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Whales are usually known for how big they are, but one species that lives in the Atlantic ocean is actually shrinking.

North Atlantic right whales are - on average - a metre shorter today than whales of the same species that lived 40 years ago, with some growing up to three metres smaller.

That means whales that are 10-years-old now are only growing to sizes seen amongst whales that were one or two-years-old during the 1980s.

For the study, the researchers compared decades old photographs taken from planes to more recent shots from drones.

Biologists had photographed what they thought was a one-year-old calf based on its size, only to realise later the whale was actually five to 10 years old.

"I was pretty shocked," said Joshua Stewart the study's lead author.

North Atlantic right whales are an endangered species. There numbers are less than 400 and each year many of the whales die after being trapped in fishing gear or are hit by ships.

"Over 85 per cent of the population has entanglement injuries, either scars or attached gear, so it's a pretty chronic problem for this population," said Amy Knowlton, co-author of the study.

For the young whales that survive collisions with ships or fishing equipment, it can still cause future problems growing up. If the nets or equipment becomes attached to the whales, then the animal can be carrying that extra load with them for several months or even years. The extra weight uses a lot of energy and can stunt their growth.

"You could sort of imagine like if we were to strap a sandbag to you and you had to drag that around for a few months or a year," said Stewart.

"You're going to have a lot less energy to devote to other things, especially if you're still growing."

Stewart believes that other species of whales could be getting smaller too, but because the North Atlantic right whales have been monitored for so long, the evidence of them shrinking has become obvious.

Campaigners are now calling on fishing boats in North America and Canada to reduce their rope size, or get rid or ropes all together. It's hoped that efforts to help the whales will see a recovery for the species, with the animals returning to sizes previously seen decades ago.