North Atlantic right whales are shrinking as a result of constantly getting caught up in fishing gear, a new study reveals.

Researchers believe the cumulative stress and damage from multiple entanglement incidents stunts their growth.

The critically endangered baleen whale can live for several decades, reaching a length well in excess of 33 feet (ten metres), and sometimes weighing in excess of 100 tonnes.

But when researchers looked at images taken from the air of 129 different individuals, they found the species has been getting smaller since the 1980s.

"On average, a whale born today is expected to reach a total length about a meter shorter than a whale born in 1980," said Joshua Stewart, lead author of the study from the National Oceanic and Atmospheric Administration in California.

This, he says, represents a seven per cent decline in the average length of these giants.

"But that’s just the average, there are also some extreme cases where young whales are several meters shorter than expected,” he adds.

The study, published in the journal Current Biology, found that fishing equipment is the biggest threat to the mammals.

Pictured, the size of different North Atlantic right whales compared to their expected bulk (dotted line). Experts say the whales are now, on average, seven per cent shorter than they were in 1980

“North Atlantic right whales have been monitored consistently since the 1980s and have been declining in number since 2011, due primarily to deaths associated with entanglements in active fishing gear and vessel strikes,” the researchers warn in their study.

As a result of their dire situation they are one of the most well-studied whale populations, with detailed information on ages, sizes, and entanglement histories.

This made them an ideal species to study the long-term impact of fishing gear.

"Fishing gear entanglements in this population are unfortunately fairly common, and entanglements resulting in attached gear and severe injuries have been generally increasing over the past several decades," Dr Stewart said.

"Previous studies have shown that the increased drag from entangling gear requires right whales to spend a lot of extra energy just to go about their normal activities, and that is energy they might otherwise spend on growth or reproduction.

“In some cases, entanglements can be lethal, but it turns out that even sub-lethal entanglements can have lasting impacts on right whales."

The researchers add that the regular encounters with rogue fishing nets not only causes smaller body sizes but this “arrested growth” may also lead to reduced reproductive success.

Right whales

Also, the whales currently only survive their clashes with fishing nets because they are big enough to avoid the small gaps and pull the webbing along, but as their bodies shrink, it increases the likelihood that, in the future, they will be unable to escape the nets and perish.

Dr Stewart said: "The smaller you are, the less energetic reserves you have, and the harder it might be to survive a serious entanglement or sustained food shortage.

“This really makes me wonder about how large whales worldwide are being impacted by entanglements.

"This is by no means a problem unique to right whales. Entanglements are a major threat for whales, marine mammals, and other marine species worldwide.

"My guess is that many other species are being similarly affected, but we just don’t have the ability to detect it in less well-studied populations."