After an incident early this month in which a humpback whale and a whale-tour vessel collided near Maui, personnel for the Hawaiian Islands Humpback Whale National Marine Sanctuary have begun posting improved yellow, black and white signs at small boat harbors around the islands.

The signs, which replace those installed last year, better highlight the collision hazard and warn boaters to be vigilant and to remain at the helm at all times during the November-through-May whale season.

The whale sanctuary is managed by the National Oceanic and Atmospheric Administration in partnership with the state Department of Land and Natural Resources.

Judith Fogarty, special agent in charge of the NOAA Office of Law Enforcement on O'ahu, said the whale-vessel incident happened on Jan. 2 about 12 miles off Lahaina.

"After the strike, the whale surfaced and we did not see blood in the water or any physical evidence of harm to the whale," said Fogarty, who added that it's still possible the whale could have been injured.

"That's why we're asking boaters to be vigilant."

Jeffrey Walters, co-manager of the sanctuary for the DLNR, said the new signs are intended to alert boaters for their own protection as well as the whale's.

During the whale season two years ago, for example, a whale was injured, a boater was knocked unconscious and the vessel was damaged, said Walters.

And according to a DLNR statement, during that same season "a vessel-whale collision appears to have been a factor in the death of a boy aboard a whale-watch boat off O'ahu."

"These adult whales can weigh 45 tons, so hitting one is like running into a brick wall," Walters said.

"What we're worried about is that over the summer some fishermen get in the habit of setting their auto pilot and turning around to start messing with their fishing gear."

Walters said stepping away from the helm in the summer is no problem because there's little the boat could run into. But doing that in the winter whale season can lead to disaster, he said.

According to research done recently by University of Hawai'i professor Joe Mobley, as many as 6,000 humpbacks could be in Hawai'i waters this season. Each winter, the whales return to Hawai'i from their North Pacific feeding grounds to nurse their young.

Humpback calves surface to breathe as often as every three minutes and normally do not announce their presence with a large blow spout. Last year, a humpback calf was spotted in Maui waters with such deep propeller cuts in its back that Walters said it's thought to have died.

"We're trying to remind the fishermen that you've got to stay at the helm and keep your hand on the wheel at all times because you've got to be able to react in a micro-second," he said.

Reach Will Hoover at whoover@honoluluadvertiser.com.