MA'ALAEA, Maui — For the second time in less than a week, a baby humpback whale has been injured by a boat — this time in a hit-and-run collision off Maui.

The 14-foot-long calf was spotted by a tour boat Wednesday at Ma'alaea with deep gashes caused by a propeller. A Dolphin Institute research vessel that was in the area found it resting at the surface with its mother in a protective position underneath the calf. Two male escort whales were with the pair, and prowling nearby were three tiger sharks.

Lou Herman, president of the nonprofit Dolphin Institute, said the five reported whale-boat collisions this year are the most ever in a single humpback whale season.

With the whale population in Hawai'i increasing at an estimated 7 percent annually, the ratio of whales to vessels is increasing, Herman said — meaning a greater threat to calves, which are difficult to see as they rest just under the surface.

On March 9, a Pacific Whale Foundation whale-watch vessel struck a whale calf off South Maui, injuring its head and pectoral fin. The whale was later seen accompanied by its mother and behaving normally.

The humpback calf spotted Wednesday was not bleeding, and it appeared the injuries had occurred within the previous two to three days, said David Schofield, marine mammal response coordinator for the National Oceanic and Atmospheric Administration's Pacific Island Region. No one reported the collision, although it is almost certain the boat operator was aware of the impact, he said.

"When these whales hit boats or boats hit whales, whatever the case may be, there's usually severe damage to the boat. It's like a small car hitting a deer," he said.

Elia Herman of the Dolphin Institute said the baby humpback had a deep slash in front of its pectoral fin and a larger gash "where blubber was flapping" above the same fin. "It wasn't a pretty sight," she said.

"I wouldn't say it had no chance of survival, but it would take a lot of luck. The mom seemed to be doing the right kinds of things to keep it safe. We'll keep our fingers crossed."

The Dolphin Institute boat monitored the whales for about four hours, and Elia Herman was encouraged when the pod started traveling and the baby humpback became more active and was seen rolling around. The sharks didn't appear to have lingered, she said.

"We're out here studying these whales ... and when you see them being hurt as a result of humans it's always a really hard thing to see," she said.

Whale researchers find the most recent whale-boat collisions particularly troubling because they involved calves, which are more vulnerable to vessel strikes because they are inexperienced and must surface more frequently. "Like young children, they aren't always where they should be and they aren't always following along with their mothers and might wander into harm's way ...," Schofield said.

"We have two calves hit in basically same area of the body, near vital organs. Whether it's just one severe boat strike or five, it's the beginning of an alarming trend," he said.

NOAA's Office of Law Enforcement is investigating the earlier collisions, including one several weeks ago involving a Coast Guard vessel. The exact date of that incident was not immediately available.

A 20-foot-plus, rigid inflatable-hull Coast Guard vessel was returning from a routine case, traveling at less than 10 mph, when it "bumped" a whale about 200 yards from Coast Guard Station Maui at Ma'alaea Harbor, said Petty Officer Michael DeNyse. A seaman saw a whale moving away from the vessel and the crew scanned the water for blood but found none, he said. An internal investigation determined there was no damage to the boat and that the whale had not been injured, DeNyse said.

The other two collisions — Jan. 17 off Kaua'i and Jan. 2 off Maui — involved whale-watch boats.

NOAA deputy special agent Marc Cline said the enforcement office is shorthanded because of training and out-of-state cases, and he has not been able to launch an investigation yet into the latest collision. The Hawai'i office has only three agents and a uniformed officer, and they assist three others assigned to U.S.-related islands in the Pacific. Altogether, there are seven officers covering 10 million square miles of ocean, Cline said, with the expansive Northwestern Hawaiian Islands soon to be added to their jurisdiction.

An estimated 5,000 humpback whales visit Hawai'i annually to socialize, mate and calve. The majority arrive by mid-December and most leave by April for their summer feeding grounds in the north Pacific.

Humpback whales are protected by the Endangered Species Act and the Marine Mammal Protection Act. Vessels are required to maintain a distance of at least 100 yards. The majority of reported whale strikes are accidental and prosecutions are rare.

It has been estimated that only a quarter of the incidents are reported. Schofield said it's important that officials are immediately notified of boat collisions with whales so that crews can respond quickly to check the animals for injuries. In the case of the Pacific Whale Foundation incident, the captain reported the strike right away and a team got on the water within an hour and a half to find the injured calf.

Schofield said boaters should operate at slower speeds and use lookouts to spot surfacing whales and blows.

Reach Christie Wilson at cwilson@honoluluadvertiser.com.