Humpback whales swimming in the waters off Hawaii are increasingly in danger of being hit by boats.
Whale watching season lasts from December to May
So far, seven confirmed collisions have been recorded in the current breeding season, compared with 33 over the last 30 years.
Environmentalists say they are alarmed by the growing trend, but researchers believe the increase is due to a jump in population numbers.
Around 1,000 calves are born in Hawaiian waters every year.
The population of humpback whales has been growing by around 7% annually since the mid 1990s in the North Pacific area.
"As long as the population continues to get bigger, it's going to keep happening," Joseph Mobley, a professor at the University of Hawaii-West Oahu told the Associated Press news agency.
More whales are also swimming to Hawaii from icy feeding grounds off areas such as Alaska and Canada.
The calves pose a greater risk as they need to surface more often.
"It's kind of like driving in a school zone," marine biologist David Schofield of the National Oceanic and Atmospheric Administration (NOAA) said.
Boat drivers are required to follow certain procedures, such as always travelling below 13 knots, never leaving the helm, posting a lookout and staying a minimum distance away from whales.
Boat owners have to follow strict guidelines to avoid harm
Local experts say an injured whale can often disappear and their welfare unknown.
"It's unclear what happens to injured whales, which despite their size can quickly disappear, sometimes with fatal gashes and internal wounds," said Ed Lyman, from NOAA's response team.
Some 50 ships are involved in whale watching in Hawaii, carrying 300,000 passengers a year.
Humpbacks were placed under international protection in 1966, and are also shielded under US federal law.