More than 600 volunteers on O'ahu, Kaua'i, the Big Island and Kaho'olawe counted an average of 20 humpback whales per 15-minute period during a four-hour period on Saturday as part of the annual Hawaiian Islands Humpback Whale National Marine Sanctuary Ocean Count.

The final count for the year will take place March 31.

On Saturday, between 8 a.m. and noon, volunteers at 59 sites spotted an average of three whales per 15-minute period on O'ahu; four on Kaua'i; four on the Big Island; and nine on Kaho'olawe.

Hawai'i's humpback whale population has been increasing at an annual rate of about 7 percent, sanctuary officials said, citing scientific data. Data from counts can be used to corroborate the findings, sanctuary officials said.

Hawaiian waters provide critical breeding habitat for about two-thirds of the North Pacific humpback whale population, according to sanctuary officials.

"February is the peak of whale season in the Hawaiian Islands, and this season has proved to be a busy one for the sanctuary," said Christine Brammer, Hawaiian Islands Humpback Whale National Marine Sanctuary ocean count coordinator.