HONOLULU -- For the second time in the past two weeks, a humpback whale calf has been injured by a boat propeller off the island of Maui. That makes five humpback whale calves struck so far this year. That has officials concerned.

Young humpback whales often frolic near the surface of the ocean in Hawaiian waters at this time of year. Sometimes boat drivers don't see them or are not looking for them and inadvertently run them over.

A propeller can inflict severe damage.

"There is a large amount of tissue missing from the scapula region and top of pectoral flipper," said David Schofield, of the National Oceanic and Atmospheric Administration Fisheries Division, about the most recent case. "They have a skin layer and blubber area that insulates them from harsh saltwater environment. They are mammals like us and are susceptible to microbial infection."

The protective layer has been broken, meaning the whale might suffer an infection. Sharks might also kill it. Three tiger sharks were seen trailing the injured calf.

Humpback whales travel to Hawaiian waters each year to raise their young. Mother whales keep pretty close tabs on their calves, but calves must surface more often to breathe.

The calves also don't have the learned experience of hearing boat noise. That is why boat captains operating within the National Humpback Whale Sanctuary must be especially alert this time of year.

"We want to ask boat captains to stay at the helm, don't leave the helm and keep the attention on the helm," Schofield said.

If a boat does strike a whale, officials want the boat captains to contact the U.S. Coast Guard or NOAA Fisheries immediately.