VIRGINIA BEACH, Va. (AP) - Federal ocean managers are investigating the death of an endangered right whale off of the coast of Virginia.

The National Oceanic and Atmospheric Administration said the whale was found dead off the coast near the North Carolina border on Monday. North Atlantic right whales are one of the most endangered marine mammals, and they are coming off a year of high mortality in 2017.

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NOAA said the whale appeared to have become entangled in some kind of line. Entanglement in fishing line and ship strikes are two causes of right whale deaths.

There are less than 500 North Atlantic right whales left. NOAA said in a news release there are currently only about 100 females of breeding age in the population and more females seem to be dying than males. Births have also been declining in recent years, and to date, no new calves have been spotted in the calving grounds off Florida this year.

NOAA said the animal found dead off of Virginia is the first reported right whale mortality of 2018. At least 17 died in U.S. and Canadian waters last year.

Shortly after receiving the report of the dead right whale, NOAA officials were alerted to the presence of four live right whales in the same general area, prompting the establishment of a voluntary vessel speed restriction zone (Dynamic Management Area or DMA).

Less than 500 North Atlantic right whales are left in the ocean, according to fisheries managers.

The DMA has been established 86 nautical miles east-southeast of Virginia Beach to protect the aggregation of right whales sighted in this area by a U.S. military ship on Jan. 23, NOAA said in the news release.

The restriction zone is in effect until Feb. 7, it said.

Mariners are requested to route around this area or transit through it at 10 knots or less.