When a North Atlantic wild whales, endangered, spend months, even years, escaping from fishing nets, there is not much energy for mating and nursing calves.
Addressing these debris, along with ship collisions and other forms of human encroachment, has greatly exacerbated the rehabilitation of majestic marine mammals long after the explosions of harpoons and factories exploded, according to a study published on Wednesday.
After counting the tens of thousands, the population of the northern whale, running around 450 today, climbed slowly from 1990, but began to fall again around 2010.
If the Canadian and American waters that had flown during this fourth century were virgin and clear from human traffic, "the numbers of the species would be almost double that they are now and their current range would not be so intense, "by Peter Corkeron of the NOAA Northeastern Fisheries Science Center in Massachusetts.
More specifically, there will be double female whales: "The overall gradient of the recovery pathway is driven by women's mortality," they added.
From 1970 to 2009, 80% of the 122 known whales' deaths in the North Atlantic were caused by human objects or activities.
The species has not been hunted for more than half a century.
But beyond the number of whales that were killed, the question was whether the species could have been less severely restricted than humans.
To find out, Corkeron compared birth rates with the South Right Whale, a sister species in the southern hemisphere – estimated to number about 15,000 – that is in a much better condition and far less exposed to harmful human explosions.
Data gathered over the last three decades has made it possible to count the number of new calves born in different subpopulations at both poles.
Northern and southern whales have long been regarded as one species until genetic analysis has shown differently.
As they suspected, the three whales of the South – from the shores of eastern South America, South Africa and southwestern Australia – produced descendants at twice the rate of their northern relative.
Other signs that the North Atlantic environment was burdened were the poor health of females and calves, according to the study.
"This female whale whale resigns from reproduction in response to poor body conditions is established," the authors said.
What caused injuries, decreased body weight and apparent reluctance to share?
The most likely culprit is "ghost nets," griddle guns often made of synthetic fibers as strong as they are long-lasting, the study concluded.
More than 80 percent of all North Atlantic whales are known to have been entangled in an abandoned net at least once and more than half there were two or more.
"Discomforts can last from months to years and recovery may take a similar time," writes the authors Royal Society of Open Science.
For South whales, the problem is non-existent.
After being numbered in the hundreds of thousands, the slow moving right whales – migrating along the shores – were as easy as a preferred hunt for whalers and in the 20th century.
The species can reach 20 feet (65 feet) and weigh 100 tonnes, more than a full commercial aircraft.
They are also obedient and full of the viscous from which whale oil was made.
Threatened whale washed on a Belgian beach
The recovery of the North Atlantic whales, Eubalaena glacialis, has been limited by man-made mortality, Royal Society of Open Science, rsos.royalsocietypublishing.or … /10.1098/rsos.180892