A spate of collisions between vessels and endangered humpback whales has prompted some scientists to call for stricter rules — including boating speed limits — in the animals’ winter migratory waters in Hawaii.

But some commercial vessel operators argue the rules are unnecessary because the humpbacks are thriving. One estimate, the number of whales migrating between Alaska and Hawaii, has increased at least 15 times in a little more than 30 years.

"The bottom line really is the whales have recovered fabulously," said David Jung, who has operated his Molokai Princess ferry for 25 years. "And what is the sense of piling on more laws that are absolutely not needed?"

Researcher Marsha Green said the Hawaiian Islands Humpback Whale National Marine Sanctuary is the only place designated as a protected habitat for humpback whales, and a scientific report indicates there are more vessel-whale collisions occurring than reported.

Environmentalists fear more collisions will occur in the future if safeguards aren’t instituted.

"What’s the point of having a sanctuary if it’s not safe?" Green said.

Scientists say the challenge is in determining the gravity of collisions. In most instances the whales disappear before their injuries, if any, can be assessed.

In late January a vessel struck a humpback whale about a half-mile out of Lahaina Harbor, leaving blood in the water. A second whale strike by a boat was reported the next morning off Maalaea, Maui. That whale also disappeared.

"We only saw one whale with propeller wounds but can’t tie it with the two cases under investigation," said David Schofield, a federal marine mammal manager.

Schofield said there is evidence that some currents between islands in Hawaii would take injured or dead whales away from the coastline into the open ocean.

Scientists say that reports of vessel-whale collisions increased from one to four per year in 2001-2005 to five to 11 per year in 2006-2010.

In a proposed management plan under review by state and federal officials, an advisory group has recommended a 14-knot (16 mph) speed limit on boats in the 1,370-square-mile sanctuary.

The advisory group is also suggesting sanctuary officials consider expanding its boundary and its protection to other species, and imposing seasonal quiet zones at highly frequented humpback whale areas such as Maalaea Bay, the Auau Channel (between Maui and Lanai) and Penguin Bank, the undersea shelf that extends from West Molokai.

Designated by Congress in 1992, the sanctuary serves as a major calving area for female humpback whales from Alaska.

To reduce the chances of harassment and injury, a federal rule requires vessels to maintain a distance of 100 yards from a humpback whale in sanctuary waters, and the state has imposed a ban on thrill crafts (such as jet skis) in the West and South Maui coastal waters during migratory season from Dec. 15 through May 15.

Environmentalists and recreational vessel operators differ on what action, if any, to take in a sanctuary where thousands of people rely on the ocean to make a living.

Those against imposing vessel speed limits say that even without speed rules the number of humpbacks has increased since hunting them was banned and the Endangered Species Act was enacted in 1973.

The number of humpback whales, Megaptera novaeangliae, that frequent Hawaii has jumped to an estimated 10,000 to 12,000 — a significant increase from the 1,000 estimated in 1979 but still below the original 15,000 to 20,000 before whaling, federal officials said.

The 30,000 to 40,000 humpbacks worldwide represent only 30 percent to 35 percent of the original population, according to the American Cetacean Society.

Vessel owners say the higher numbers of whales and calves have contributed to the increase in vessel-whale collisions.

And whale behavior has been changing. Whales are no longer fleeing boats, but are frequently swimming up to them.

Vessel owners said federal officials need to redefine the meaning of vessel-whale collisions, especially when whales swim up and bump vessels.

Tim Lyons, executive director of the Ocean Tourism Coalition, said his association is opposed to the proposed 14-knot speed limit because boat operators would have to limit distances for tours.

"It has pretty severe repercussions," said Lyons, who represents 300 members statewide.

Maui Adventure Cruises Capt. Pat Stenger said his boat handles better traveling at 17 knots because it reaches a speed capable of taking ocean troughs.

Stenger, who conducts snorkeling tours off Lanai, said reducing the speed to 14 knots would lessen the riding comfort for passengers.

Greg Kaufman, executive director of the nonprofit Pacific Whale Foundation, said, "My concern is they’re targeting recreational boaters that have a minimal effect on the whales."

A number of other national marine sanctuaries are considering or have imposed vessel speed limits.

As a means to reduce right whale collisions, the National Oceanic and Atmospheric Administration imposed requirements in areas of the East Coast in 2008, limiting vessels 65 feet and greater in length to speeds of 10 knots (11.5 mph) or less during certain times of the year.

Vessels may operate at greater speeds if necessary to maintain safe maneuvering but must enter the deviations in their log.

The restrictions on the East Coast exempt U.S. vessels owned or operated by the federal government, including the Navy and Coast Guard, and state-authorized rescue vessels.

In the Florida Keys National Marine Sanctuary, boats are limited to 4 knots in areas within 100 yards of shallow reefs throughout the year.

On the West Coast, federal and shipping industry officials are expected to make changes later this year in routes that cross three national marine sanctuaries to protect whales.