the biggest swell so far of the season,” said Genki Kino, a National Weather Service meteorologist who also surfs. “We typically see one swell like this per season — today was definitely the day.”

As of 4:15 p.m. Saturday, the city’s Ocean Safety lifeguards had performed 89 rescues, 31 on the North Shore and 58 on the West Side, according to the Emergency Services Department. Lifeguards also issued 5,170 safety warnings and undertook roughly 2,500 preventative actions to keep people out of harm’s way on the West Side and 2,670 at North Shore beaches.

In the Nakakuli incident, the man and his grandson were fishing near a surf spot known as Monuments, located close to the intersection of Hakimo Road and Farrington Highway, when the two were swept off the rocks just after 11 a.m. Saturday.

The teenager managed to get out on his own and tried to help his grandfather, who had gotten caught in the rocks and was unconscious by the time Honolulu Fire Department personnel and Ocean Safety lifeguards arrived, said acting HFD spokesman Carl Otsuka. After rescuers freed the man from the rocks, Emergency Medical Services staff provided advanced medical treatment and transported him in critical condition to a hospital, where he later died.

The Honolulu Medical Examiner’s Office identified the man as David Borje of the Waianae Coast.

Otsuka said an HFD captain at the scene told him the waves were the largest he’d seen in Nanakuli in more than a decade.

HFD is warning people that if waves are big or the rocks are wet, “don’t go out there and don’t go fishing” or exploring in tidepools, Otsuka said.

Emergency Services spokeswoman Shayne Enright said the surf will continue to be large and dangerous today.

“We strongly advise staying far away from the shoreline, staying off wet rocks and not turning your back to the ocean. Also, if you are at an unguarded beach, call 911 as soon as you see someone in trouble.”

The National Weather Serv­ice reported at 4 p.m. Saturday that the largest northwest swell of the winter season was rapidly moving down the island chain, with peak surf heights of 40 to 50 feet and locally higher waves. The swell peaked late Saturday afternoon to early evening for Oahu, but a new northwest swell was due to arrive Monday and will likely push surf to warning levels once again, with peak surf heights lower than on Saturday.

Some of the other surf-­related emergency calls Saturday included a report at about 11 a.m. of a 37-year-old man who was riding on the back of a personal watercraft when he was injured at a surf spot known as Himalayas, outside Laniakea.

Then at 2 p.m., lifeguards rescued an injured surfer off Makua Beach. He refused transport to a hospital.

Lifeguards responded to an injured teenager who had been slammed into the shore by a wave at 3:30 p.m. at a spot called I Don’t Knows on the Yokohama Bay side of the beach. He also refused transport to the hospital.

Police were busy as well, dealing with hordes of motorists trying to jam into Waimea Bay Beach Park, slowing traffic on Kamehameha Highway.

Waves washed over the highway near the Rock Piles surf spot, but the road remained open.

Kino said huge waves were rolling in on the outer reefs at three North Shore surf spots, known as Log Cabins, north of Waimea Bay; Himalayas, which is accessed by personal watercraft; and Avalanche by Haleiwa. When swells are breaking that far out, they are difficult to see from the shore, but he estimated wave heights of between 50 and 60 feet.

On Maui’s north shore, Hookipa was breaking at 20 to 30 feet, according to lifeguards. In anticipation of dangerous surf, county officials earlier announced the closure of that beach park in Haiku and Baldwin Beach Park in Paia through Monday.