Data gathered over the last three decades made it possible to count the number of new calves born in different sub-populations at both poles.
The Northern and Southern whales were long thought to be one species until genetic analysis showed otherwise.
As suspected, the three groups of Southern whales — off the coasts of eastern South America, southern Africa and southwest Australia — produced offspring at twice the rate as their northern kin.
Further evidence that the North Atlantic environment was taking a toll was the poor health of females and their calves, the study found.
‘Ghost nets’
“That female baleen whales forgo reproduction in response to poor body conditions is well established,” the authors said.
What caused the lacerations, reduced body weight, and apparent unwillingness to mate?
The most likely culprit is “ghost nets”, sprawling webs of fishing gear often made of synthetic fibres as strong as they are long-lasting, the study concluded.
More than 80 percent of all North Atlantic right whales are known to have been entangled in abandoned netting at least once, and well over half have been there twice or more.
“Entanglements can last from months to years, and recovery can take a similar time,” the authors wrote in Royal Society Open Science.
For the Southern whales, the problem is non-existent.
Once numbered in the hundreds of thousands, slow moving right whales — migrating along coastlines — were both easy and preferred prey for whalers well into the 20th century.
The species can grow to 20 metres (65 feet) and weigh 100 tonnes, more than a fully-loaded commercial jet.
They are also docile and full of the blubber from which whale oil was made.