WAILUKU, Maui —Federal officials are asking boaters to report any sightings of an injured humpback whale calf spotted off Ma'alaea on Wednesday with deep wounds that may have been caused by a boat propeller.

National Oceanic and Atmospheric Administration officials did not attempt to find the 14-foot-long animal because of the difficulty of locating it so long after it was first seen but are asking for reports, said David Schofield, NOAA marine mammal response coordinator.

There were no sightings of the calf yesterday.

Researchers who saw the injuries were hesitant to predict the baby whale's chances of survival.

Dave Matilla, science and rescue coordinator for the Hawaiian Islands Humpback Whale National Marine Sanctuary, said he's seen whales with much worse injuries survive, and those with barely visible wounds die from internal injuries.

"If it's just blubber, it stands a chance, but if there are internal injuries" the odds of survival are reduced, he said.

Sharks are another threat. Three tiger sharks were seen near the injured baby whale Wednesday, but Elia Herman of the Dolphin Institute said it did not appear the predators followed the calf when it swam off with its mother.

Officials believe the whale was hit by a boat two to three days before it was spotted Wednesday. The collision was not reported.

There have been five known whale-boat strikes this year, four off Maui and one off Kaua'i. Three involved whale-watch boats and one a Coast Guard rigid-hull inflatable vessel. Only the last two incidents — this week's unreported collision and a March 9 incident between a Pacific Whale Foundation boat and a calf — are believed to have caused injury to the whales.

The whale in the March 9 collision injured its head and pectoral fin, and was last seen behaving normally with its mother.

Humpback whales are protected by state and federal laws. Vessels, kayakers and other ocean users are required to stay at least 100 yards from whales, but because the animals are on the move and often underwater, it can be difficult to avoid close encounters. Boaters are not required to report collisions.

Officials say most whale strikes are accidental and prosecutions for whale harassment are rare. Federal penalties can range up to $10,000.

If there's good news to be found in the recent collisions, it's that the unfortunate incidents are a sign that Hawai'i's whale population is steadily growing.

Matilla estimated the state's humpback whale population at upwards of 10,000 — double the estimates of 10 to 15 years ago — with as many as 1,000 calves born here each season from December to May, creating a "minefield" for boaters.

Baby whales are more vulnerable to boat strikes because of their inexperience and the need to surface more frequently. Mother whales can stay submerged for 15 to 25 minutes, but calves need to breathe three or four times during that period, Matilla said.

"A lot of calves pop up in front of you all alone because they don't know any better," he said.

"Most people are still in a mindset that they are an endangered species and that they are pretty rare out there. But with 1,000 calves being born here each year, it's almost like a thousand submerged reefs that can pop up in front of you."

Animal behavior expert Joe Mobley of the University of Hawai'i-West O'ahu, who has studied humpbacks, said another reason for whale-vessel collisions could be that the animals are getting used to boats.

"In the 1980s, we would have to chase after them just to take pictures of their tails, but now they're coming up to the boats. They're less fearful and allowing boats much closer approaches, and whales are approaching boats and putting themselves in harm's way," Mobley said.

Matilla said he's disappointed that increased public awareness campaigns that included posting signs at harbors reminding boaters to proceed cautiously when whales are present didn't prevent the recent collisions. Vessel operators are advised to use slower speeds and spotters to point out whales.

"During the peak of the season you have to be very careful. You can't assume they'll get out of your way," he said.

Reach Christie Wilson at cwilson@honoluluadvertiser.com.