The most endangered large whale species in the Atlantic is threatened by increasing rates of lethal and debilitating entanglements and a dramatic 40% decline in birth rates since 2010. About 500 North Atlantic right whales still survive after two decades period of modest annual growth, but the two new emerging trends are casting doubt on the species overall recovery.

That is the conclusion of a new study published in "Frontiers in Marine Science" by Dr. Scott D. Kraus, Vice President and Senior Adviser, Anderson Cabot Center for Ocean Life at the New England Aquarium, along with researchers from the University of Rhode Island, Provincetown Center for Coastal Studies, University of North Carolina, Woods Hole Oceanographic Institution, and Duke University.

"Right whales need immediate and significant management intervention to reduce mortalities and injuries from fishing gear," the authors concluded in the study. "Managers need a better understanding about the causes of reduced calving rates before this species can be considered on the road to recovery. Failure to act on this new information will lead to further declines in this population's number and increase its vulnerability to extinction."

Since 1935, when North Atlantic right whales neared extinction and whaling for this species became illegal, right whales rebounded to about 295 living whales in 1992. The number of whales then increased by about 2.8 percent a year, to an estimated 500 right whales in 2010.

But, the numbers of calves born each year dropped dramatically in the following five years. "Why? We have a couple of strong suspicions, but nothing confirmed." Dr. Kraus said. Some data suggest that in addition to the direct mortality cause by entanglements, non-lethal entanglement episodes are having long-term physical and reproductive health effects on right whales.

Other information shows that prey species have been shifting due to climate and environmental changes, potentially making harder for right whales to feed adequately. Finally, there is some concern over the potential for long-lasting implications from a disease event in the 1990s. All of these areas are being actively studied by Dr. Kraus and other whale scientists.

For the study published in "Frontiers," the researchers analyzed the rising numbers of right whales that died due to human action. From 2009 to 2013, an average of 4.3 right whales died each year, mostly due to deadly entanglements in fishing ropes and gear, according to 2015 National Marine Fisheries Service data. By comparison, between 1970 and 2009, 44 percent of right whales died from ship strikes and 35 percent died from entanglements.

Those numbers are now reversing with entanglements becoming the more prevalent killer of whales at 85 percent between 2010 and 2015 compared to 15 percent of ship strikes. Recent US and Canadian governmental action to slow ships and to move shipping lanes out of migratory paths of whales have been successful at reducing ship strikes.

In an effort to reduce entanglements, Aquarium scientists Amy Knowlton and Tim Werner are leading promising research on ropes that break more readily when whales become entangled. Knowlton and her colleagues at the Aquarium just received a state grant for $180,000 to fund research on the development of fishing ropes that can reduce whale deaths. This work will be done in collaboration with fishermen and rope manufacturers.

Other aspects of the research reveal that North Atlantic right whales may be struggling more than other right whales around the world. Here, they are growing at only 2 to 3 percent a year compared to 6 to 7 percent in other regions, the study finds.

In January 2016, the National Oceanic and Atmospheric Administration (NOAA) suggested that the species' population decline was reversing, and there were signs of recovery. But, Dr. Kraus and his fellow authors believe that the opposite might now be true. "In contrast to this optimistic view of right whale recovery, our review of the recent science suggests that fishing gear entanglements are increasing in number and severity, and that this source of injury and mortalities may be overwhelming recovery efforts."