When the rarest female Atlantic whales spend months, even years, separating themselves from fishing nets, there is not much energy left to mate and breastfeed.
Facing such debris, along with ship collisions and other forms of human encroachment, has hampered the recovery of magnificent long-standing marine mammals after explosive spears and factory ships nearly wiped them out, according to a study published Wednesday.
Once the number reaches tens of thousands, the northern whale population – hovering around 450 today – rose slowly from 1990, but began to fall again around 2010.
If the Canadian and US water they spent for a quarter of a century was pure and uncluttered by human traffic, "the number of species will almost double now, and their current state of emergency will not be so terrible," the scientists lead. by Peter Corkeron of the NOAA Northeastern Fisheries Science Center in Massachusetts reported.
More importantly, there will be twice as many female whales: "The general slope of the recovery trajectory is driven by the death of women," they added.
From 1970 to 2009, 80 percent of 122 known North Atlantic whale deaths were caused by objects or human activity.
This species has not been hunted for more than half a century.
But beyond the number of whales killed is the question whether the population of the species may have been restricted in a more subtle way by people.
To find out, Corkeron compares birth rates with Southern right whales, siblings in the southern hemisphere – estimated at around 15,000 – which are far better and far less exposed to dangerous human emanations.
Data collected over the past three decades allows counting the number of new calves born in various sub-populations in both poles.
North and South Whales have long been regarded as one species until genetic analysis shows otherwise.
As expected, three Southern whale groups – off the coast of eastern South America, southern Africa, and southwest Australia – produced twice as many offspring as their northern relatives.
Further evidence that the North Atlantic environment takes casualties is the poor health of their females and calves, the study found.
& # 39; Ghost net & # 39;
"That female balin whales forget reproduction in response to poor body conditions is well established," the authors said.
What causes torn sores, reduced weight, and reluctance to mate?
The most likely perpetrators are "ghost nets", fishing equipment nets that are often made from synthetic fibers as strong as they are durable, the study concluded.
More than 80 percent of all North Atlantic whales are known to have been trapped in abandoned nets at least once, and more than half have been twice or more.
"Engagement can take place from month to year, and recovery can take the same time," the authors wrote Royal Society of Open Science.
For the Southern whale, the problem is not there.
After hundreds of thousands, slow-moving right whales – migrating along the coastline – were easy and preferred prey for whalers until the 20th century.
Species can grow up to 20 meters (65 feet) and weigh 100 tons, more than a complete commercial jet.
They are also benign and full of fat from which whale oil is made.
Endangered fin whales are swept on the coast of Belgium
The recovery of the North Atlantic whale, Eubalaena glacialis, has been limited by human deaths, Royal Society of Open Science, rsos.royalsocietypublishing.or … /10.1098/rsos.180892