The number of collisions between vessels and humpback whales this year is already up to six, prompting discussions about speed limits on the oceans around Hawai'i as a way to protect both humans and the whales.

Researchers estimate that 10,000 of the big animals visit Hawaiian waters each year — a number increasing by about 7 percent a year — but the number of collisions between boats and whales is increasing faster. The six collisions, and another that is unconfirmed, compare with four incidents in all of 2005.

"If we keep having this many collisions every year, we're probably going to be looking at some form of (speed) zoning," said Jeff Walters, Hawai'i state manager for the Hawaiian Islands Humpback Whale National Marine Sanctuary.

The issue hasn't been widely discussed, so there hasn't been much reaction to the idea. It is not even clear who might have the power to enact oceanic speed zones, but Walters said it might not be popular among boaters who need to move quickly.

"Some people are going to freak out," he said.

His federal counterpart, Naomi McIntosh, federal manager of the sanctuary, said speed and congestion both may be issues.

"Whale populations are increasing, the number of boats on the water is increasing, human interest in whales is increasing. As a result ... we're going to have to look at new management strategies," McIntosh said.

"We think we can say with confidence that speed is a known factor elsewhere, but we're still gathering data here."

Others say the facts are already pretty clear.

Going 15 mph or slower "greatly reduces the chances a whale will die from the interaction. ... We need to slow down out there," said David Schofield, Marine Mammal Response Network Coordinator with NOAA's Pacific Islands Regional Office.

One of the key areas for whale research today involves discerning how many whales there are, where they hang out, and how they behave — information that could be used to help establish rules to minimize collisions. NOAA's permits office in Washington, D.C., has issued 11 permits for humpback whale researchers to approach the animals closer than the legal limit of 100 yards. Several of those permits are for research that could yield data to help determine where collision risks are highest.

Calves are a particular issue. McIntosh said that about 1,000 calves are born in Hawaiian waters each year, meaning they might represent one-tenth of the population in the Islands during the whale season. But of 14 confirmed collisions from 2004 to present, six are confirmed or believed to have involved calves. The statistic suggests the youngest whales are the most vulnerable to collisions.

Several whale researchers and regulators said that it appears that mothers and young calves are most common in near-shore waters that are protected from strong winds and currents, and that these might be the most appropriate waters for marine speed limits.

"Calves tend to hang out in shallower waters where the moms are resting. A lot of the time they are between 5 and 20 feet, just below the surface. They are not visible and they don't really have a visible blow," said Ann Zoidis, a federally permitted whale researcher with Cetos Research.

But many kinds of whale behavior still aren't fully understood. Humpbacks come to Hawai'i to give birth and raise their calves, but no one clearly knows where they give birth, how deep, at what time of day, what their behaviors are during birthing and a lot of other things.

Chris Yates, head of NOAA's Protected Resources Division in the Islands, said detailed information is needed to drive the protection of the whales.

Calves are also less experienced than adults and may behave unpredictably. In February 2003 off Kaua'i, a calf breached behind a whalewatching boat and landed across its stern, breaking a passenger's leg. Calves also have been spotted with fresh propeller cuts in their backs.

"In Hawai'i, ship strikes are clearly my No. 1 concern. ... " Yates said. "We're looking to identify zones where calves are most likely to be."

While speed is believed to be a problem in whale strikes, most of this year's six confirmed collisions involved boats going slower than 15 mph. In at least one case in March 2006, a boat was hit by a whale while it was in neutral.

Yates said that it is the high-speed collisions that appear to cause the most damage.

"There are studies that show that the speed of the vessel makes a big difference in the severity of the injury," Yates said.

One might think that whale-watching boats ought to be the least likely to be involved in collisions, said Greg Kaufman, president of the Maui-based Pacific Whale Foundation. The boats tend to go slow around whales. They have lots of people on board actively looking for whales. And yet they do hit whales, and whales hit them.

"There are actually fewer and fewer whale-watching boats out there," although they tend to be bigger boats, Kaufman said.

They end up involved in collisions simply because they spend a lot of time on the water in areas where whales are present, he said. "The math is against commercial whale-watching operators," he said.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.