PORTLAND, Maine - Marine authorities in the U.S. and Canada said they will marshal resources to try to find out what's behind a string of deaths of endangered North Atlantic right whales.

The animals are among the rarest marine mammals in the world, with only about 500 still living. The countries will collaborate on a report that could help craft future regulations that protect the vulnerable whales, representatives said Friday.

Representatives of the National Oceanic and Atmospheric Administration and Fisheries and Oceans Canada both said ship strikes and entanglement in fishing gear have played roles in the deaths of the whales, and that other factors also could have played a role.

The goal of the countries is to find out more about why 13 of the whales have been found dead this year and respond with solutions, said David Gouveia, protected species monitoring program branch chief for NOAA Fisheries Greater Atlantic Region.

The report will take months to assemble, and a budget for the investigation has not been developed, officials said.

The effort will involve collecting data on each whale that died and considering factors such as changes to the environment and habitat, they said.

An average number of dead right whales would be about four, Gouveia said.