BOSTON – The rare North Atlantic right whale recorded a third straight year of strong births, with 18 new calves spotted in a population that numbers only about 350.

The number of births is down from the past two years, when 21 and a record 31 calves were counted, respectively. But it exceeds the 20-year average of about 12 and continues a strong rebound from the low point in 2000, when just a single calf was born.

“It’s definitely an encouraging sign,” said Lisa Conger, a right whale specialist at the New England Aquarium. “In the big picture, why this population is not coming back is still a big mystery.”

The whale has been protected since 1935 after it was nearly hunted to extinction. Its recovery has been exceptionally slow and researchers are looking into pollution and environmental changes as possible factors, Conger said.

It’s also unknown why births have seen a recent surge, she added. Some scientists believe there may be a link between calving rates and the amount of zooplankton – which whales eat – in the North Atlantic. Others theorize the births follow a natural cycle that swings up and down every three to four years, meaning the births may be due to drop, Conger said.

Ship strikes and fishing gear entanglements are among the greatest threats to right whales. Conger said five of the calves born in 2001 died, two from gear entanglements. None of the 2003 births are known to have died, she said.

The right whales migrate south to the waters off Georgia and Florida in the winter to deliver and nurse their calves, then follow their food supply north in the summer.