The humpback whale season got off to a bumpy start with three reported vessel strikes in December, an unusual number so early in the season, according to NOAA officials.

The whale season generally peaks from late January through March, although the first sightings have been as early as September and the endangered marine mammals have been known to linger as late as June.

Last season there were six vessel strikes, and four in each of the previous three seasons.

On Dec. 28, a 65-foot boat struck an adult or sub-adult whale off Kaua'i's Na Pali coast, according to David Schofield, Marine Mammal Response Network coordinator for NOAA's Pacific Islands Regional Office. The whale sustained no visible injuries, Schofield said.

"It came across the bow from left to right. They were operating at a safe speed of 13 to 15 knots and there was no time to stop and they banged into the whale," he said.

Witnesses on board the vessel stated the collision was unavoidable "and that the crew acted with concern and professionalism," he said.

Schofield would not reveal whether the vessel was a tour boat because the case is under investigation. The Dec. 28 incident is the only report that's been confirmed by authorities.

On Dec. 29, officials received a report that a 10- to 12-foot whale calf with what looked like fresh propeller wounds was spotted off Lahaina and that there was blood in the water. NOAA's Large Whale Response Team was unable to confirm the report, Schofield said.

A second injured-whale report the following day involved a sub-adult humpback seen off Lahaina with a V-shaped cut that could have been caused by a vessel strike. That report also was unconfirmed.

ALARMING TREND

Schofield said it is highly unusual to have so many vessel strike reports so early in the humpback whale season. He called it an alarming trend that could be explained by the growing number of whales in Hawaiian waters and an education campaign to encourage boat captains and others to report the incidents.

In the case of the Kaua'i strike, Schofield said the boat owner and the captain "went above and beyond" what is normally expected in response to a collision. He said the vessel was placed in neutral and checked for damage to protect passengers; the NOAA hot line for marine mammal-vessel strikes was contacted and a written report filed with NOAA's Pacific Islands Regional Office and Office for Law Enforcement; and that witness statements were gathered.

The crew also searched for injured whales near the site of collision, he said.

Contact with a boat's hull can cause a whale to suffer contusions, concussions, broken bones, internal bleeding and other injuries that can't be seen externally, and propeller blades can slice open wounds that can lead to infection and excessive blood loss that can attract sharks.

There is also a risk to humans. On Christmas Day 2003, a 3-year-old boy suffered fatal injuries when the whale-watch boat he was on struck a whale off Diamond Head. The following month, a Maui fisherman was knocked unconscious when his 18-foot boat accidentally slammed into a whale.

Boaters, kayakers and other ocean users are required by law to keep a distance of at least 100 yards from humpback whales. Officials said most collisions are unintentional, occurring when a whale surfaces unexpectedly in front of a vessel.

But there are ways to reduce the likelihood of that happening, Schofield said, and going slowly is the one of them. Speeds of 13 knots — 15 mph — or less have been shown to decrease the seriousness of injuries to whales, he said.

He also advised use of propeller guards.

10,000 WHALES A YEAR

Scientific studies have shown that the Islands' humpback whale population is increasing at an annual rate of approximately 7 percent. Experts estimate 10,000 humpback whales migrate to Hawai'i annually, up from estimates of 5,000 to 7,000 just a few years ago.

The whales swim here from their colder feeding grounds in the North Pacific to breed, calve and socialize.

Last weekend's Ocean Count, sponsored by the Hawaiian Islands Humpback Whale National Marine Sanctuary, showed that humpback whales are prevalent throughout the island chain during this time of year.

More than 600 volunteers gathered data from 59 sites on O'ahu, Kaua'i, the Big Island and Kaho'olawe. Participants tallied humpback whale sightings and documented the animals' surface behavior during the survey.

Preliminary data show there were an average of four whales sighted off O'ahu per 15-minute count period, an average of five whales per count period for Kaua'i and the Big Island, and 23 per count period off Kaho'olawe in the heart of the humpback whales' calving and breeding grounds.

The best spots for whale-watching were the eastern coastline from Hanauma Bay to Makapu'u Point. Also reporting a high number of sightings were spotters at Shark's Cove, Turtle Bay and La'ie on the North Shore, and Ma'ili Point and Makua Cave on the Wai'anae Coast.

Two more Ocean Counts are scheduled for Feb. 24 and March 31. A whale count on Maui, conducted by the Pacific Whale Foundation, is scheduled for Feb. 24.

Reach Christie Wilson at cwilson@honoluluadvertiser.com.