Every year off the coast of California, Oregon and Washington, approximately 18 blue whales, 22 humpbacks and 43 fin whales are hit and killed by ships, according to a report by a Petaluma-based conservation group. 

The study conducted by Point Blue Conservation Science, with help from Washington's Cascadia Research Collective, shows the endangered marine mammals are being killed at higher rates than is acceptable to keep their populations growing. 

"We did our very best to be conservative with our estimates so in all likelihood the numbers are actually higher than that," said Cotton Rockwood, a senior marine ecologist with Point Blue Conservation Science. 

A blue whale washed up on a Marin County beach in May after being hit by a ship. The whale had 10 broken ribs, a badly fractured spine, and showed signs of skull trauma, according to the Marine Mammal Center. 

In June, a boat was caught on camera colliding with a humpback whale in the San Francisco Bay. Less than two weeks later, a humpback was seen barely dodging an oncoming sailboat near the Golden Gate Bridge. 

WATCH: Humpback barely dodges oncoming sailboat

These recent incidents all made headlines, but Rockwood said many more whale deaths go completely unnoticed.

"Most of them when they get hit, they usually sink. And if the water is cold enough and deep enough, they never float again," he said.

Rockwood estimates less than 17 percent of whales that are fatally struck by vessels ever surface or wash to shore. Plus, ships that strike whales often do so without anyone on board knowing. 

"There have been cases when ships come into port with the body of a whale draped over the bulbous bow and the vessel captain has no idea it is there," Rockwood said. 

So the researchers used a model to estimate how many whales are being killed annually, using what they know about whale population movements and ships' travel routes to calculate how often the two would collide. They then calculated how likely it is a whale would die when hit. 

A spokesperson for the National Oceanic and Atmospheric Administration said the agency has received a copy of the report but hasn't had a chance to review it. 

NOAA Public Affairs Officer Jim Milbury explained that on the whole, confirmed ship-strikes off the coast of California are trending downward. 

"From 2006 to 2011 the average was almost eight a year and since then the average is less than 3.5," Milbury said. "But that's only the ones we have seen and confirmed as ship strikes off California. It's just very difficult to know exactly how many may or may not have been struck." 

Rockwood and his fellow researchers hope that the study's findings will inspire change in the shipping industry, expanding the area where ships are encouraged to slow down and watch for wildlife.

John Berge, vice president of the Pacific Merchant Shipping Association in Oakland, told the San Jose Mercury News that the industry already follows voluntary speed limits where they are in place and has also helped create an app to track areas where whales are located. 

"People on a ship are just like anybody else. If we can do things to reduce [the risk of hitting a whale] we view that as a win."
