The number of collisions between whales and boats, or other marine vessels, in the southern Gulf of Maine, may actually be far greater than previous estimates, a new study suggests.

Published in the journal Marine Mammal Science, the new study showed that nearly 15 percent of the humpback whales that come to feed in the southern Gulf of Maine every spring were struck by commercial and recreational marine vessels.

"Vessel strikes are a significant risk to both whales and to boaters," said Alex Hill, a scientist at the Whale and Dolphin Conservation and lead author of the study, in a report from Portland Press Herald. "Long-term studies can help us figure out if our outreach programs to boaters are effective, what kinds of management actions are needed and help to assess the health of the population."

For the study, the researchers analyzed the injuries on 624 humpback whales photographed in the southern Gulf of Maine from 2004 to 2013. A total of 210,733 photos were evaluated by multiple reviewers for five categories of injury consistent with a ship collision.

Out of the 624 individual humpback whales included in the study, 14.7 percent or 92 of them have injuries consistent with one or more collisions with marine vessels. Most of them were struck four or fewer times. Seven of the humpback whales collided with marine vessels at least three times, while two whales showed signs that they may have been struck at least four times.

The researchers can't exactly determine if the injuries were caused by a single event or from multiple ship collisions. However, one calf showed clear signs that it got its injuries from two separate vessel strike.

Ship collision does not only endanger the life of whales but also of the boaters. Mini vessels can easily shrink by the collision. The researchers noted that their findings could be an understatement of the real numbers of ship-whale collision as the study did not include whales that were killed by vessel strike.

Humpback whales are listed as endangered species under the US Endangered Species Act. Aside from ship strikes, humpback whales face several other threats, including entanglement, harassment from whale watchers, loss of habitat and commercial whaling.